Minimize Parker Solar Probe

Parker Solar Probe - former SPP (Solar Probe Plus) Spacecraft Mission

Spacecraft     Launch    Mission Status     Sensor Complement    References

The Solar Probe Plus mission is part of NASA's LWS (Living With a Star) Program. The program is designed to understand aspects of the sun and Earth's space environment that affect life and society. The program is managed by NASA/GSFC (Goddard Space Flight Center). The Johns Hopkins University Applied Physics Laboratory (JHU/APL) in Laurel, MD., is the prime contractor for the spacecraft. In September 2010, NASA selected the Solar Probe Plus mission for development. A launch of the mission is planned for 2018. 1)

NASA's first mission to go to the sun, the Parker Solar Probe, is named after Eugene Parker who first theorized that the sun constantly sends out a flow of particles and energy called the solar wind.

On May 31, 2017, NASA has renamed the Solar Probe Plus spacecraft humanity's first mission to a star, which will launch in 2018 as the ”Parker Solar Probe” in honor of astrophysicist Eugene Parker. The announcement was made at a ceremony at the University of Chicago, where Parker serves as the S. Chandrasekhar Distinguished Service Professor Emeritus, Department of Astronomy and Astrophysics.

In 1958, Parker — then a young professor at the university’s Enrico Fermi Institute — published an article in the Astrophysical Journal called “Dynamics of the interplanetary gas and magnetic fields.” Parker believed there was high speed matter and magnetism constantly escaping the sun, and that it affected the planets and space throughout our solar system.

This phenomenon, now known as the solar wind, has been proven to exist repeatedly through direct observation. Parker’s work forms the basis for much of our understanding about how stars interact with the worlds that orbit them.

“This is the first time NASA has named a spacecraft for a living individual,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington. “It’s a testament to the importance of his body of work, founding a new field of science that also inspired my own research and many important science questions NASA continues to study and further understand every day. I’m very excited to be personally involved honoring a great man and his unprecedented legacy.”

“The solar probe is going to a region of space that has never been explored before,” said Parker. “It’s very exciting that we’ll finally get a look. One would like to have some more detailed measurements of what’s going on in the solar wind. I’m sure that there will be some surprises. There always are.”

In the 1950s, Parker proposed a number of concepts about how stars — including our sun — give off energy. He called this cascade of energy the solar wind, and he described an entire complex system of plasmas, magnetic fields and energetic particles that make up this phenomenon. Parker also theorized an explanation for the superheated solar atmosphere, the corona, which is — contrary to what was expected by physics laws — hotter than the surface of the sun itself. Many NASA missions have continued to focus on this complex space environment defined by our star — a field of research known as heliophysics.

“Parker Solar Probe is going to answer questions about solar physics that we’ve puzzled over for more than six decades,” said Parker Solar Probe Project Scientist Nicola Fox, of the Johns Hopkins University Applied Physics Laboratory. “It’s a spacecraft loaded with technological breakthroughs that will solve many of the largest mysteries about our star, including finding out why the sun’s corona is so much hotter than its surface. And we’re very proud to be able to carry Gene’s name with us on this amazing voyage of discovery.”

NASA missions are most often renamed after launch and certification; in this case, given Parker’s accomplishments within the field, and how closely aligned this mission is with his research, the decision was made to honor him prior to launch, in order to draw attention to his important contributions to heliophysics and space science.

Born on June 10, 1927, in Michigan, Eugene Newman Parker received a Bachelor of Science in physics from Michigan State University and a doctorate from Caltech. He then taught at the University of Utah, and since 1955, Parker has held faculty positions at the University of Chicago and at its Fermi Institute. He has received numerous awards for his research, including the George Ellery Hale Prize, the National Medal of Science, the Bruce Medal, the Gold Medal of the Royal Astronomical Society, the Kyoto Prize, and the James Clerk Maxwell Prize.

Table 1: NASA has renamed the Solar Probe Plus mission to Parker Solar Probe 2)

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Figure 1: NASA’s first mission to go to the sun, the Parker Solar Probe, is named after Eugene Parker who first theorized that the sun constantly sends out a flow of particles and energy called the solar wind (image credit: NASA, JHU/APL)

The SPP science objectives are: 3) 4) 5)

1) Determine the structure and dynamics of the magnetic fields at the sources of the fast and slow solar wind.

2) Trace the flow of energy that heats the corona and accelerates the solar wind.

3) Determine what mechanisms accelerate and transport energetic particles.

4) Explore dusty plasma phenomena in the near-sun environment and their influence on the solar wind and energetic particle formation.

Background: 6) 7) 8) 9) 10)

• The concept for a “solar probe” dates back to “Simpson’s CommiIee” of the Space Science Board (National Academy of Sciences, 24 October 1958). The need for extraordinary knowledge of Sun from remote observations, theory, and modeling to answer the questions:

- Why is the solar corona so much hotter than the photosphere?

- How is the solar wind accelerated?

• SPP was a NASA concept study in 2008. The challanging objective of the mission is to explore the near-Sun environment for a better understanding of solar physics. So far, no missions have penetrated closer to the Sun than 0.3 AU (Astronomical Units).

• Helios 1 and 2 were a pair of cooperative US and German deep space probes (launch Dec. 10, 1974 and Jan. 15, 1976, respectively) which set the record for the closest approach to the Sun, at ~45 million km, slightly inside the orbit of Mercury.

• The NASA MESSENGER mission (launch Aug. 3, 2004) was the first spacecraft to orbit planet Mercury. The data of the Sun are unique representing the only in situ measurements of the inner heliosphere as close as 60 solar radii (RS). The unexplored region within this distance is where the corona is accelerated to form the supersonic solar wind, and is critical to our understanding of the Sun’s impact on the solar system.

First definitions of Solar Probe missions (studies) at NASA/JPL were started in 1978. The original Solar Probe mission concept of 2005, based on a Jupiter gravity assist trajectory, was no longer feasible under the new guidelines given to the mission. A complete redesign of the mission was required to meet the mission constraints, which called for the development of alternative mission trajectories that excluded a flyby of Jupiter.

In mid-2007, NASA asked JHU/APL to consider another concept for Solar Probe that would perform all science objectives of the 2005 concept, implemented as a non-nuclear powered spacecraft, and executed under a New Frontiers-like cost cap. The resulting mission is called Solar Probe+ in recognition of the potential gains in science of the current concept over predecessors. 11) 12) 13)

In March 2012, the SPP project advanced to Phase-B. 14)

Two key technical challenges make a solar probe much more difficult than other missions: 15)

1) The extremely high temperature and harsh environment in the Sun’s proximity, which the spacecraft cannot survive without adequate thermal protection

2) The extreme difficulty of getting close to the Sun, as an enormous amount of velocity must be canceled out from the Earth orbital velocity in order for a probe to get close to the Sun.

SPP will sample the solar corona to reveal how it is heated and the solar wind and solar energetic particles are accelerated. Solving these problems has been a top science goal for over 50 years. 16) During the seven-year mission, seven Venus gravity assist (VGA) maneuvers will gradually lower the perihelia to <10 RS (Radius of sun ~700,000 km), the closest any spacecraft has come to the Sun. Throughout the 7-year nominal mission duration, the spacecraft will spend a total of 937 hours inside 20 RS , 440 hours inside 15 RS , and 14 hours inside 10 RS, sampling the solar wind in all its modalities (slow, fast, and transient) as it evolves with rising solar activity toward an increasingly complex structure. SPP will orbit the Sun in the ecliptic plane, and so will not sample the fast wind directly above the Sun’s polar regions (Figure 1). However, the current mission design compensates for the lack of in-situ measurements of the fast wind above the polar regions by the relatively long time SPP spends inside 20 RS.17) - This will allow extended measurement of the equatorial extensions of high-latitude coronal holes and equatorial coronal holes. At a helioradius ~35 RS , there are two periods per orbit (one inbound and one outbound) when SPP will be in quasi-corotation with the Sun and will cross a given longitudinal sector slowly. In these intervals, known as fast radial scans, the spacecraft will sample the solar wind over large radial distances within a given flux tube before moving across the sector. These measurements will yield additional information on the spatial/temporal dependence of structures in the solar wind and on how they merge in the inner heliosphere.

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Figure 2: Solar wind speed as a function of heliographic latitude illustrating the relationship between the structure of the solar wind and coronal structure at solar minimum (a, c) and solar maximum (b). Ulysses SWOOPS solar wind data are superposed on composite solar images obtained with the SOHO EIT and LASCO C2 instruments and with the Mauna Loa K-coronameter. (d) Solar cycle evolution (image credit: D. J. McComas et al.) .18)

Science overview:

The SPP mission targets processes and dynamics that characterize the Sun’s expanding corona and solar wind. SPP will explore the inner region of the heliosphere through in-situ and remote sensing observations of the magnetic field, plasma, and energetic particles. The solar magnetic field plays a defining role in forming and structuring the solar corona and the heliosphere. In the corona, closed magnetic field lines confine the hot plasma in loops, while open magnetic field lines guide the solar wind expansion in the inner corona. The energy that heats the corona and drives the wind derives from photospheric motions, and is channeled, stored, and dissipated by the magnetic fields that emerge from the convection zone and expand in the corona where they dominate almost all physical processes therein. Examples of these are waves and instabilities, magnetic reconnection, and turbulence, which operate on a vast range of spatial and temporal scales. Magnetic fields play also a critical role in coronal heating and solar wind acceleration. They are conduits for waves, store energy, and propel plasma into the heliosphere through complex forms of magnetic activity [e.g., CMEs (Coronal Mass Ejections), flares, and small-scale features such as spicules and jets]. How solar convective energy couples to magnetic fields to produce the multifaceted heliosphere is central to SPP science.

SPP will make in-situ and remote measurements from <10 RS to at least 0.25 AU (53.7 RS ). Measurements of the region where the solar wind originates and where the most hazardous solar energetic particles are energized will improve our ability to characterize and forecast the radiation environment of the inner heliosphere. SPP will measure local particle distribution functions, density and velocity field fluctuations, and electromagnetic fields within 0.25 AU of the Sun. These data will help answer the basic questions of how the solar corona is powered, how the energy is channeled into the kinetics of particle distribution functions in the solar corona and wind, and how such processes relate to the turbulence and wave-particle dynamics observed in the heliosphere. Cross-correlation of velocity, density, and electromagnetic fluctuations will allow a partial separation of spatial and temporal effects.

The physical conditions of the region below 20 RS are important in determining largescale properties such as solar wind angular momentum loss and global heliospheric structure. The Alfvénic critical surface, where the solar wind speed overtakes the Alfvén speed, is believed to lie in this region. This surface defines the point beyond which the plasma ceases to corotate with the Sun, i.e., where the magnetic field loses its rigidity to the plasma. In this region solar wind physics changes because of the multi-directionality of wave propagation (waves moving sunward and anti-sunward can affect the local dynamics including the turbulent evolution, heating and acceleration of the plasma). This is also the region where velocity gradients between the fast and slow speed streams develop, forming the initial conditions for the formation, further out, of CIRs (Corotating Interaction Regions).

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Figure 3: SPP, shown along its orbit (dashed curve) near a perihelion pass, will measure solar energetic ions and electrons from a vantage point very near the site where these particles are accelerated. The illustration sketches the occurrence of a solar flare and a CME extending a few RS from the Sun. The shock at the front edge of the CME and the compressed sheath plasma behind the shock form as the CME, with its entrained flux rope (tangled pink lines), pushes outward from the Sun through the ambient solar wind. Swept-up magnetic field lines are refracted and compressed across the shock and draped around the CME. Energetic particles accelerated at both the flare and CME shock are shown spiraling away from the Sun (yellow spirals) along the magnetic field. For simplicity, magnetic field lines around the shock are depicted as smooth. However, it is expected that the field ahead of CME shock and in the sheath will highly structured.Waves ahead of the shock that are produced by high intensities of shock-accelerated ions streaming away from the shock are sketched for the uppermost magnetic field line connected to the CME shock (image credit: Ref.16)

SPP participation:

• 31 institutions participate in SPP science teams

- 23 in the US, 8 foreign

- 17 educational, 5 non-profit, 8 government labs

• 106 science team members

- 69 PIs and Co-Is

- 37 additinal scientists

- Next generation graduate students and post-docs.

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Figure 4: Participating organizations in SPP (image credit: JHU/APL)

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Figure 5: This illustration of NASA's Parker Solar Probe depicts the spacecraft traveling through the Sun’s outer atmosphere (image credit: JHU/APL)

To accomplish the science objectives of addressing the fundamental questions about the Sun by acquiring critical data and measurements to answer questions that cannot be answered by observations from satellites in Earth orbit and from other interplanetary space probes, a solar probe must approach the Sun closely. A solar orbit approach to within the range of 10 solar radii (Rs) from Sun’s center must be considered to conduct the necessary in situ measurements and investigations.

Getting directly to the Sun from Earth would require a launch energy C3 as large as 423 km2/s2. This is beyond the capability of launch vehicles currently available (Atlas V, Delta IV Heavy) or to be developed in the near future. The highest launch C3 ever achieved was 164 km2/s2 for the New Horizons mission to Pluto (launch Jan. 2006).

After an extensive analysis by NASA and JHU/APL, the trajectory option 5 was chosen as the baseline trajectory for the new solar probe. The redesigned mission is named SPP (Solar Probe Plus) for its significant advantages in both technical implementation and science accomplishments as compared with the original Solar Probe mission.

The mission design utilizes seven Venus gravity assists to gradually reduce perihelion (Rp) from 35 solar radii (Rs) in the first orbit to < 10 Rs for the final three orbits. The SPP orbit consists of two primary orbit phases, a science phase (0.25AU to perihelion) and a cruise/data downlink phase (0.25AU to aphelion).

Parameter

Solar Probe (2005)

Solar Probe Plus (2008)

Minimum perihelion

4 Rs

9.5 Rs

Inclination

90º from ecliptic

3.4º from ecliptic

Number of solar passes

2

24

Total time within 20 Rs

96 hours

961 hours

Time between passes

4.6 years

88 to 150 days

Time from launch to first perihelion

4.1 years

3 months

Mission duration

8.8 years

6.9 years

Aphelion

5.5 AU

1 AU

Table 2: Comparison of Solar Probe and Solar Probe Plus (Ref. 15)

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Figure 6: Reference Mission: Launch and Mission Design Overview (image credit: JHU/APL, NASA)

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Figure 7: Detail of solar encounter timeline for a typical orbit (image credit: NASA, JHU/APL)

Figure 7 shows the orbit within ± 10 days of perihelion, and an expansion of the region ± 20 Rs. This figure also shows the time spent in each part of the solar encounter of scientific interest for one of the final orbits. In total, SPP will spend more than 2100 hrs closer than 30 Rs, nearly 1000 hours below 20 Rs, and 27 hrs in the region below 10 Rs.

SPP is an ambitious mission, requiring significant technology development in several major areas. Table 3 is a summary of the technology readiness assessment for SPP and gives an indication of the basis for technology. For each area, technology development plans have been established, and in each case, significant progress has been made to achieve TRL (Technology Readiness Level) 6 by PDR (Preliminary Design Review).

Item

TRL (Technology Readiness Level)

Comment

TPS (Thermal Protection System)

4

NASA sponsored technology development

Solar array

4

Combines space heritage and concentrator cells

Cooling system

4

Adapted from heritage systems

X-/Ka-band transponder

4

NASA sponsored technology development

LEON3 processor

5

Qualified product in new JHU/APL application

Table 3: Technology development for SPP (shows only items with TRL lower than 6)


A short presentation of the two solar missions: Parker SP (Parker Solar Probe) of NASA and Solar Orbiter of ESA

• May 16, 2018: Two upcoming missions will soon take us closer to the Sun than we’ve ever been before, providing our best chance yet at uncovering the complexities of solar activity in our own solar system and shedding light on the very nature of space and stars throughout the universe. 19)

- Together, NASA’s Parker Solar Probe and ESA’s (the European Space Agency) Solar Orbiter may resolve decades-old questions about the inner workings of our nearest star. Their comprehensive, up-close study of the Sun has important implications for how we live and explore: Energy from the Sun powers life on Earth, but it also triggers space weather events that can pose hazard to technology we increasingly depend upon. Such space weather can disrupt radio communications, affect satellites and human spaceflight, and — at its worst — interfere with power grids. A better understanding of the fundamental processes at the Sun driving these events could improve predictions of when they’ll occur and how their effects may be felt on Earth.

- “Our goal is to understand how the Sun works and how it affects the space environment to the point of predictability,” said Chris St. Cyr, Solar Orbiter project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is really a curiosity-driven science.”

- Parker Solar Probe is slated to launch in the summer of 2018, and Solar Orbiter is scheduled to follow in 2020. These missions were developed independently, but their coordinated science objectives are no coincidence: Parker Solar Probe and Solar Orbiter are natural teammates.

- Both missions will take a closer look at the Sun's dynamic outer atmosphere, called the corona. From Earth, the corona is visible only during total solar eclipses, when the Moon blocks the Sun's most intense light and reveals the outer atmosphere’s wispy, pearly-white structure. But the corona isn’t as delicate as it looks during a total solar eclipse — much of the corona’s behavior is unpredictable and not well understood.

- The corona’s charged gases are driven by a set of laws of physics that are rarely involved with our normal experience on Earth. Teasing out the details of what causes the charged particles and magnetic fields to dance and twist as they do can help us understand two outstanding mysteries: what makes the corona so much hotter than the solar surface, and what drives the constant outpouring of solar material, the solar wind, to such high speeds.

- We can see that corona from afar, and even measure what the solar wind looks like as it passes by Earth — but that’s like measuring a calm river miles downstream from a waterfall and trying to understand the current’s source. Only recently have we had the technology capable of withstanding the heat and radiation near the Sun, so for the first time, we’re going close to the source.

- “Parker Solar Probe and Solar Orbiter employ different sorts of technology, but — as missions — they’ll be complementary,” said Eric Christian, a research scientist on the Parker Solar Probe mission at NASA Goddard. “They’ll be taking pictures of the Sun’s corona at the same time, and they’ll be seeing some of the same structures — what's happening at the poles of the Sun and what those same structures look like at the equator.”

- Parker Solar Probe will traverse entirely new territory as it gets closer to the Sun than any spacecraft has come before — as close as 3.8 million miles from the solar surface. If Earth were scaled down to sit at one end of a football field, and the Sun at the other, the mission would make it to the 4-yard line. The current record holder, Helios B, a solar mission of the late 1970s, made it only to the 29-yard line.

- From that vantage point, Parker Solar Probe’s four suites of scientific instruments are designed to image the solar wind and study magnetic fields, plasma and energetic particles — clarifying the true anatomy of the Sun’s outer atmosphere. This information will shed light on the so-called coronal heating problem. This refers to the counterintuitive reality that, while temperatures in the corona can spike upwards of a few million degrees Celsius, the underlying solar surface, the photosphere, hovers around just 6,000ºC. To fully appreciate the oddity of this temperature difference, imagine walking away from a campfire and feeling the air around you get much, much hotter.

- Solar Orbiter will come within 26 million miles of the Sun — that would put it within the 27-yard line on that metaphorical football field. It will be in a highly tilted orbit that can provide our first-ever direct images of the Sun’s poles — parts of the Sun that we don’t yet understand well, and which may hold the key to understanding what drives our star’s constant activity and eruptions.

- Both Parker Solar Probe and Solar Orbiter will study the Sun’s most pervasive influence on the solar system: the solar wind. The Sun constantly exhales a stream of magnetized gas that fills the inner solar system, called solar wind. This solar wind interacts with magnetic fields, atmospheres, or even surfaces of worlds throughout the solar system. On Earth, this interaction can spark auroras and sometimes disrupt communications systems and power grids.

- Data from previous missions have led scientists to believe the corona contributes to the processes that accelerate particles, driving the solar wind’s incredible speeds — which triple as it leaves the Sun and passes through the corona. Right now, the solar wind travels some 92 million miles by the time it reaches the spacecraft that measure it — plenty of time for this stream of charged gases to intermix with other particles traveling through space and lose some of its defining features. Parker Solar Probe will catch the solar wind just as it forms and leaves the corona, sending back to Earth some of the most pristine measurements of solar wind ever recorded. Solar Orbiter’s perspective, which will provide a good look at the Sun’s poles, will complement Parker Solar Probe’s study of the solar wind, because it allows scientists to see how the structure and behavior of the solar wind varies at different latitudes.

- Solar Orbiter will also make use of its unique orbit to better understand the Sun’s magnetic fields; some of the Sun’s most interesting magnetic activity is concentrated at the poles. But because Earth orbits on a plane more or less in line with the solar equator, we don’t typically get a good view of the poles from afar. It’s a bit like trying to see the summit of Mount Everest from the base of the mountain.

- That view of the poles will also go a long way toward understanding the overall nature of the Sun’s magnetic field, which is lively and extensive, stretching far beyond the orbit of Neptune. The Sun’s magnetic field is so far-reaching largely because of the solar wind: As the solar wind streams outward, it carries the Sun’s magnetic field with it, creating a vast bubble, called the heliosphere. Within the heliosphere, the solar wind determines the very nature of planetary atmospheres. The heliosphere’s boundaries are shaped by how the Sun interacts with interstellar space. Since Voyager 1’s passage through the heliopause in 2012, we know these boundaries dramatically protect the inner solar system from incoming galactic radiation.

- It’s not yet clear how exactly the Sun’s magnetic field is generated or structured deep inside the Sun — though we do know intense magnetic fields around the poles drives variability on the Sun, causing solar flares and coronal mass ejections. Solar Orbiter will hover over roughly the same region of the solar atmosphere for several days at a time while scientists watch tension build up and release around the poles. Those observations may lead to better awareness of the physical processes that ultimately generate the Sun’s magnetic field.

- Together, Parker Solar Probe and Solar Orbiter will refine our knowledge of the Sun and heliosphere. Along the way, it’s likely these missions will pose even more questions than they answer — a problem scientists are very much looking forward to.

- "There are questions that have been bugging us for a long time," said Adam Szabo, mission scientist for Parker Solar Probe at NASA Goddard. "We are trying to decipher what happens near the Sun, and the obvious solution is to just go there. We cannot wait — not just me, but the whole community."




Spacecraft:

At 9.5 Rs, the solar intensity is 512 times that at 1AU. SPP is packaged behind the carbon-carbon TPS (Thermal Protection System), a 11 cm thick heat shield, to protect it from this extreme solar environment and allow it to operate at standard space thermal environments while the TPS experiences temperatures of 1400ºC on its sun-facing surface. SPP utilizes actively cooled solar arrays for power generation maintaining the solar cells within required temperature limits (Ref. 3). 20) 21)

Solar Probe Plus is a 3-axis stabilized spacecraft, shown in Figure 8, with functional block diagram in Figure 10.

TPS: The most prominent feature is the 2.3 m diameter TPS, with associated structure used to attach the shield to the spacecraft. The TPS protects the bus and payload within its umbra during solar encounter. The conceptual science instruments are mounted either directly to the bus, on a stand-off bracket near the fairing attachment, or on a science boom extended from the rear of the spacecraft.

In general, the payload is protected from the effects of solar exposure by the TPS. Two notable exceptions are the SPC (Solar Probe Cup), part of the SWEAP investigation, and the electric field antennas carried as part of the FIELDS investigation. Both sensor packages extend beyond the TPS and see the same environment as the TPS sunward-looking face. Both sensors are of high heritage; however the solar environment during solar encounter is significantly more severe than all previous experience. Therefore, technology development programs for each have been implemented to demonstrate the operation of each in the expected SPP environment.

Three deployable conceptual carbon-carbon plasma wave antennas are mounted 120º apart on the side of the bus. These antennas will partially protrude beyond the umbra during encounter. The solar array cooling system dissipates the high solar flux absorbed by solar array wings during closest approach to the sun enabling the solar cells to operate within their temperature constraints while providing the required electrical power. Water in the cooling system is pumped from the outboard-most edge of the solar array substrate, or platen, up through channels in the solar array wings into the four cooling system radiators mounted under the TPS and back through the pump located on the top deck of the spacecraft. The system can dissipate 6000 W of heat at perihelion, and is designed and operated to prevent freezing at aphelion.

The new configuration uses a single pair of arrays to generate power. The bulk of the solar array panel is filled with “primary cells” similar to cells used on the MESSENGER mission to Mercury, while the angled panel on the end of the solar arrays use cells designed to withstand the high illumination during perihelion.

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Figure 8: Illustration of the Solar Probe Plus spacecraft configuration (image credit: JHU/APL)

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Figure 9: Spacecraft overview (image credit: JHU/APL, Ref. 10)

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Figure 10: Block diagram of the Solar Probe Plus spacecraft (image credit: JHU/APL, Ref. 4)

At aphelion, the entire array is exposed to sunlight. As the spacecraft nears the sun, the array is tilted toward the spacecraft body until at perihelion only the end of the array is exposed to sunlight in the penumbra created by the TPS knife edge. The array substrate is a titanium plate with channels running under the cells. Water pumped through this panel carries heat to radiators mounted on the TPS support structure. Figure 11 shows the configuration at perihelion.

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Figure 11: Solar array illumination at perihelion (image credit: JHU/APL)

Technology development work on TPS has resulted in several changes to the design. The TPS remains a carbon-carbon and carbon foam sandwich, with a ceramic coating on the Sun-facing surface to control reflectance and emissivity properties. The shape and size of the TPS has changed to optimize mass while considering manufacturability and the need for longer knife edges for illumination control of the solar arrays. In particular, the TPS has shrunk from nearly 3 m in diameter to reflect more efficient packaging of the spacecraft.

The spacecraft in Figure 8 also reflects the new antenna configuration, including a 0.6 m HGA (High Gain Antenna) mounted on the body of the spacecraft instead of a boom. In addition to mass optimization, this change removes the need to deploy and retract the HGA each orbit to protect it from thermal damage at perihelion, thus increasing the reliability of the system.

The design uses a blowdown monopropellant hydrazine system for propulsion, with thrusters for attitude control and trajectory correction. Star Trackers and an internally redundant IRU (Inertial Measurement Unit) are included for guidance and control. The avionics suite is based on the APL IEM (Integrated Electronics Module) and PDU (Power Distribution Unit) used in most APL missions over the last decade or more. The IEM houses the command and data handling processor, solid-state recorder, interface to the guidance and control instruments, and payload interface. The PDU is an internally redundant box that includes all power switching. RIO (Remote I/O) devices are used to collect spacecraft telemetry, and communicate with the avionics suite through serial data links.

Avionics and SpaceWire Network: 22)

• SpaceWire selected over 1553: SpaceWire offers greater bandwidth and lower emissions

• Redundant processor module: (prime, hot spare, warm spare)

• Redundant electronic modules: SSRs (Solid State Recorders) are cross strapped

• Two cross strapped transponders.


Communication Coverage Profile:

Adequate communication links between ground and spacecraft are essential for mission operations. Transmissions of spacecraft operation commands, spacecraft telemetry, science observation sequences, and instrument measurement data between the SPP spacecraft and ground are through the spacecraft Telecomm system and NASA’s DSN (Deep Space Network) of tracking stations located at Goldstone in the United States, Canberra in Australia, and Madrid in Spain. Besides the data transmission, navigation of the SPP spacecraft will rely on regular and periodic tracking of the spacecraft through the DSN. The communication coverage of the spacecraft over the mission duration directly affects the spacecraft’s operation, science data download, navigation, and the control of the flight trajectory. Due to launch and navigation errors, the flight trajectory needs to be periodically adjusted by applying a TCM (Trajectory Correct Maneuver). Availability of adequate navigation tracking and communication links to the spacecraft dictates the placement of the trajectory correction maneuvers, which has direct impacts on the onboard ΔV budget (Ref. 21).

A comprehensive study of detailed communication coverage over the entire mission was conducted in Phase B across multiple SPP subsystem teams. Because of the unique operation environment of the SPP mission, many factors must be understood in order to maintain adequate communications with the spacecraft. First, the highly elliptical solar orbits across the inner solar system cause frequent solar conjunctions, sometimes with extended periods. And secondly, the spacecraft’s TPS obstructs the view of the antenna and causes extra outage of communication times.

The X-band is baselined for spacecraft tracking for navigation and works for both uplink and downlink modes. The Ka-band is mainly for science data downlink and works only for the downlink mode.

Besides the communication outage due to the solar conjunctions attributed to the viewing geometry of Sun, Earth, and the spacecraft, the TPS of the spacecraft sometimes causes additional outage. Because of the extremely high heat radiated from the Sun, the spacecraft bus must be constantly protected from direct solar radiation to prevent overheating. When spacecraft solar distance is less than 0.7 AU, the spacecraft must be oriented with the TPS pointed at the Sun, so that the spacecraft bus and components including the antennas are behind the TPS and are protected inside the TPS umbra. Since the antennas are behind the TPS, some of the radio transmission is obstructed by the TPS. About 14° of the field of view from the center of the TPS is blocked. When the direction of Earth is near the direction of the Sun and within the 14° cone angle about the TPS center, the view from the SPP antenna to Earth is obstructed by the TPS, thereby preventing communication between Earth and the spacecraft.

The survive the extreme solar radiation conditions, the TPS must remain pointed toward the sun at all times. The flight software is required to be capable of controlling attitude within 5 seconds of a processor reset or demotion. The spacecraft has three flight processors (prime, hot spare, and backup spare) to meet this requirement. The tight TPS pointing requirements cause geometric challenges for communications with earth resulting in severely limited communication availability and bandwidth. The SPP spacecraft will use Ka-band downlink transmissions which provide high throughput with CFDP (CCSDS File Delivery Protocol) to return as much data as possible. The SPP spacecraft will use X-band uplink with CFDP to provide efficient guaranteed delivery of commands and save uplink bandwidth when deploying command loads to multiple processors. 23)

Commanding: The SPP flight software reuses heritage code from JHU/APL missions designed to use CCSDS Telecommand packets for commanding. The SPP Ground Software has a database of commands which can create telecommand packets and package them into CLTUs (Command Link Transmission Units). SPP supports the unreliable delivery Expedited Service (BD Service) of the CCSDS Communications Operations Procedure-1 (COP-1) commanding protocol. SPP does not support the reliable Sequence-Controlled Service (AD Service) of COP-1 commanding. The COP-1 AD Service is not well suited for deep space without modification as it provides a limited maximum number of commands without acknowledgment and requires significant retransmission if a single command is dropped.

SPP is a decoupled mission where each SOC (Science Operations Center) can command their instrument as they see fit. Aside from a limited set of calibration activities and earth pointing for communications (when allowed), the spacecraft pointing is fixed at the sun. There is no coordination required between the SOCs and the MOC (Mission Operations Center) to point the spacecraft. The MOC validates that instrument commands are well-formed, targeted to the right destination, and have an APID (Application Identifier) within the assigned instrument APID range prior to allowing transmission to the spacecraft. The MOC does not perform any further validation of instrument commands. The flight software will only send instrument CF contents to the target instrument interface. Instruments can only be commanded via files sent to the MOC by SFTP. These command files are queued and later uplinked to the spacecraft. A separate sequence number will be used for each instrument interface. This guarantees the ordering of files sent to the instrument interface while not impacting sequencing of CDH (Command and Data Handling) or other instrument command files. Due to power constraints, instruments are off during Ka telemetry downlinks, but files can still be uplinked during this period and later streamed to the instrument when it is powered on. Figure 12 highlights the steps involved in sending instrument commands to the target instrument.

The MOC runs a file queue management application that is responsible for initiating the uplink file transfers. The spacecraft CDH and instrument files are all stored in separate queues in this application. Instrument files have an enable time when it is considered acceptable to send them to the spacecraft and a time-out time when an opportunity would have been missed and it no longer makes sense to uplink the file. MOC files are queued in realtime by a contact plan and do not have time out times. Each queue can be enabled for file selection or disabled by the MOC. This application will select the next file by checking for priority files first and then doing a round-robin selection between each enabled command interface with a file that is ready to send.

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Figure 12: Instrument command flow (image credit: JHU/APL)

Telemetry: JHU/APL uses the SLE (Space Link Extension)) Return All Frames (RAF) service to receive CCSDS telemetry frames from the spacecraft. Virtual channels are assigned for real-time telemetry, recorded data on SSR (Solid State Recorder), and realtime fill frames. The process of prioritizing and playing back SSR telemetry via CFDP has been used quite successfully on the MESSENGER and Van Allen Probes missions. SSR Housekeeping telemetry is ingested into the MOC telemetry archive. Instrument SSR telemetry files will be provided directly to SOCs with no processing by the MOC.

SPP will record telemetry immediately before a low data rate contact and use CFDP to guarantee delivery of the most critical data during this contact. Figure 13 shows the high level flow of instrument telemetry from creation to the SOC.

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Figure 13: Instrument telemetry flow (image credit: JHU/APL)

Frontier Radio on SPP mission: 24)

NASA has selected the Frontier Radio DS (Deep Space) version, developed by JHU/APL, for the communication for the SPP (Solar Probe Plus) mission. The VAP (Van Allen Probes) mission successfully transitioned the Frontier Radio technology to TRL-9 in an S-band duplex configuration for Near-Earth applications (Frontier NE). The successful VAP effort and TRL-6 X/X/Ka-band development efforts provided a deep space Frontier Radio (Frontier DS) with high heritage from the TRL-9 near-Earth unit. The low-SWaP (Size, Weight, and Power) and intrinsically high radiation tolerance of the Frontier Radio DS uniquely qualified it for the SPP application and resulted in the mission baselining this radio. As with VAP for the near-Earth radio, the SPP effort supported the maturation of the deep space radio enhancements, including the necessary compatibility testing with the DSN (Deep Space Network). Flight Frontier Radios for the SPP mission (Figure 14) have completed qualification as of August 2016 and will be integrated into the spacecraft during the remainder of 2016. 25)

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Figure 14: A flight Frontier Radio for SPP (image credit: JHU/APL)

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Figure 15: Configuration of the Avionics and SpaceWire Network (image credit: JHU/APL)




Development status of the project:

• July 5, 2018: The launch of Parker Solar Probe, the mission that will get closer to the Sun than any human-made object has ever gone, is quickly approaching, and on June 27, 2018, Parker Solar Probe’s heat shield — called the TPS (Thermal Protection System) — was installed on the spacecraft. 26)

- A mission 60 years in the making, Parker Solar Probe will make a historic journey to the Sun’s corona, a region of the solar atmosphere. With the help of its revolutionary heat shield, now permanently attached to the spacecraft in preparation for its August 2018 launch, the spacecraft’s orbit will carry it to within 4 million miles of the Sun's fiercely hot surface, where it will collect unprecedented data about the inner workings of the corona.

- The eight-foot-diameter (2.44 m) heat shield will safeguard everything within its umbra, the shadow it casts on the spacecraft. At Parker Solar Probe’s closest approach to the Sun, temperatures on the heat shield will reach nearly 2,500 degrees Fahrenheit (1370 ºC), but the spacecraft and its instruments will be kept at a relatively comfortable temperature of about 85 degrees Fahrenheit (29ºC).

- The heat shield is made of two panels of superheated carbon-carbon composite sandwiching a lightweight 4.5-inch-thick (11.4 cm) carbon foam core. The Sun-facing side of the heat shield is also sprayed with a specially formulated white coating to reflect as much of the Sun’s energy away from the spacecraft as possible.

- The heat shield itself weighs only about 72 kg — here on Earth, the foam core is 97 percent air. Because Parker Solar Probe travels so fast — 430,000 miles per hour at its closest approach to the Sun, fast enough to travel from Philadelphia to Washington, D.C., in about one second — the shield and spacecraft have to be light to achieve the needed orbit.

- The reinstallation of the Thermal Protection System — which was briefly attached to the spacecraft during testing at the Johns Hopkins Applied Physics Lab in Laurel, Maryland, in fall 2017 — marks the first time in months that Parker Solar Probe has been fully integrated. The heat shield and spacecraft underwent testing and evaluation separately at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, before shipping out to Astrotech Space Operations in Titusville, Florida, in April 2018. With the recent reunification, Parker Solar Probe inches closer to launch and toward the Sun.

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Figure 16: Parker Solar Probe’s heat shield, the TPS, is lifted and realigned with the spacecraft’s truss as engineers from the Johns Hopkins Applied Physics Lab prepare to install the eight-foot-diameter heat shield on June 27, 2018 (image credit: NASA/Johns Hopkins APL/Ed Whitman)

• May 16, 2018: As we develop more and more powerful tools to peer beyond our solar system, we learn more about the seemingly endless sea of faraway stars and their curious casts of orbiting planets. But there’s only one star we can travel to directly and observe up close — and that’s our own: the Sun. 27)

- Two upcoming missions will soon take us closer to the Sun than we’ve ever been before, providing our best chance yet at uncovering the complexities of solar activity in our own solar system and shedding light on the very nature of space and stars throughout the universe.

- Together, NASA’s Parker Solar Probe and ESA’s (the European Space Agency) Solar Orbiter may resolve decades-old questions about the inner workings of our nearest star. Their comprehensive, up-close study of the Sun has important implications for how we live and explore: Energy from the Sun powers life on Earth, but it also triggers space weather events that can pose hazard to technology we increasingly depend upon. Such space weather can disrupt radio communications, affect satellites and human spaceflight, and — at its worst — interfere with power grids. A better understanding of the fundamental processes at the Sun driving these events could improve predictions of when they’ll occur and how their effects may be felt on Earth.

- “Our goal is to understand how the Sun works and how it affects the space environment to the point of predictability,” said Chris St. Cyr, Solar Orbiter project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is really a curiosity-driven science.”

Figure 17: Parker Solar Probe will swoop to within 4 million miles of the Sun's surface, facing heat and radiation like no spacecraft before it. Launching in 2018, Parker Solar Probe will provide new data on solar activity and may make critical contributions to our ability to forecast major space-weather events that affect life on Earth (video credit: NASA, JHU/APL)

• April 30, 2018: The Parker Solar Probe's Faraday cup, a key sensor aboard the $1.5 billion NASA mission launching this summer, earned its stripes last week by enduring testing in a homemade contraption designed to simulate the sun. 28)

- The cup will scoop up and examine the solar wind as the probe passes closer to the sun than any previous manmade object. Justin Kasper, University of Michigan associate professor of climate and space sciences and engineering, is principal investigator for Parker's SWEAP (Solar Wind Electrons Alphas and Protons) investigation.

- In order to confirm the cup will survive the extreme heat and light of the sun's surface, researchers previously tortured a model of the Faraday cup at temperatures exceeding 1650 ºC, courtesy of the Oak Ridge National Laboratory's Plasma Arc Lamp. The cup, built from refractory metals and sapphire crystal insulators, exceeded expectations.

- But the final test took place last week, in a homemade contraption Kasper and his research team call the Solar Environment Simulator. While being blasted with roughly 10 kilowatts of light on its surface—enough to heat a sheet of metal to 980ºC in seconds—the Faraday cup model ran through its paces, successfully scanning a simulated stream of solar wind.

- "Watching the instrument track the signal from the ion beam as if it was plasma flowing from the sun was a thrilling preview of what we will see with Parker Solar Probe," Kasper said.

- Roilings in the sun's atmosphere can violently fling clouds of plasma into space, known as coronal mass ejections, sometimes directly at Earth. Without precautionary measures, such clouds can set up geomagnetic oscillations around Earth that can trip up satellite electronics, interfere with GPS and radio communications and—at their worst—can create surges of current through power grids that can overload and disrupt the system for extended periods of time, up to months.

- By understanding what makes up the solar corona and what drives the constant outpouring of solar material from the sun, scientists on Earth will be better equipped to interpret the solar activity we see from afar and create a better early-warning system.

- To test the cup model, researchers had to create something new. Their simulator sits in a first-floor lab at the Smithsonian Astrophysical Observatory in Cambridge, MA, and embodies the adage that necessity is the mother of invention.

- It has the look of a makeshift operating room, with a metal frame holding up thick blue tarps around three sides creating a 16 x 8 workspace.

- Inside the area, recreating the sun's heat and light fell to a quartet of modified older model IMAX projectors that Kasper's team purchased on eBay for a few thousand dollars apiece. These are not the digital machines you find in today's Cineplexes, but an earlier generation that utilized bulbs.

- "It turns out a movie theater bulb on an IMAX projector runs at about the same 5,700 degrees Kelvin—the same effective temperature as the surface of the sun," Kasper said. "And it gives off nearly the same spectrum of light as the surface."

- Space offers essentially no atmosphere, meaning a proper testing environment for the Faraday cup would have as little air as possible. So researchers placed the cup in a metal vacuum chamber for testing.

- All four of the IMAX projectors sit atop wheeled tables, and to set up for the test, researchers rolled them into place, with their beams pointed through the vacuum tube window directly at the Faraday cup.

- The final element of the simulator is its ability to generate the kinds of particles the Faraday cup will need to sense and evaluate. To do that, the team attached an ion gun to the vacuum tube hatch, with the "barrel" of the device reaching inside and pointed at the cup.

- In this final test, the Faraday cup took the heat and delivered—putting Parker Solar Probe on track for its summer launch.

- Kelly Korreck, a U-M alumna and astrophysicist at the institute, serves as head of science operations on Parker's SWEAP investigation as well as SWEAP activities for the Smithsonian. "As for the test today, it confirmed what I had suspected—when you take an amazing team of scientists and engineers, give them a complex, difficult, interesting project and the motivation of exploring a region of the universe humankind has never been to, before remarkable things happen," she said.

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Figure 18: Researchers use a quartet of IMAX projectors to create the light and heat the Parker Solar Probe cup will experience during its trips through the sun's atmosphere. The cup sits inside a vacuum chambers set up in a lab at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts (image credit: Levi Hutmacher, Michigan Engineering)

• April 6, 2018: NASA’s Parker Solar Probe has arrived in Florida to begin final preparations for its launch to the Sun, scheduled for July 31, 2018. 29)

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Figure 19: The custom shipping container holding NASA’s Parker Solar Probe is prepared for unloading from the C-17 of the United States Air Force’s 436th Airlift Wing after landing at Space Coast Regional Airport in Titusville, Florida, on the morning of April 3, 2018 (image credit: NASA/JHU-APL/Ed Whitman)

- In the middle of the night on April 2, the spacecraft was driven from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, to nearby Joint Base Andrews in Maryland. From there, it was flown by the United States Air Force’s 436th Airlift Wing to Space Coast Regional Airport in Titusville, Florida, where it arrived at 10:40 a.m. EDT. It was then transported a short distance to Astrotech Space Operations, also in Titusville, where it will continue testing, and eventually undergo final assembly and mating to the third stage of the Delta IV Heavy launch vehicle.

- Parker Solar Probe is humanity’s first mission to the Sun. After launch, it will orbit directly through the solar atmosphere – the corona – closer to the surface than any human-made object has ever gone. While facing brutal heat and radiation, the mission will reveal fundamental science behind what drives the solar wind, the constant outpouring of material from the Sun that shapes planetary atmospheres and affects space weather near Earth.

- “Parker Solar Probe and the team received a smooth ride from the Air Force C-17 crew from the 436th,” said Andy Driesman, Parker Solar Probe project manager from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “This is the second most important flight Parker Solar Probe will make, and we’re excited to be safely in Florida and continuing pre-launch work on the spacecraft.”

- At Astrotech, Parker Solar Probe was taken to a clean room and removed from its protective shipping container on Wednesday, April 4. The spacecraft then began a series of tests to verify that it had safely made the journey to Florida. For the next several months, the spacecraft will undergo comprehensive testing; just prior to being fueled, one of the most critical elements of the spacecraft, the TPS (Thermal Protection System), or heat shield, will be installed. The TPS is the breakthrough technology that will allow Parker Solar Probe to survive the temperatures in the Sun’s corona, just 3.8 million miles from the surface of our star.

- “There are many milestones to come for Parker Solar Probe and the amazing team of men and women who have worked so diligently to make this mission a reality,” said Driesman. “The installation of the TPS will be our final major step before encapsulation and integration onto the launch vehicle.”

- Parker Solar Probe will be launched from Launch Complex-37 at NASA’s Kennedy Space Center, Florida. The two-hour launch window opens at approximately 4 a.m. EDT on July 31, 2018, and is repeated each day (at slightly earlier times) through Aug. 19.

- Throughout its seven-year mission, Parker Solar Probe will explore the Sun's outer atmosphere and make critical observations to answer decades-old questions about the physics of stars. Its data will also be useful in improving forecasts of major eruptions on the Sun and the subsequent space weather events that impact technology on Earth, as well as satellites and astronauts in space. The mission is named for University of Chicago Professor Emeritus Eugene N. Parker, whose profound insights into solar physics and processes have guided the discipline. It is the first NASA mission named for a living individual.

- Parker Solar Probe is part of NASA’s Living With a Star Program to explore aspects of the connected Sun-Earth system that directly affect life and society. Living With a Star is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. Johns Hopkins APL designed, built and manages the mission for NASA. Instrument teams are led by researchers from the University of California, Berkeley; the University of Michigan in Ann Arbor; Naval Research Laboratory in Washington, D.C.; Princeton University in New Jersey; and the Smithsonian Astrophysics Observatory in Cambridge, Massachusetts.

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Figure 20: NASA’s Parker Solar Probe is wheeled into position in a clean room at Astrotech Space Operations (image credit: NASA/JHU-APL/Ed Whitman)

• March 6, 2018: NASA is inviting people around the world to submit their names online to be placed on a microchip aboard NASA’s historic Parker Solar Probe mission launching in summer 2018. The mission will travel through the Sun’s atmosphere, facing brutal heat and radiation conditions — and your name will go along for the ride. 30)

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Figure 21: Eugene Parker, professor emeritus at the University of Chicago, visits the spacecraft that bears his name, NASA’s Parker Solar Probe, on Oct. 3, 2017. Engineers in the clean room at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, where the probe was designed and built, point out the instruments that will collect data as the mission travels directly through the Sun’s atmosphere (image credit: NASA, JHU/APL)

• On 17 January 2018, NASA’s Parker Solar Probe was lowered into the 12 m tall thermal vacuum chamber at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland. The spacecraft will remain in the chamber for about seven weeks, coming out in mid-March for final tests and packing before heading to Florida. Parker Solar Probe is scheduled to launch from NASA’s Kennedy Space Center on July 31, 2018, on a Delta IV Heavy launch vehicle. 31)

- “This is the final major environmental test for the spacecraft, and we’re looking forward to this milestone,” said Annette Dolbow, Parker Solar Probe’s integration and test lead from the Johns Hopkins Applied Physics Lab. “The results we’ll get from subjecting the probe to the extreme temperatures and conditions in the chamber, while operating our systems, will let us know that we’re ready for the next phase of our mission – and for launch.”

- During thermal balance testing, the spacecraft will be cooled to -292 degrees Fahrenheit (-180ºC). Engineers will then gradually raise the spacecraft’s temperature to test the thermal control of the probe at various set points and with various power configurations.

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Figure 22: NASA’s Parker Solar Probe descends into the thermal vacuum chamber at NASA’s Goddard Space Flight Center. The spacecraft will be inside the chamber for about seven weeks (image credit: NASA, JHU/APL, Ed Whitman)

• On 6 November 2017, NASA's Parker Solar Probe spacecraft arrived at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, for environmental tests. During the spacecraft’s stay at Goddard, engineers and technicians will simulate extreme temperatures and other physical stresses that the spacecraft will be subjected to during its historic mission to the Sun. 32)

- Before arriving at Goddard, Parker Solar Probe was at the JHU/APL (Johns Hopkins University /Applied Physics Laboratory) in Laurel, Maryland, where it was designed and built.

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Figure 23: Parker Solar Probe arrives at the integration and testing facility at NASA/GSFC in Greenbelt, Maryland (image credit: NASA/JHU/APL, Ed Whitman)

• September 27, 2017: Now less than one year away from launch, the Parker Solar Probe began as an idea in the Outer Planet/Solar Probe program of NASA in the 1990s. 33)

- The original mission concept, the Solar Orbiter, was canceled in 2003 as part of the George W. Bush Administration’s restructuring of NASA to focus more on research and development and address management shortcomings in the wake of the 1 February 2003 breakup of the Space Shuttle Columbia that claimed the lives of all seven astronauts aboard. — Six years later, the mission concept was resurrected as a “new mission start” in 2009 with an aim to launch a new solar probe in 2015.

- By 2012, as the mission moved into its design phase, the launch was pushed to 2018.

- Originally called the Solar Probe Plus (SPP), the mission was renamed earlier this year on 31 May 2017, and in so doing NASA radically departed from of its previous mission naming practices.

• On Sept. 21, 2017, the revolutionary heat shield that will protect the first spacecraft to fly directly into the Sun's atmosphere was installed for final integrated vehicle testing ahead of launch. This is the only time the spacecraft will have its thermal protection system—which will reach temperatures of 2,500 degrees F (1370ºC) while at the Sun—attached until just before launch. 34)

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Figure 24: On 21 Sept. 2017, engineers at JHU/APL in Laurel, Maryland, lowered the thermal protection system – the heat shield – onto the spacecraft for a test of alignment as part of integration and testing (image credit: NASA/JHUAPL)

• July 14, 2016: Following a successful NASA management review on July 7, the Solar Probe Plus mission — which will send a spacecraft on several daring data-collecting runs through the sun’s atmosphere — is moving into the system assembly, integration, test and launch stage of the project. NASA terms this period as Phase D, during which the mission team will finish building the spacecraft, install its science instruments, test it under simulated launch and space conditions, and launch it. 35)

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Figure 25: Engineers at JHU/APL in Laurel, Maryland, prepare the developing Solar Probe Plus spacecraft for thermal vacuum tests that simulate conditions in space. Today, the spacecraft includes the primary structure and its propulsion system; still to be installed over the next several months are critical systems such as power, communications and thermal protection, as well as science instruments. The probe is scheduled for launch in summer 2018 (image credit: NASA, JHU/APL)

• April 8, 2015: NASA’s SPP (Solar Probe Plus) mission reached a major milestone in March when it successfully completed its CDR (Critical Design Review). An independent NASA review board met at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, from March 16 to 20 to review all aspects of the mission plan; APL has designed and will build and operate the spacecraft for NASA. The CDR certifies that the Solar Probe Plus mission design is at an advanced stage and that fabrication, assembly, integration and testing of the many elements of the mission may proceed. 36)

• In March 2014, Solar Probe Plus will begin advanced design, development and testing — a step NASA designates as Phase C — following a successful design review in which an independent assessment board deemed that the mission team, led by JHU/APL ( Johns Hopkins University/Applied Physics Laboratory) in Laurel, MD, was ready to move ahead with full-scale spacecraft fabrication, assembly, integration and testing. 37)


Launch: The Parker Solar Probe spacecraft was launched on 12 August 2018 (07:31 UTC) from SLC-37B (Space Launch Complex -7 B) of the Cape Canaveral Air Force Station, Florida. The launch vehicle was a Delta-4 Heavy rocket of ULA (United Launch Alliance), augmented by Orbital ATK’s Star-48 solid motor as a third stage, in order to cope with the extremely high energy required for this flagship mission. 38) 39) 40)

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Figure 26: The United Launch Alliance Delta IV Heavy rocket launches NASA's Parker Solar Probe to touch the Sun, Sunday, Aug. 12, 2018, from Launch Complex 37 at Cape Canaveral Air Force Station, Florida. Parker Solar Probe is humanity’s first-ever mission into a part of the Sun’s atmosphere called the corona. Here it will directly explore solar processes that are key to understanding and forecasting space weather events that can impact life on Earth (image credit: NASA, Bill Ingalls)

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Figure 27: Renowned physicist Eugene Parker (at 91) watches the launch of the spacecraft that bears his name – NASA’s Parker Solar Probe – early in the morning on 12 August, 2018, from Launch Complex 37 at Cape Canaveral Air Force Station in Florida (image credit: NASA, Glenn Benson)

Orbit: The trajectory able to send the spacecraft within 10 RS (Solar Radii) of the Sun center is a Venus-Venus-Venus-Venus-Venus-Venus-Venus-Gravity-Assist (V7GA) trajectory, a unique trajectory developed to enable the Parker Solar Probe mission without a Jupiter gravity assist. Even with the most powerful launch vehicle and upper stage, a spacecraft cannot get close to the Sun from Earth directly. Extra energy must be shed off the spacecraft’s orbit to further reduce its heliocentric orbital velocity in order to encounter the Sun under 10 RS. The V7GA trajectory allows for the spacecraft to reduce the necessary orbital speed via multiple Venus gravity assists.

The amount of required orbital speed reduction required at aphelion is too large to come from one or two Venus flybys. Attaining the aphelion orbital speed reduction will require seven Venus flybys. Following each Venus flyby, the orbital speed at aphelion will decrease, resulting in a smaller orbit with a shorter perihelion distance. After seven Venus flybys, orbit perihelion distance will gradually decrease to 9.86 RS, the minimum solar distance required for the baseline mission. Throughout the mission there are no additional deep space maneuvers; all the orbit changes as well as the phasing (Venus-to-Venus transfer location and timing) between each Venus flyby are achieved through the control of the Venus flybys by appropriate selection of the Venus flyby target parameters. To minimize the mission duration, both resonant and non-resonant Venus flybys are utilized in this trajectory design (Ref. 21).

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Figure 28: Overview of the V7GA mission trajectory (image credit: JHU/APL)

The SPP mission is comprised of 24 highly elliptical, heliocentric orbits with decreasing orbital periods from 168 days for orbit 1, settling into an 88 day orbit period midway through the mission. Each orbit is broken into two distinct periods, the Solar Encounter period and the Cruise/Downlink period. Figure 29 highlights the primary characteristics of each period (Ref.23) .

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Figure 29: SPP Orbital Operations Concept (image credit: JHU/APL)

Figure 30: Illustration of NASA’s Parker Solar Probe at the Sun (video credit: NASA's Goddard Space Flight Center)


It's hard to go to the Sun 41)

The Sun contains 99.8 percent of the mass in our solar system. Its gravitational pull is what keeps everything here, from tiny Mercury to the gas giants to the Oort Cloud, 186 billion miles away. But even though the Sun has such a powerful pull, it's surprisingly hard to actually go to the Sun: It takes 55 times more energy to go to the Sun than it does to go to Mars.

Why is it so difficult? The answer lies in the same fact that keeps Earth from plunging into the Sun: Our planet is traveling very fast — about 67,000 miles/hr (107,820 km/hr) — almost entirely sideways relative to the Sun. The only way to get to the Sun is to cancel that sideways motion.

The extreme difficulty of getting close to the Sun — at first an enormous amount of velocity must be canceled out from the Earth orbital velocity in order for a probe to leave Earth’s orbit and head towards the Sun — then the tremendous gravitational acceleration towards the Sun has to be partially offset with the seven Venusian (decelerating) ‘slingshots’.

Since the Parker Solar Probe will skim through the Sun's atmosphere, it only needs to drop 53,000 miles/hr (85,300 km/hr) of sideways motion to reach its destination, but that's no easy feat. In addition to using a powerful rocket, the Delta IV Heavy, the Parker Solar Probe will perform seven Venus gravity assists over its seven-year mission to shed sideways speed into Venus' well of orbital energy.

These gravity assists will draw the Parker Solar Probe's orbit closer to the Sun for a record approach of just 3.83 million miles from the Sun's visible surface on the final orbits.

Though it's shedding sideways speed to get closer to the Sun,the Parker Solar Probe will pick up overall speed, bolstered by the Sun's extreme gravity - so it will also break the record for the fastest-ever human-made objects, clocking in at 430,000 miles/hr (692,000 km/hr) on its final orbits.

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Figure 31: Illustration of NASA’s Parker Solar Probe at the Sun (image credit: NASA/GSFC)




Mission status

• July 10, 2020: Early on July 11, 2020 (UTC), the spacecraft will perform its first outbound flyby of Venus, passing approximately 516 miles above the surface as it curves around the planet. Such Venus gravity assists play an integral role in the Parker Solar Probe mission. The spacecraft relies on the planet to rid itself of orbital energy, which in turn allows it to travel ever closer to the Sun after each Venus flyby. The mission’s previous two Venus flybys swooped past the Sun-facing side of the planet, and this will be Parker Solar Probe’s first pass on Venus’ night side. 42)

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Figure 32: Parker Solar Probe performs its third Venus flyby on July 11, 2020 (UTC), setting the spacecraft up for another record-breaking close approach to the Sun in September 2020 (image credit: NASA/Johns Hopkins APL/Steve Gribben)

- Parker Solar Probe will witness a brief 11-minute solar eclipse during the maneuver while passing through the shadow of the planet. Utilizing powerful telescopes, the Apache Point Observatory in New Mexico, Lick Observatory in California, and the Keck Observatory in Hawaii will search for Venus aurora from the ground in coordination with Parker Solar Probe’s pass around the planet, weather permitting. Scientists will combine these ground-based observations with data collected by Parker Solar Probe during the flyby to take an unprecedented look at the interactions between Venus and the solar wind.

- This Venus flyby sets Parker Solar Probe up for its sixth close pass by the Sun, slated for September 27. During this perihelion, Parker Solar Probe will travel even closer to the Sun, setting a new record when it passes approximately 8.3 million miles from the solar surface, more than 3 million miles closer than the previous perihelion at 11.6 million miles from the solar surface. The spacecraft’s seventh perihelion is slated for January 17, 2021.

• June 12, 2020: At the heart of understanding our space environment is the knowledge that conditions throughout space — from the Sun to the atmospheres of planets to the radiation environment in deep space — are connected. Studying this connection – a field of science called heliophysics — is a complex task: Researchers track sudden eruptions of material, radiation, and particles against the background of the ubiquitous outflow of solar material. 43)

- A confluence A confluence of events in early 2020 created a nearly ideal space-based laboratory, combining the alignment of some of humanity's best observatories — including Parker Solar Probe, during its fourth solar flyby — with a quiet period in the Sun's activity, when it's easiest to study those background conditions. These conditions provided a unique opportunity for scientists to study how the Sun influences conditions at points throughout space, with multiple angles of observation and at different distances from the Sun.

- The Sun is an active star whose magnetic field is spread out through the solar system, carried within the Sun's constant outflow of material called the solar wind, which affects spacecraft and shapes the environments of worlds throughout the solar system. We've observed the Sun, space near Earth and other planets, and even the most distant edges of the Sun's sphere of influence for decades. And 2018 marked the launch of a new, game-changing observatory: Parker Solar Probe, with a plan to ultimately fly to about 3.83 million miles from the Sun's visible surface.

- Parker has now had four close encounters of the Sun. (The data from Parker's first encounters with the Sun has already revealed a new picture of its atmosphere.) During its fourth solar encounter, spanning parts of January and February 2020, the spacecraft passed directly between the Sun and Earth. This gave scientists a unique opportunity: The solar wind that Parker Solar Probe measured when it was closest to the Sun would, days later, arrive at Earth, where the wind itself and its effects could be measured by both spacecraft and ground-based observatories. Furthermore, solar observatories on and near Earth would have a clear view of the locations on the Sun that produced the solar wind measured by Parker Solar Probe.

- "We know from Parker data that there are certain structures originating at or near the solar surface. We need to look at the source regions of these structures to fully understand how they form, evolve, and contribute to the plasma dynamics in the solar wind," said Nour Raouafi, project scientist for the Parker Solar Probe mission at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. "Ground-based observatories and other space missions provide supporting observations that can help draw the full picture of what Parker is observing."

- This celestial alignment would be of interest to scientists under any circumstances, but it also coincided with another astronomical period of interest to scientists: solar minimum. This is the point during the Sun's regular, approximately 11-year cycles of activity when solar activity is at its lowest level — so sudden eruptions on the Sun such as solar flares, coronal mass ejections and energetic particle events are less likely. And that means that studying the Sun near solar minimum is a boon for scientists who can watch a simpler system and thus untangle which events cause which effects.

- "This period provides perfect conditions to trace the solar wind from the Sun to Earth and the planets," said Giuliana de Toma, a solar scientist at the High Altitude Observatory in Boulder, Colorado, who led coordination among observatories for this observation campaign. "It is a time when we can follow the solar wind more easily, since we don't have disturbances from the Sun."

- For decades, scientists have pulled together observations during these periods of solar minimum, an effort co-led by Sarah Gibson, a solar scientist at the High Altitude Observatory, and other scientists. For each of the past three solar minimum periods, scientists pooled observations from an ever-expanding list of observatories in space and on the ground, hoping the wealth of data on the undisturbed solar wind would unveil new information about how it forms and evolves. For this solar minimum period, scientists began gathering coordinated observations starting in early 2019 under the umbrella Whole Heliosphere and Planetary Interactions, or WHPI for short.

- This particular WHPI campaign comprised a broader-than-ever swath of observations: covering not only the Sun and effects on Earth, but also data gathered at Mars and the nature of space throughout the solar system — all in concert with Parker Solar Probe's fourth and closest-yet flyby of the Sun.

- The WHPI organizers brought together observers from all over the world — and beyond. Combining data from dozens of observatories on Earth and in space gives scientists a chance to paint what might be the most comprehensive picture ever of the solar wind: from images of its birth with solar telescopes, to samples shortly after it leaves the Sun with Parker Solar Probe, to multi-point observations of its changing state throughout space.

- Some samples of the kinds of data captured during this international collaboration of Sun and space observatories.

Parker Solar Probe

Figure 33: This animated sequence of visible-light images from Parker Solar Probe's WISPR instrument shows a coronal streamer, observed when Parker Solar Probe was near perihelion on Jan. 28, 2020 (image credits: NASA/Johns Hopkins APL/Naval Research Lab/Parker Solar Probe)

- Early data from Parker Solar Probe's close pass by the Sun during the WHPI campaign shows a solar wind system more dynamic than what's visible in observations near Earth. In particular, scientists hope the full set of data — downlinked to Earth in May 2020 — will reveal dynamic structures, like tiny coronal mass ejections and magnetic flux ropes in their early stages of development, that can't be seen with other observatories watching from farther away. Connecting structures like this, previously too small or too distant to see, with solar wind and near-Earth measurements may help scientists better understand how the solar wind changes throughout its lifetime and how its origins near the Sun affect its behavior throughout the solar system.

Mauna Loa Solar Observatory

- Parker Solar Probe's close-up views of solar wind structures are complemented by solar observatories on Earth and in space, which have a larger field of view to capture solar wind structures.

- Data from the Mauna Loa Solar Observatory in Hawaii shows a jet of material being ejected near the Sun's south pole on Jan. 21, 2020. Coronal jets like this are one solar wind feature that scientists hope to observe more closely with Parker Solar Probe, as the mechanisms that create them could shed more light on the solar wind's birth and acceleration.

- "It would be extremely fortunate if Parker Solar Probe observed this jet, since it would provide information on plasma and the field in and around the jet not long after its formation," said Joan Burkepile, lead scientist for the Coronal Solar Magnetism Observatory K-coronagraph instrument at the Mauna Loa Solar Observatory, which captured these images.

Figure 34: Data from the Mauna Loa Solar Observatory in Hawaii shows a jet of material being ejected near the Sun's south pole on Jan. 21, 2020 (UTC). This difference image is created by subtracting the pixels of the previous image from the current image to highlight changes (image credit: Mauna Loa Solar Observatory/K-Cor)

Solar and Terrestrial Relations Observatory

- Along with observations of the solar wind from Parker Solar Probe and near Earth, scientists also have detailed images of the Sun and its atmosphere from spacecraft like NASA's SDO (Solar Dynamics Observatory) and the STEREO (Solar and Terrestrial Relations Observatory). NASA's Solar and Terrestrial Relations Observatory, or STEREO, has a distinct view of the Sun from its vantage point about 78 degrees away from Earth.

- During this WHPI campaign, scientists took advantage of this unique viewing angle. From Jan. 21-23 — when Parker Solar Probe and STEREO were aligned — the STEREO mission team increased the exposure length and frequency of images taken by its coronagraph, revealing fine structures in the solar wind as they speed out from the Sun.

- These difference images are created by subtracting the pixels of a previous image from the current image to highlight changes — here, revealing a small CME that would otherwise be difficult to see.

Figure 35: NASA's STEREO, took extra images with longer exposure times to improve views of structure in the solar wind. These difference images, spanning Jan. 21-23, 2020, are created by subtracting the pixels of a previous image from the current image to highlight changes (video credit: NASA/STEREO)

Solar Dynamics Observatory

Figure 36: NASA's SDO keeps a constant eye on the Sun. These images, captured in a wavelength of extreme ultraviolet light, span Jan. 15 – Feb. 11, 2020 (video credit: NASA/SDO)

- SDO takes high-resolution views of the entire Sun, revealing fine details on the solar surface and the lower solar atmosphere. These images were captured in a wavelength of extreme ultraviolet light at 171 Angstroms, highlighting the quiet parts of the Sun's outer atmosphere, the corona. This data — along with SDO's images in other wavelengths — maps much of the Sun's activity, allowing scientists to connect solar wind measurements from Parker Solar Probe and other spacecraft with their possible origins on the Sun.

Modeling the Data

- Ideally, scientists could use these images to readily pinpoint the region on the Sun that produced a particular stream of solar wind measured by Parker Solar Probe — but identifying the source of any given solar wind stream observed by a spacecraft is not simple. In general, the magnetic field lines that guide the solar wind's movement flow out of the Northern half of the Sun point in the opposite direction than they do in the Southern half. In early 2020, Parker Solar Probe's position was right at the boundary between the two – an area known as the heliospheric current sheet.

- "For this perihelion, Parker Solar Probe was very close to the current sheet, so a little nudge one way or the other would make the magnetic footpoint shift to the south or north pole," said Nick Arge, a solar scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We were on the tipping point where sometimes it went north, sometimes south."

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Figure 37: The Sun's "open" magnetic field — shown in this model in blue and red, with looped or closed field shown in yellow — primarily comes from near the Sun's north and south poles during solar minimum, but it spreads out to fill space converging near the Sun's equator (image credit: NASA/Nick Arge)

- Predicting which side of the tipping point Parker Solar Probe was on was the responsibility of the modeling teams. Using what we know about the Sun's magnetic field and the clues we can glean from distant images of the Sun, they made day-by-day predictions of where, precisely, on the Sun birthed the solar wind that Parker would fly through on a given day. Several modeling groups made daily attempts to answer just that question.

- Using measurements of the magnetic field at the Sun's surface, each group made a daily prediction for the source region producing the solar wind that Parker Solar Probe was flying through.

- Arge worked with Shaela Jones, a solar scientist at NASA Goddard who did daily forecasting during the WHPI campaign, using a model originally developed by Arge and colleagues Yi-Ming Wang and Neil Sheeley, called the WSA model. According to their forecasts, the predicted source of the solar wind switched between hemispheres suddenly during the observation campaign, because Earth's orbit at the time was also closely aligned with the heliospheric current sheet – that region where the direction of magnetic polarity and the source of the solar wind switches between north and south. They predicted that Parker Solar Probe, flying in a similar plane as Earth, would experience similar switches in solar wind source and magnetic polarity as it flew near the Sun.

Figure 38: This model run — produced by Nick Arge and Shaela Jones using the WSA model — illustrates the predicted origin for solar wind that will impact Earth days later, spanning Jan. 10 – Feb. 3, 2020. The colored regions near the Sun's north and south poles show the regions from which the solar wind flows out, with red regions showing a faster flow and blue regions showing a slower flow. The yellow lines on the Sun divide areas of opposite magnetic polarity. The white lines indicate the predicted points of origin for the solar wind arriving at Earth at the given date. The black and white underlaid image shows a map of the magnetic field at the Sun's surface, the basis for the model's predictions. The black regions are where the magnetic field points inward, toward the Sun, and white regions are where the field points outward, away from the Sun (image credits: NASA/Nick Arge/Shaela Jones)

- Solar wind models rely on daily measurements of the Sun's surface magnetic field — the black and white image underlaid. This particular model used measurements from the National Solar Observatory's Global Oscillation Network Group and a model that focuses on predicting how the Sun's surface magnetic field will change over several days. Creating these magnetic surface maps is a complicated and imperfect process unto itself, and some of the modeling groups participating in the WHPI campaign also used magnetic measurements from multiple observatories. This, along with differences in each group's models, created a spread of predictions that sometimes placed the source of Parker Solar Probe's solar wind stream in two different hemispheres of the Sun. But given the inherent uncertainty in modeling the solar wind's source, these different predictions can actually make for more robust operations.

- ”If you can observe the Sun in two different places with two telescopes, you have a better chance to get the right spot," said Jones.

Poker Flat Incoherent Scatter Radar

- The solar wind carries with it both an enormous amount of energy and the embedded magnetic field of the Sun. When it reaches Earth, it can ring our planet's natural magnetic field like a bell, making it bend and deform — which produces a measurable change in magnetic field strength at certain points on Earth's surface. We track those changes because magnetic field oscillations can lead to a host of space weather effects that interfere with spacecraft or even, occasionally, utility grids on the ground.

- A host of ground-based magnetometers have tracked these effects since the 1850s, and they're one of the many sets of data scientists are gathering in connection with this campaign. Other ground-based instruments can reveal the invisible effects of space weather in our atmosphere. One such system is the PFISR (Poker Flat Incoherent Scatter Radar) — a radar system based at the Poker Flat Research Range near Fairbanks, Alaska.

- This radar is specially tuned to detect one of most reliable indicators of a disturbance in Earth's magnetic field: electrons in Earth's upper atmosphere. These electrons are created when particles trapped in the magnetosphere are sent zooming into Earth's atmosphere by a complex series of events, a set of circumstances known as a magnetospheric substorm.

- On Jan. 16, PFISR measured the changing electrons in Earth's upper atmosphere during one such substorm. During a substorm, particles cascade into the upper atmosphere, not only creating the shower of electrons measured by the radar, but driving a more visible effect: the aurora. PFISR uses multiple beams of radar oriented in different directions, which allowed scientists to build up a three-dimensional picture of how electrons in the atmosphere changed throughout the substorm.

Figure 39: The Poker Flat Incoherent Scatter Radar in Poker Flat, Alaska, makes 3D measurements of electrons in Earth's upper atmosphere. These electrons are produced by the same process that produces aurora, seen here by the Poker Flat All-Sky Camera, which images aurora over Alaska, on Jan. 16, 2020 [video credits: Poker Flat Incoherent Scatter Radar (NSF)/Poker Flat All-Sky Camera (University of Alaska Fairbanks)/Don Hampton]

- Because this substorm took place so early in the observation campaign — only one day after data collection began — it's unlikely that it was caused by conditions on the Sun observed during the campaign. But even so, the connection between magnetospheric substorms and the broader, global-scale effects created by the solar wind — called geomagnetic storms — isn't entirely understood.

- "This substorm didn’t happen during a geomagnetic storm time," said Roger Varney, principal investigator for PFISR at SRI International in Menlo Park, California. "The solar wind during this event is fluctuating, but not particularly strongly — it's basically background noise. But solar wind is basically never steady; it's constantly putting some energy into the magnetosphere."

- This deposit of energy into Earth's magnetic system has far-reaching effects: for one, changes in the composition and density of Earth's upper atmosphere can garble communications and navigation signals, an effect often characterized by total electron content. Changes in density can also affect the orbits of satellites to great degree, introducing uncertainty about precise position.

MAVEN

- Earth isn't the only planet where the solar wind has measurable effects — and studying other worlds in our solar system can help scientists understand some of the solar wind's effects on Earth and how it influenced the evolution of Earth and other worlds throughout the solar system's history.

- At Mars, the solar wind coupled with Mars' lack of a global magnetic field may be a major factor in the dry, barren world the Red Planet is today. Though Mars was once much like Earth — warm, with liquid water and a thick atmosphere — the planet has changed drastically over the course of its four-billion-year history, with most of its atmosphere being stripped away to space. With similar processes observed here on Earth, scientists leverage understanding of solar-planetary interactions at Mars to determine how processes leading to atmospheric escape has the ability to change whether a planet is habitable or not. Today, the MAVEN (Mars Atmosphere and Volatile Evolution mission) studies these processes at Mars. MAVEN observations at Mars are available for this latest WHPI campaign.

- Over the coming months, heliophysicists around the world will begin to study data from these observatories in depth, hoping to draw connections that reveal new knowledge about the Sun and its changes that influence Earth and space across the solar system.

- Parker Solar Probe is part of the NASA Heliophysics Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living with a Star program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the spacecraft and manages the mission for NASA.

- The research discussed in this story includes work supported by the Poker Flat Incoherent Scatter Radar which is a major facility funded by the National Science Foundation through cooperative agreement AGS-1840962 to SRI International and work at the National Center for Atmospheric Research funded by the National Science Foundation through cooperative agreement AGS-1852977. Support for the WHPI Campaigns is provided through the NASA’s Heliophysics System Observatory Connect (HSO Connect) program.

• January 14, 2020: There’s a wind that emanates from the Sun, and it blows not like a soft whistle but like a hurricane’s scream. Made of electrons, protons and heavier ions, the solar wind courses through the solar system at roughly 1 million mph (1.6 million km/h), barreling over everything in its path. 44)

Figure 40: Parker Solar Probe’s FIELDS instrument can eavesdrop on the electric and magnetic fluctuations caused by plasma waves, and it can “hear” when the waves and particles interact with one another, recording frequency and amplitude information about these plasma waves that scientists can transform into sound files. (video credit: NASA/Johns Hopkins APL)

- Yet through the wind’s roar, NASA’s Parker Solar Probe hears the small chirps, squeaks and rustles that hint at the origin of this mysterious and ever-present wind. And now the Parker Solar Probe team is getting their first chance to hear them, too.

- “We are looking at the young solar wind, being born around the Sun,” said mission Project Scientist Nour Raouafi, from the Johns Hopkins Applied Physics Laboratory (APL), in Laurel, Maryland. “And it’s completely different from what we see here near Earth.”

- Scientists have studied the solar wind for more than 60 years, but they’re still puzzled over some of its behaviors. While they know it comes from the Sun’s million-degree upper atmosphere called the corona, the solar wind, for example, doesn’t slow down as it leaves the Sun — it speeds up, and it has a sort of internal heater that keeps it from cooling as it zips through space. With growing concern about the solar wind’s ability to interfere with GPS satellites and disrupt power grids on the ground, it’s become imperative to better understand it.

- Just 17 months since launch, and after three orbits around the Sun, Parker Solar Probe — designed, built and now operated by Johns Hopkins APL for NASA — has not disappointed.

- “We expected to make big discoveries because we’re going into uncharted territory,” Raouafi said. “What we’re actually seeing is beyond anything anybody imagined.”

- Researchers suspected that plasma waves within the solar wind could be responsible for some of the wind’s odd characteristics. Just as fluctuations in air pressure cause winds that force rolling waves on the ocean, fluctuations in electric and magnetic fields can cause waves that roll through the clouds of electrons, protons and other charged particles that make up the plasma racing away from the Sun. Particles can ride these plasma waves much like the way a surfer rides an ocean wave, propelling them to higher speeds.

- “Plasma waves certainly play a part in heating and accelerating the particles,” Raouafi said. Scientists just don’t know how much of a part.

- That’s where Parker Solar Probe comes in. The spacecraft’s FIELDS instrument can eavesdrop on the electric and magnetic fluctuations caused by plasma waves. And it can “hear” when the waves and particles interact with one another, recording frequency and amplitude information about these plasma waves that scientists could then play as sound waves. And it results in some striking sounds.

- Take, for example, whistler-mode waves. These are caused by energetic electrons bursting out of the Sun’s corona. These electrons follow magnetic field lines that stretch from the Sun out to the solar system’s farthest edge, spinning around them like they’re riding a carousel. When a plasma wave’s frequency matches how frequently those electrons are spinning, they amplify one another. And it sounds like a scene out of “Star Wars.”

- “Some theories suggest that part of the solar wind’s acceleration is due to these escaping electrons,” said David Malaspina, a member of the FIELDS team and an assistant professor at the University of Colorado, Boulder, and the Laboratory for Atmospheric and Space Physics. They could also be a critical clue to understanding one process that heats the solar wind.

- “We can use observations of these waves to work our way backward and probe the source of these electrons in the corona,” Malaspina said.

- Another example are dispersive waves. These plasma waves quickly shift from one frequency to another as they move through the solar wind. These shifts create a sort of “chirp” that sounds like wind rushing over a microphone.

- “These waves haven’t been detected in the solar wind before, at least not in any large numbers,” Malaspina explained. They’re rare near the Earth, so researchers thought they were unimportant. But closer to the Sun, these waves are everywhere. - “Nobody knows what causes these chirping waves or what they do to heat and accelerate the solar wind. That’s what we’re going to be determining,” Malaspina said. “I think it’s incredibly exciting.”

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Figure 41: Scientists have studied the solar wind for more than 60 years, but they’re still puzzled over some of its behaviors. NASA’s Parker Solar Probe — designed and built by APL — hears the small chirps, squeaks and rustles that hint at the origin of this mysterious and ever-present wind. And now the Parker Solar Probe team is getting their first chance to hear them, too (image credit: NASA/Johns Hopkins APL)

- Raouafi commented that seeing all of this wave activity very close to the Sun is why this mission is so critical. “We are seeing new, early behaviors of solar plasma we couldn’t observe here at Earth, and we’re seeing that the energy carried by the waves is being dissipated somewhere along the way, to heat and accelerate the plasma.”

- But it wasn’t just plasma waves that Parker Solar Probe heard. While barreling through a cloud of microscopic dust, the spacecraft’s instruments also captured a sound resembling old TV static. That static-like sound is actually hundreds of microscopic impacts happening every day as dust from asteroids torn apart by the Sun’s gravity and solar heating and particles stripped from comets as they graze the star strike the spacecraft at speeds close to a quarter of a million miles per hour.

- As Parker Solar Probe cruised through this dust cloud, the spacecraft didn’t just crash into these particles — it obliterated them. Each grain’s atoms burst apart into electrons, protons and other ions in a mini puff of plasma that the FIELDS instrument can “hear.”

- Each collision, however, also chips away a tiny bit of the spacecraft. “It was well understood that this would happen,” Malaspina explained. “What was not understood was how much dust was going to be there.” APL engineers used models and remote observations to estimate how bad the situation might be well before the spacecraft launched. But in this uncharted territory, the number was bound to have some margin of error.

- James Kinnison, the Parker Solar Probe mission system engineer at APL, said this discrepancy in dust density is just one more reason why Parker being close to the Sun is so useful. “We protected almost everything from the dust,” Kinnison said. And although the dust is denser than expected, nothing right now points to dust impacts being a concern for the mission, he added.

- Parker Solar Probe is scheduled to make another 21 orbits around the Sun, using five Venus flybys to draw itself increasingly closer. That will allow researchers to even better understand how these waves change their behavior closer to the Sun and allow scientists to build a more complete evolutionary picture of the solar wind.

• December 11, 2019: Nearly a year and a half into its mission, Parker Solar Probe has returned gigabytes of data on the Sun and its atmosphere. Following the release of the very first science from the mission, five researchers presented additional new findings from Parker Solar Probe at the fall meeting of the American Geophysical Union on Dec. 11, 2019. Research from these teams hints at the processes behind both the Sun's continual outflow of material — the solar wind — and more infrequent solar storms that can disrupt technology and endanger astronauts, along with new insight into space dust that creates the Geminids meteor shower. 45)

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Figure 42: Parker Solar Probe's WISPR instruments captured the first-ever view of a dust trail in the orbit of asteroid Phaethon. This dust trail creates the Geminids meteor shower, visible each December (image credit: Brendan Gallagher/Karl Battams/NRL)

- “Before the launch of the Parker Solar Probe mission, we knew Parker would be venturing into an uncharted region of the solar system and that the potential for discovery was enormous,” said Parker Solar Probe Project Scientist Nour E. Raouafi of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “What we are learning from the initial data is beyond what we could imagine. We are discovering a new face of the nascent solar wind. Going to the source (the solar corona) is revealing pieces of what we have been missing for decades.”

- Images presented at this press conference are available at https://svs.gsfc.nasa.gov/13494

The young solar wind

- The solar wind carries the Sun's magnetic field with it, shaping space weather throughout the solar system as it flows out from the Sun at around a million miles per hour. Some of Parker Solar Probe's primary science goals are to pinpoint the mechanisms that send the solar wind streaming out into space at such high speeds.

- One clue lies in disturbances in the solar wind that could point to the processes that heat and accelerate the wind. These structures — pockets of relatively dense material — have been glimpsed in data from earlier missions spanning decades. They are several times the size of Earth's entire magnetic field, which stretches tens of thousands of miles into space — meaning these structures can compress Earth's magnetic field on a global scale when they crash into it.

- "When structures in the solar wind reach Earth, they can drive dynamics in Earth's magnetosphere, including particle precipitation from Earth's radiation belts," said Nicholeen Viall, a space scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who presented new findings on solar wind structures from Parker Solar Probe at the AGU meeting. Particle precipitation can cause a range of effects, like setting off the aurora and interfering with satellites.

- Near the Sun, Parker Solar Probe made better-than-ever measurements of these solar wind structures, using both imagers to take pictures from afar and in situ instruments to measure the structures as they pass over the spacecraft. To get a more complete picture of these solar wind structures, Viall went one step further, combining observations from Parker, satellites near Earth, and NASA's STEREO-A spacecraft to examine these structures from multiple angles.

- STEREO-A carries an instrument called a coronagraph, which uses a solid disk to block out the bright light of the Sun, letting the camera capture images of the relatively faint outer atmosphere, the corona. From its vantage point about 90 degrees away from Earth, STEREO-A could see the regions of the corona that Parker was flying through — allowing Viall to combine the measurements in a novel way and get a better view of solar wind structures as they flowed out from the Sun. Alongside Parker Solar Probe's images, scientists now have a better view of magnetic disturbances in the solar wind.

- Parker's instruments are also shedding new light on the invisible processes in the solar wind, revealing a surprisingly active system near the Sun.

- "We think of the solar wind — as we see it near Earth — as very smooth, but Parker saw surprisingly slow wind, full of little bursts and jets of plasma," said Tim Horbury, a lead researcher on Parker Solar Probe's FIELDS instruments based at Imperial College London.

- Horbury used data from Parker Solar Probe's FIELDS instruments — which measure the scale and shape of electric and magnetic fields near the spacecraft — to examine in detail one particularly odd event: magnetic "switchbacks," sudden clusters of events when the solar magnetic field bends back on itself, first described with Parker Solar Probe's initial results on Dec. 4, 2019.

- The exact origin of the switchbacks isn't certain, but they may be signatures of the process that heats the Sun's outer atmosphere, the corona, to millions of degrees, hundreds of times hotter than the visible surface below. The cause of this counterintuitive jump in temperature is a longstanding question in solar science — referred to as the coronal heating mystery — and is closely related to questions about how the solar wind is energized and accelerated.

- We think the switchbacks are probably related to individual energetic energy releases on the Sun — what we call jets," said Horbury. "If these are jets, there must a very large population of small events happening on the Sun, so they would contribute a large fraction of the total energy of the solar wind."

A look inside solar storms

- Along with the solar wind, the Sun also releases discrete clouds of material called CMEs (Coronal Mass Ejections). Denser and sometimes faster than the solar wind, CMEs can also trigger space weather effects on Earth, or cause problems for satellites in their path.

- CMEs are notoriously hard to predict. Some of them are simply not visible from Earth or from STEREO-A — the two positions where we have instruments capable of seeing CMEs from afar — because they erupt from parts of the Sun out of view of both spacecraft. Even when they are spotted by instruments, it's not always possible to predict which CMEs will disturb Earth's magnetic field and trigger space weather effects, as the magnetic structure within the cloud of material plays a crucial role.

- Our best shot at understanding the magnetic properties of any given CME relies on pinpointing the region on the Sun from which the CME exploded — meaning that one type eruption called a stealth CME poses a unique challenge for space weather forecasters.

- Stealth CMEs are visible in coronagraphs — instruments that look only at the Sun's outer atmosphere — but don't leave clear signatures of their eruption in images of the Sun's disk, making it difficult to ascertain from where, exactly, they lifted off.

- But during Parker Solar Probe's first solar flyby in November 2018, the spacecraft was hit by one of these stealth CMEs.

- "Flying close to the Sun, Parker Solar Probe has a unique chance to see young CMEs that haven't been processed from traveling tens of millions of miles," said Kelly Korreck, head of science operations for Parker's SWEAP instruments, based at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. "This was the first time we were able to stick our instruments inside one of these coronal mass ejections that close to the Sun."

- In particular, Korreck used data from Parker's FIELDS and SWEAP instruments to get a snapshot of the internal structure of the CME. SWEAP, the mission's solar wind instruments, measures characteristics like velocity, temperature, and electron and proton densities of the solar wind. These measurements not only provide one of the first looks inside a CME so close to the Sun, but they may help scientists learn to trace stealth CMEs back to their sources.

- Another type of solar storm consists of extremely energetic particles moving near the speed of light. Though often related to CME outbursts, these particles are subject to their own acceleration processes — and they move much faster than CMEs, reaching Earth and spacecraft in a matter of minutes. These particles can damage satellite electronics and endanger astronauts, but their speed makes them more difficult to avoid than many other types of space weather.

- These bursts of particles often, but not always, accompany other solar events like flares and CMEs, but predicting just when they'll make an appearance is difficult. Before particles reach the near-light speeds that makes them hazardous to spacecraft, electronics and astronauts, they go through a multi-stage energization process — but the first step in this process, near the Sun, hadn't been directly observed.

- As Parker Solar Probe traveled away from the Sun in April 2019, after its second solar encounter, the spacecraft observed the largest-yet energetic particle event seen by the mission. Measurements by the energetic particle instrument suite, ISOIS, have filled in one missing link in the processes of particle energization.

- "The regions in front of coronal mass ejections build up material, like snowplows in space, and it turns out these 'snowplows' also build up material from previously released solar flares," said Nathan Schwadron, a space scientist at the University of New Hampshire in Durham.

- Understanding how solar flares create populations of seed particles that feed energetic particle events will help scientists better predict when such events might happen, along with improving models of how they move through space.

Asteroid fingerprints

- Parker Solar Probe's WISPR instruments are designed to capture detailed images of the faint corona and solar wind, but they also picked up another difficult-to-see structure: a 60,000-mile-wide dust trail following the orbit of the asteroid Phaethon, which created the Geminids meteor shower. In 2019, the Geminids meteor shower peaks on the night of Dec. 13-14.

- This trail of dust grains peppers Earth's atmosphere when our planet intersects with Phaethon's orbit each December, burning up and producing the spectacular show we call the Geminids. Though scientists have long known that Phaethon is the parent of the Geminids, seeing the actual dust trail hasn't been possible until now. Extremely faint and very close to the Sun in the sky, it has never been picked up by any previous telescope, despite several attempts — but WISPR is designed to see faint structures near the Sun. WISPR's first-ever direct view of the dust trail has given new information about its characteristics.

- "We calculate a mass on the order of a billion tons for the entire trail, which is not as much as we’d expect for the Geminids, but much more than Phaethon produces near the Sun," said Karl Battams, a space scientist at the U.S. Naval Research Lab in Washington, D.C. "This implies that WISPR is only seeing a portion of the Geminid stream – not the entire thing – but it’s a portion that no one had ever seen or even knew was there, so that’s very exciting!”

- With three orbits under its belt, Parker Solar Probe will continue its exploration of the Sun over the course of 21 progressively-closer solar flybys. The next orbit change will occur during the Venus flyby on Dec. 26, bringing Parker to about 11.6 million miles from the Sun's surface for its next close approach to the Sun on Jan. 29, 2020. With direct measurements of this never-before-measured environment — closer to the Sun than ever before — we can expect to learn even more about these phenomena and uncover entirely new questions.

- Parker Solar Probe is part of NASA’s Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living with a Star program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. Johns Hopkins APL designed, built and operates the spacecraft.

• December 4, 2019: In August 2018, NASA's Parker Solar Probe launched to space, soon becoming the closest-ever spacecraft to the Sun. With cutting-edge scientific instruments to measure the environment around the spacecraft, Parker Solar Probe has completed three of 24 planned passes through never-before-explored parts of the Sun's atmosphere, the corona. On Dec. 4, 2019, four new papers in the journal Nature describe what scientists have learned from this unprecedented exploration of our star — and what they look forward to learning next. 46)

- These findings reveal new information about the behavior of the material and particles that speed away from the Sun, bringing scientists closer to answering fundamental questions about the physics of our star. In the quest to protect astronauts and technology in space, the information Parker has uncovered about how the Sun constantly ejects material and energy will help scientists re-write the models we use to understand and predict the space weather around our planet and understand the process by which stars are created and evolve.

- “This first data from Parker reveals our star, the Sun, in new and surprising ways,” said Thomas Zurbuchen, associate administrator for science at NASA Headquarters in Washington. “Observing the Sun up close rather than from a much greater distance is giving us an unprecedented view into important solar phenomena and how they affect us on Earth, and gives us new insights relevant to the understanding of active stars across galaxies. It’s just the beginning of an incredibly exciting time for heliophysics with Parker at the vanguard of new discoveries.”

Figure 43: NASA's Parker Solar Probe mission has returned unprecedented data from near the Sun, culminating in new discoveries published on Dec. 4, 2019, in the journal Nature. Among the findings are new understandings of how the Sun's constant outflow of material, the solar wind, behaves. Seen near Earth — where it can interact with our planet's natural magnetic field and cause space weather effects that interfere with technology — the solar wind appears to be a relatively uniform flow of plasma. But Parker Solar Probe's observations reveal a complicated, active system not seen from Earth (video credit: NASA's Goddard Space Flight Center)

- Though it may seem placid to us here on Earth, the Sun is anything but quiet. Our star is magnetically active, unleashing powerful bursts of light, deluges of particles moving near the speed of light and billion-ton clouds of magnetized material. All this activity affects our planet, injecting damaging particles into the space where our satellites and astronauts fly, disrupting communications and navigation signals, and even — when intense — triggering power outages. It’s been happening for the Sun's entire 5-billion-year lifetime, and will continue to shape the destinies of Earth and the other planets in our solar system into the future.

- “The Sun has fascinated humanity for our entire existence,” said Nour E. Raouafi, project scientist for Parker Solar Probe at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, which built and manages the mission for NASA. “We’ve learned a great deal about our star in the past several decades, but we really needed a mission like Parker Solar Probe to go into the Sun’s atmosphere. It’s only there that we can really learn the details of these complex solar processes. And what we’ve learned in just these three solar orbits alone has changed a lot of what we know about the Sun.”

- What happens on the Sun is critical to understanding how it shapes the space around us. Most of the material that escapes the Sun is part of the solar wind, a continual outflow of solar material that bathes the entire solar system. This ionized gas, called plasma, carries with it the Sun's magnetic field, stretching it out through the solar system in a giant bubble that spans more than 10 billion miles.

The dynamic solar wind

- Observed near Earth, the solar wind is a relatively uniform flow of plasma, with occasional turbulent tumbles. But by that point it’s traveled over ninety million miles — and the signatures of the Sun's exact mechanisms for heating and accelerating the solar wind are wiped out. Closer to the solar wind's source, Parker Solar Probe saw a much different picture: a complicated, active system.

- “The complexity was mind-blowing when we first started looking at the data,” said Stuart Bale, the University of California, Berkeley, lead for Parker Solar Probe’s FIELDS instrument suite, which studies the scale and shape of electric and magnetic fields. “Now, I’ve gotten used to it. But when I show colleagues for the first time, they’re just blown away.” From Parker’s vantage point 15 million miles from the Sun, Bale explained, the solar wind is much more impulsive and unstable than what we see near Earth.

- Like the Sun itself, the solar wind is made up of plasma, where negatively charged electrons have separated from positively charged ions, creating a sea of free-floating particles with individual electric charge. These free-floating particles mean plasma carries electric and magnetic fields, and changes in the plasma often make marks on those fields. The FIELDS instruments surveyed the state of the solar wind by measuring and carefully analyzing how the electric and magnetic fields around the spacecraft changed over time, along with measuring waves in the nearby plasma.

- These measurements showed quick reversals in the magnetic field and sudden, faster-moving jets of material — all characteristics that make the solar wind more turbulent. These details are key to understanding how the wind disperses energy as it flows away from the Sun and throughout the solar system.

Figure 44: Parker Solar Probe observed switchbacks — traveling disturbances in the solar wind that caused the magnetic field to bend back on itself — an as-yet unexplained phenomenon that might help scientists uncover more information about how the solar wind is accelerated from the Sun (image credit: NASA's Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez)

- One type of event in particular drew the eye of the science teams: flips in the direction of the magnetic field, which flows out from the Sun, embedded in the solar wind. These reversals — dubbed "switchbacks" — last anywhere from a few seconds to several minutes as they flow over Parker Solar Probe. During a switchback, the magnetic field whips back on itself until it is pointed almost directly back at the Sun. Together, FIELDS and SWEAP, the solar wind instrument suite led by the University of Michigan and managed by the Smithsonian Astrophysical Observatory, measured clusters of switchbacks throughout Parker Solar Probe's first two flybys.

- “Waves have been seen in the solar wind from the start of the space age, and we assumed that closer to the Sun the waves would get stronger, but we were not expecting to see them organize into these coherent structured velocity spikes," said Justin Kasper, principal investigator for SWEAP — short for Solar Wind Electrons Alphas and Protons — at the University of Michigan in Ann Arbor. "We are detecting remnants of structures from the Sun being hurled into space and violently changing the organization of the flows and magnetic field. This will dramatically change our theories for how the corona and solar wind are being heated.”

- The exact source of the switchbacks isn't yet understood, but Parker Solar Probe's measurements have allowed scientists to narrow down the possibilities.

- Among the many particles that perpetually stream from the Sun are a constant beam of fast-moving electrons, which ride along the Sun’s magnetic field lines out into the solar system. These electrons always flow strictly along the shape of the field lines moving out from the Sun, regardless of whether the north pole of the magnetic field in that particular region is pointing towards or away from the Sun. But Parker Solar Probe measured this flow of electrons going in the opposite direction, flipping back towards the Sun — showing that the magnetic field itself must be bending back towards the Sun, rather than Parker Solar Probe merely encountering a different magnetic field line from the Sun that points in the opposite direction. This suggests that the switchbacks are kinks in the magnetic field — localized disturbances traveling away from the Sun, rather than a change in the magnetic field as it emerges from the Sun.

- Parker Solar Probe's observations of the switchbacks suggest that these events will grow even more common as the spacecraft gets closer to the Sun. The mission's next solar encounter on Jan. 29, 2020, will carry the spacecraft nearer to the Sun than ever before, and may shed new light on this process. Not only does such information help change our understanding of what causes the solar wind and space weather around us, it also helps us understand a fundamental process of how stars work and how they release energy into their environment.

The rotating solar wind

- Some of Parker Solar Probe's measurements are bringing scientists closer to answers to decades-old questions. One such question is about how, exactly, the solar wind flows out from the Sun.

- Near Earth, we see the solar wind flowing almost radially — meaning it's streaming directly from the Sun, straight out in all directions. But the Sun rotates as it releases the solar wind; before it breaks free, the solar wind was spinning along with it. This is a bit like children riding on a playground park carousel – the atmosphere rotates with the Sun much like the outer part of the carousel rotates, but the farther you go from the center, the faster you are moving in space. A child on the edge might jump off and would, at that point, move in a straight line outward, rather than continue rotating. In a similar way, there's some point between the Sun and Earth, the solar wind transitions from rotating along with the Sun to flowing directly outwards, or radially, like we see from Earth.

- Exactly where the solar wind transitions from a rotational flow to a perfectly radial flow has implications for how the Sun sheds energy. Finding that point may help us better understand the lifecycle of other stars or the formation of protoplanetary disks, the dense disks of gas and dust around young stars that eventually coalesce into planets.

- Now, for the first time — rather than just seeing that straight flow that we see near Earth — Parker Solar Probe was able to observe the solar wind while it was still rotating. It's as if Parker Solar Probe got a view of the whirling carousel directly for the first time, not just the children jumping off it. Parker Solar Probe's solar wind instrument detected rotation starting more than 20 million miles (32 million km) from the Sun, and as Parker approached its perihelion point, the speed of the rotation increased. The strength of the circulation was stronger than many scientists had predicted, but it also transitioned more quickly than predicted to an outward flow, which is what helps mask these effects from where we usually sit, about 150 km million km from the Sun.

- “The large rotational flow of the solar wind seen during the first encounters has been a real surprise," said Kasper. "While we hoped to eventually see rotational motion closer to the Sun, the high speeds we are seeing in these first encounters is nearly ten times larger than predicted by the standard models."

Dust near the Sun

- Another question approaching an answer is the elusive dust-free zone. Our solar system is awash in dust — the cosmic crumbs of collisions that formed planets, asteroids, comets and other celestial bodies billions of years ago. Scientists have long suspected that, close to the Sun, this dust would be heated to high temperatures by powerful sunlight, turning it into a gas and creating a dust-free region around the Sun. But no one had ever observed it.

- For the first time, Parker Solar Probe's imagers saw the cosmic dust begin to thin out. Because WISPR — Parker Solar Probe's imaging instrument, led by the Naval Research Lab — looks out the side of the spacecraft, it can see wide swaths of the corona and solar wind, including regions closer to the Sun. These images show dust starting to thin a little over 7 million miles from the Sun, and this decrease in dust continues steadily to the current limits of WISPR's measurements at a little over 4 million miles from the Sun.

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Figure 45: Parker Solar Probe saw cosmic dust (illustrated here) — scattered throughout our solar system — begin to thin out close to the Sun, supporting the idea of a long-theorized dust-free zone near the Sun ( image credit: NASA's Goddard Space Flight Center/Scott Wiessinger)

- "This dust-free zone was predicted decades ago, but has never been seen before," said Russ Howard, principal investigator for the WISPR suite — short for Wide-field Imager for Solar Probe — at the Naval Research Laboratory in Washington, D.C. "We are now seeing what's happening to the dust near the Sun."

- At the rate of thinning, scientists expect to see a truly dust-free zone starting a little more than 2-3 million miles from the Sun — meaning Parker Solar Probe could observe the dust-free zone as early as 2020, when its sixth flyby of the Sun will carry it closer to our star than ever before.

Putting space weather under a microscope

- Parker Solar Probe's measurements have given us a new perspective on two types of space weather events: energetic particle storms and coronal mass ejections.

- Tiny particles — both electrons and ions — are accelerated by solar activity, creating storms of energetic particles. Events on the Sun can send these particles rocketing out into the solar system at nearly the speed of light, meaning they reach Earth in under half an hour and can impact other worlds on similarly short time scales. These particles carry a lot of energy, so they can damage spacecraft electronics and even endanger astronauts, especially those in deep space, outside the protection of Earth's magnetic field — and the short warning time for such particles makes them difficult to avoid.

- Understanding exactly how these particles are accelerated to such high speeds is crucial. But even though they zip to Earth in as little as a few minutes, that's still enough time for the particles to lose the signatures of the processes that accelerated them in the first place. By whipping around the Sun at just a few million miles away, Parker Solar Probe can measure these particles just after they've left the Sun, shedding new light on how they are released.

- Already, Parker Solar Probe's ISOIS instruments, led by Princeton University, have measured several never-before-seen energetic particle events — events so small that all trace of them is lost before they reach Earth or any of our near-Earth satellites. These instruments have also measured a rare type of particle burst with a particularly high number of heavier elements — suggesting that both types of events may be more common than scientists previously thought.

- "It’s amazing – even at solar minimum conditions, the Sun produces many more tiny energetic particle events than we ever thought," said David McComas, principal investigator for the ISOIS (Integrated Science Investigation of the Sun suite), at Princeton University in New Jersey. "These measurements will help us unravel the sources, acceleration, and transport of solar energetic particles and ultimately better protect satellites and astronauts in the future."

Figure 46: Parker Solar Probe has made new observations of energetic particles — like those seen here impacting a detector on ESA and NASA's SOHO (Solar and Heliospheric Observatory) — which will help scientists better understand how these events are accelerated (image credit: ESA/NASA/SOHO)

- Data from the WISPR instruments also provided unprecedented detail on structures in the corona and solar wind — including coronal mass ejections, billion-ton clouds of solar material that the Sun sends hurtling out into the solar system. CMEs can trigger a range of effects on Earth and other worlds, from sparking auroras to inducing electric currents that can damage power grids and pipelines. WISPR's unique perspective, looking alongside such events as they travel away from the Sun, has already shed new light on the range of events our star can unleash.

Figure 47: Parker Solar Probe's imagers look out sideways from behind the spacecraft's heat shield, watching structures as they develop in the corona (video credit: NASA/JHUAPL/Naval Research Lab/Parker Solar Probe)

- "Since Parker Solar Probe was matching the Sun's rotation, we could watch the outflow of material for days and see the evolution of structures," said Howard. "Observations near Earth have made us think that fine structures in the corona segue into a smooth flow, and we're finding out that's not true. This will help us do better modeling of how events travel between the Sun and Earth."

- As Parker Solar Probe continues on its journey, it will make 21 more close approaches to the Sun at progressively closer distances, culminating in three orbits a mere 3.83 million miles (6.16 million km) from the solar surface.

- “The Sun is the only star we can examine this closely,” said Nicola Fox, director of the Heliophysics Division at NASA Headquarters. “Getting data at the source is already revolutionizing our understanding of our own star and stars across the universe. Our little spacecraft is soldiering through brutal conditions to send home startling and exciting revelations.”

- Data from Parker Solar Probe's first two solar encounters is available to the public online.

• August 12, 2019: Since NASA's Parker Solar Probe launched on Aug. 12, 2018, Earth has made a single trip around the Sun — while the daring solar explorer is well into its third orbit around our star. With two close passes by the Sun already under its belt, Parker Solar Probe is speeding toward another close solar approach on Sept. 1, 2019. 47)

- Parker Solar Probe is named for Eugene Parker, the physicist who first theorized the solar wind — the constant outflow of particles and magnetic fields from the Sun — in 1958. Parker Solar Probe is the first NASA mission to be named for a living person.

Figure 48: Nicky Fox, director of NASA's Heliophysics Division, reflects on Parker Solar Probe's first year in space with Eugene Parker, after whom the mission is named. In 1958, Parker published the first scientific paper theorizing the existence of the solar wind, now studied by the spacecraft that bears his name (video credit: University of Chicago)

- In the year since launch, Parker Solar Probe has collected a host of scientific data from two close passes by the Sun.

- "We're very happy," said Nicky Fox, director of NASA's Heliophysics Division at NASA Headquarters in Washington, D.C. "We've managed to bring down at least twice as much data as we originally suspected we’d get from those first two perihelion passes."

- The spacecraft carries four suites of scientific instruments to gather data on the particles, solar wind plasma, electric and magnetic fields, solar radio emission, and structures in the Sun's hot outer atmosphere, the corona. This information will help scientists unravel the physics driving the extreme temperatures in the corona — which is counterintuitively hotter than the solar surface below — and the mechanisms that drive particles and plasma out into the solar system.

Figure 49: Parker Solar Probe's WISPR instrument saw the solar wind streaming past during the spacecraft's first solar encounter in November 2018 (video credit: NASA/Naval Research Laboratory/Parker Solar Probe)

- Parker Solar Probe's WISPR instrument captures images of solar wind structures as they stream out from the Sun, allowing scientists to connect them with Parker's in situ measurements from its other instruments.

- This video, which spans Nov. 6-10, 2018, combines views from both WISPR telescopes during Parker Solar Probe's first solar encounter. The Sun is out of frame past the combined image's left side, so the solar wind flows from left to right past the view of the telescopes. The bright structure near the center of the left edge is what's known as a streamer — a relatively dense, slow flow of solar wind coming from the Sun — originating from near the Sun's equator.

- The video appears to speed up and slow down throughout the movie because of the ways data is stored at different points in Parker Solar Probe's orbit. Near perihelion, the closest approach to the Sun, the spacecraft stores more images — and more frames for a given section make the video appear to slow down. These images have been calibrated and processed to remove background noise.

- The Milky Way's galactic center is visible on the right side of the video. The planet visible on the left is Mercury. The thin white streaks in the image are particles of dust passing in front of WISPR's cameras.

- The mission team is currently in the process of analyzing data from Parker Solar Probe's first two orbits, which will be released to the public in 2019.

- "The data we’re seeing from Parker Solar Probe’s instruments is showing us details about solar structures and processes that we have never seen before,” said Nour Raouafi, Parker Solar Probe project scientist at the Johns Hopkins Applied Physics Laboratory, which built and operates the mission for NASA. “Flying close to the Sun — a very dangerous environment — is the only way to obtain this data, and the spacecraft is performing with flying colors.”

• April 5, 2019: Parker Solar Probe has successfully completed its second close approach to the Sun, called perihelion, and is now entering the outbound phase of its second solar orbit. At 6:40 p.m. EDT on April 4, 2019, the spacecraft passed within 15 million miles of our star, tying its distance record as the closest spacecraft ever to the Sun; Parker Solar Probe was traveling at 213,200 miles per hour during this perihelion. 48)

- The Parker Solar Probe mission team at JHU/APL in Laurel, Maryland scheduled a contact with the spacecraft via the Deep Space Network for four hours around the perihelion and monitored the health of the spacecraft throughout this critical part of the encounter. Parker Solar Probe sent back beacon status “A” throughout its second perihelion, indicating that the spacecraft is operating well and all instruments are collecting science data.

- “The spacecraft is performing as designed, and it was great to be able to track it during this entire perihelion,” said Nickalaus Pinkine, Parker Solar Probe mission operations manager at APL. “We’re looking forward to getting the science data down from this encounter in the coming weeks so the science teams can continue to explore the mysteries of the corona and the Sun.”

- Parker Solar Probe began this solar encounter on March 30, and it will conclude on April 10. The solar encounter phase is roughly defined as when the spacecraft is within 0.25 AU — or 23,250,000 miles — of the Sun. One AU, or astronomical unit, is about 93 million miles (150 million km), the average distance from the Sun to Earth.

• On January 19, 2019, just 161 days after its launch from Cape Canaveral Air Force Station in Florida, NASA’s Parker Solar Probe completed its first orbit of the Sun, reaching the point in its orbit farthest from our star, called aphelion. The spacecraft has now begun the second of 24 planned orbits, on track for its second perihelion, or closest approach to the Sun, on April 4, 2019. 49)

- Parker Solar Probe entered full operational status (known as Phase E) on January 1, with all systems online and operating as designed. The spacecraft has been delivering data from its instruments to Earth via the Deep Space Network, and to date more than 17 Gbit of science data has been downloaded. The full dataset from the first orbit will be downloaded by April.

- “It’s been an illuminating and fascinating first orbit,” said Parker Solar Probe Project Manager Andy Driesman, of the Johns Hopkins University Applied Physics Laboratory. “We’ve learned a lot about how the spacecraft operates and reacts to the solar environment, and I’m proud to say the team’s projections have been very accurate.” APL designed, built, and manages the mission for NASA.

- “We’ve always said that we don’t know what to expect until we look at the data,” said Project Scientist Nour Raouafi, also of APL. “The data we have received hints at many new things that we’ve not seen before and at potential new discoveries. Parker Solar Probe is delivering on the mission’s promise of revealing the mysteries of our Sun.”

- The Parker Solar Probe team is not only focused on analyzing the science data but also preparing for the second solar encounter, which will take place in about two months.

- In preparation for that next encounter, the spacecraft’s solid state recorder is being emptied of files that have already been delivered to Earth. In addition, the spacecraft is receiving updated positional and navigation information (called ephemeris) and is being loaded with a new automated command sequence, which contains about one month’s worth of instructions.

- Like the mission's first perihelion in November 2018, Parker Solar Probe’s second perihelion in April will bring the spacecraft to a distance of about 15 million miles from the Sun – just over half the previous close solar approach record of about 27 million miles set by Helios 2 in 1976.

- The spacecraft’s four instrument suites will help scientists begin to answer outstanding questions about the Sun's fundamental physics — including how particles and solar material are accelerated out into space at such high speeds and why the Sun's atmosphere, the corona, is so much hotter than the surface below.

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Figure 50: Parker Solar Probe’s position, speed and round-trip light time as of Jan. 28, 2019 (image credit: NASA, JHU/APL, Track the spacecraft online)

• December 12, 2018: Weeks after Parker Solar Probe made the closest-ever approach to a star, the science data from the first solar encounter is just making its way into the hands of the mission's scientists. It's a moment many in the field have been anticipating for years, thinking about what they'll do with such never-before-seen data, which has the potential to shed new light on the physics of our star, the Sun. 50)

- On Dec. 12, 2018, four such researchers gathered at the fall meeting of the American Geophysical Union in Washington, D.C., to share what they hope to learn from Parker Solar Probe.

- Heliophysicists have been waiting more than 60 years for a mission like this to be possible," said Nicola Fox, director of the Heliophysics Division at NASA Headquarters in Washington. Heliophysics is the study of the Sun and how it affects space near Earth, around other worlds and throughout the solar system. "The solar mysteries we want to solve are waiting in the corona."

- From Oct. 31 to Nov. 11, 2018, Parker Solar Probe completed its first solar encounter phase, speeding through the Sun's outer atmosphere — the corona — and collecting unprecedented data with four suites of cutting-edge instruments.

- "This is the first NASA mission to be named for a living individual," said Fox. "Gene Parker’s revolutionary paper predicted the heating and expansion of the corona and solar wind. Now, with Parker Solar Probe we are able to truly understand what drives that constant flow out to the edge of the heliosphere.”

- Other worlds in our solar system experience their own versions of these effects, and far beyond the planets, the Sun's material butts up against the interstellar medium, which fills the space between the stars. The interaction in this region plays a role in how often high-energy galactic cosmic rays shoot into our solar system. All of these effects result from complicated systems — but they all start back at the Sun, making it critical to grasp the fundamental physics that drive our star's activity.

- Parker Solar Probe is designed to address three major questions about the physics of the Sun. First: How is the Sun's outer atmosphere, the corona, heated to temperatures about 300 times higher than the visible surface below? Second — how is the solar wind accelerated so quickly to the high speeds we observe? And finally, how do some of the Sun's most energetic particles rocket away from the Sun at more than half the speed of light?

- "Parker Solar Probe is providing us with the measurements essential to understanding solar phenomena that have been puzzling us for decades," said Nour Raouafi, Parker Solar Probe project scientist at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland. "To close the link, local sampling of the solar corona and the young solar wind is needed and Parker Solar Probe is doing just that."

- Parker's instruments are designed to look at these phenomena in question in ways that haven't been possible before, giving scientists the opportunity to make new strides in the study of the solar atmosphere.

- For instance, Parker Solar Probe's imagers, in the WISPR suite, will have a new perspective on the young solar wind, capturing a view of how it evolves as Parker Solar Probe travels through the solar corona.

- The spacecraft's ISOIS suite will help scientists dig down into the causes of energetic particle acceleration. Right now, theories diverge on how solar energetic particles are accelerated within the thin shock wave structures usually driven by fast coronal mass ejections — but energetic particle measurements gathered as the spacecraft travels through such waves will help shed light on this problem.

- The electric field antennas of the spacecraft's FIELDS instrument suite can pick up radio bursts that could shed light on the causes of coronal heating.

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Figure 51: This image from Parker Solar Probe's WISPR (Wide-field Imager for Solar Probe) instrument shows a coronal streamer, seen over the east limb of the Sun on 8 November 2018, at 1:12 a.m. EST. Coronal streamers are structures of solar material within the Sun's atmosphere, the corona, that usually overlie regions of increased solar activity. The fine structure of the streamer is very clear, with at least two rays visible. Parker Solar Probe was about 16.9 million miles from the Sun's surface when this image was taken. The bright object near the center of the image is Mercury, and the dark spots are a result of background correction (image credit: NASA/Naval Research Laboratory/Parker Solar Probe)

- The Solar Probe Cup instrument — which extends beyond the spacecraft's heat shield and is exposed to the full solar environment — measures the thermal properties of different ion species in the solar wind. Coupled with data from the FIELDS suite, these measurements could help reveal how the solar wind is heated and accelerated.

- The science team also expects to be surprised by some of what they learn. "We don't know what to expect so close to the Sun until we get the data, and we'll probably see some new phenomena," said Raouafi. "Parker is an exploration mission — the potential for new discoveries is huge."

- Parker Solar Probe's reports indicate that good science data was collected during the first solar encounter, and the data itself began downlinking to Earth on 7 December. Because of the relative positions of Parker Solar Probe, the Sun and Earth and their effects on radio transmission, some of the science data from this encounter will not downlink until after the mission's second solar encounter in April 2019.

- The mission team did get a chance for some real-world instrument tests during Parker Solar Probe's Venus flyby in September 2018. Parker Solar Probe made a close pass at the planet while performing a gravity assist to draw its orbit closer to the Sun. Though not expected to study the environment around Venus, Parker's instruments successfully recorded data, giving scientists an early look at what their instruments are capable of in the harsh environment of space.

- As the newest addition to NASA's fleet of heliophysics missions, Parker Solar Probe works alongside prolific solar and heliospheric research satellites like NASA's Solar Dynamics Observatory, the STEREO (Solar and Terrestrial Relations Observatory) and ACE (Advanced Composition Explorer). For years — or decades, in some cases — these observatories have scrutinized the Sun and its outflowing material, changing the way we see our star. But they are limited by where they live.

- Even as Parker uncovers new information, scientists working with its data will rely on the rest of NASA's heliophysics fleet to put those details in context.

- "Parker Solar Probe is going to a region we've never visited before," said Terry Kucera, a solar physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Meanwhile, from a distance, we can observe the Sun's corona, which is driving the complex environment around Parker Solar Probe."

Figure 52: This video clip shows actual data from NASA's Solar and Terrestrial Relations Observatory Ahead (STEREO-A) spacecraft, along with the location of Parker Solar Probe as it flies through the Sun’s outer atmosphere during its first solar encounter phase in November 2018. Such images will allow us to provide key context for understanding Parker Solar Probe's observations (image credit: NASA/STEREO)

- The distinct perspectives of these observatories should be a boon for contextualizing Parker's observations. While SDO is in geosynchronous Earth orbit, STEREO orbits the Sun at slightly less than 1 AU — one astronomical unit is the average distance between Earth and the Sun — making it just a little bit faster than Earth. That means STEREO usually observes the Sun from a different angle than we do here on Earth. Along with Parker's measurements close to the Sun and often from a different angle than any of our other satellites, this will give scientists a fuller picture of how solar events change and develop as they propagate out into the solar system.

- "The STEREO mission is all about observing the heliosphere from different locations and Parker is a part of that – making measurements from a perspective we've never had before,” said Kucera.

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Figure 53: Parker Solar Probe will give scientists another new perspective on the Sun, joining those from other Sun-observing spacecraft (image credit: NASA/GSFC)

- Modeling is another critical tool for painting the complete picture around Parker's observations."Our simulation results provide a way to interpret both the localized measurements from the in situ instruments, like FIELDS and SWEAP, as well the more global images produced by WISPR," said Pete Riley, a research scientist at Predictive Science Inc., in San Diego, California.

- Models are a good way to test theories about the underlying physics of the Sun. By creating a simulation that relies on a certain mechanism to explain coronal heating — for instance, a certain kind of plasma wave called an Alfvén wave — scientists can check the model's prediction against actual data from Parker Solar Probe to see if they line up. If they do, that means the underlying theory may be what's actually happening.

- "We’ve had a lot of success predicting the structure of the solar corona during total solar eclipses," said Riley. "Parker Solar Probe will provide unprecedented measurements that will further constrain the models and the theory that’s embedded within them."

- Parker Solar Probe is in a unique position to help improve models — in part because of its record-breaking speed.

- The Sun rotates about once every 27 days as viewed from Earth, and the solar structures that drive much of its activity move along with it. That creates a problem for scientists, who can't always tell if the variability they see is driven by actual changes to the region producing the activity — temporal variation — or is caused by simply receiving solar material from a new source region — spatial variation.

Figure 54: Numerical models provide a global context for interpreting Parker Solar Probe observations. This animation is from a model showing how the solar wind flows out from the Sun, with the perspective of Parker Solar Probe’s WISPR instrument overlaid (image credits: Predictive Science Inc.)

- For part of its orbit, Parker Solar Probe will outrun that problem. At certain points, Parker Solar Probe is traveling fast enough to almost exactly match the Sun's rotational speed, meaning that Parker "hovers" over one area of the Sun for a short amount of time. Scientists can be certain that changes in data during this period are caused by actual changes on the Sun, rather than the Sun's rotation.

• On November 16, Parker Solar Probe reported that all systems are operating well in the first detailed performance and health update sent to Earth by the spacecraft since its first solar encounter. 51)

- At about 6:00 p.m. EST on Friday, Nov. 16, mission controllers at the Johns Hopkins Applied Physics Lab in Laurel, Maryland, received the report from the spacecraft, which also included information about the data collected by the four instrument suites during its first solar encounter. Parker Solar Probe’s first solar encounter phase took place Oct. 31 – Nov. 11, culminating in its first close approach to the Sun — called perihelion — on Nov. 6 at just 15 million miles from the Sun’s surface, the closest any spacecraft has ever come to our star.

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Figure 55: Illustration of the Parker Solar Probe approaching the Sun (image credit: NASA/Johns Hopkins APL/Steve Gribben)

- All Parker Solar Probe systems are operating well and as designed. The solid state recorder on the spacecraft indicated that, as planned, the four instrument suites had recorded a significant amount of data, which is scheduled to be downloaded to Earth via the Deep Space Network over several weeks starting 7 December. In addition to helping scientists begin to explore fundamental questions about the physics of our star, the data from this initial perihelion — collected closer to the Sun than any before — will help instrument teams calibrate Parker Solar Probe’s instruments and plan future observations.

- “The team is extremely proud to confirm that we have a healthy spacecraft following perihelion,” said APL’s Nick Pinkine, mission operations manager for Parker Solar Probe. “This is a big milestone, and we’re looking forward to some amazing science data coming down in a few weeks.”

- During the 11-day solar encounter, the spacecraft executed only one autonomous momentum dump – a procedure in which small thrusters are used to adjust the speed of Parker’s reaction wheels. The rate of spin of the wheels is adjusted to maintain the desired orientation of the spacecraft relative to the Sun. Momentum dumps are expected during solar encounters, as the wheels spin up to counter increasing torque from the gravitational effects of the solar environment. Executing only one dump indicates that the spacecraft is well balanced, minimizing the need for these dumps during future solar encounters, which will save propellant.

- Parker Solar Probe’s second perihelion will occur on April 4, 2019. During the seven-year mission, the spacecraft will perform a total of 24 perihelia, with the last three bringing the spacecraft to less than 4 million miles from the Sun’s surface.

• October 29, 2018: Parker Solar Probe now holds the record for closest approach to the Sun by a human-made object. The spacecraft passed the current record of 26.55 million miles from the Sun's surface on Oct. 29, 2018, at about 1:04 p.m. EDT, as calculated by the Parker Solar Probe team. 52)

- The previous record for closest solar approach was set by the German-American Helios 2 spacecraft in April 1976. As the Parker Solar Probe mission progresses, the spacecraft will repeatedly break its own records, with a final close approach of 3.83 million miles from the Sun's surface expected in 2024.

- “It’s been just 78 days since Parker Solar Probe launched, and we’ve now come closer to our star than any other spacecraft in history,” said Project Manager Andy Driesman, from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “It’s a proud moment for the team, though we remain focused on our first solar encounter, which begins on Oct. 31.”

- Parker Solar Probe is also expected to break the record for fastest spacecraft traveling relative to the Sun on Oct. 29 at about 10:54 p.m. EDT. The current record for heliocentric speed is 153,454 miles per hour, set by Helios 2 in April 1976.

- The Parker Solar Probe team periodically measures the spacecraft's precise speed and position using NASA's DSN (Deep Space Network). The DSN sends a signal to the spacecraft, which then retransmits it back to the DSN, allowing the team to determine the spacecraft's speed and position based on the timing and characteristics of the signal. Parker Solar Probe's speed and position were calculated using DSN measurements made on Oct. 24, and the team used that information along with known orbital forces to calculate the spacecraft's speed and position from that point on.

- Parker Solar Probe will begin its first solar encounter on Oct. 31, continuing to fly closer and closer to the Sun's surface until it reaches its first perihelion — the point closest to the Sun — at about 10:28 p.m. EST on Nov. 5. The spacecraft will face brutal heat and radiation conditions while providing humanity with unprecedentedly close-up observations of a star and helping us understand phenomena that have puzzled scientists for decades. These observations will add key knowledge to NASA’s efforts to understand the Sun, where changing conditions can propagate out into the solar system, affecting Earth and other worlds.

• On Oct. 3, 2018, Parker Solar Probe performed the first significant celestial maneuver of its seven-year mission. As the orbits of the spacecraft and Venus converged toward the same point, Parker Solar Probe slipped in front of the planet, allowing Venus' gravity — relatively small by celestial standards — to twist its path and change its speed. This maneuver, called a gravity assist, reduced Parker's speed relative to the Sun by 10 percent — amounting to 7,000 miles/hr — drawing the closest point of its orbit, called perihelion, nearer to the star by 4 million miles. 53)

Figure 56: Parker Solar Probe completed its first flyby of Venus on Oct. 3, 2018, during a Venus gravity assist, where the spacecraft used the planet's gravity to alter its trajectory and bring it closer to the Sun (image credit: NASA, JHU/APL)

- Performed six more times over the course of the seven-year mission, these gravity assists will eventually bring Parker Solar Probe's closest approach to a record 3.83 million miles from the Sun's surface — about a seventh the distance of the current record-holder, Helios 2, which achieved a pass of 27 million miles from the Sun in 1976. Even before its closest approach, Parker Solar Probe is expected to overtake this record and become the closest human-made object to the Sun in late October 2018.

A solar probe has been on the minds of scientists and engineers for decades, since the late '50s, when a new theory and the first satellite measurements of the Sun's constant outflow of material, called the solar wind, pointed towards previously unsuspected complexity. But if you'd asked anyone before 2007 — well after serious planning for such a mission began — Venus would not have come up as the key to the mission puzzle. For the three-plus decades that various committees and teams worked on different concepts for the solar probe mission, it was widely agreed that the only way to dive into the solar atmosphere required sending the spacecraft to Jupiter first.

"No one believed using Venus gravity assists would be possible, because the gravity assist a planetary body can provide is proportional to the body’s mass, and Venus’ mass is so much smaller — only 0.3 percent of Jupiter’s," said Yanping Guo, mission design and navigation manager for the Parker Solar Probe mission at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland. " You compare the gravity assist Venus can provide to what Jupiter can provide, and you have to do repeated flybys to achieve the same change. Then you're getting a very long mission duration."

Getting close to the Sun is more difficult than one might think. Any spacecraft launched from Earth starts off traveling at our planet's 67,000-mile-per-hour sideways pace, speed that it must counteract before it can get anywhere near the Sun. Gravity assists are one of the most powerful tools in an orbit designer's toolbox to solve this problem: Instead of using expensive, precious fuel to change direction or speed (or both), gravity assists let you harness the natural pull of a planet, with time as the only cost.

Most deep-space missions that use planetary gravity assists use them to gain speed — like OSIRIS-REx, which used Earth's gravity to rocket towards asteroid Bennu — or to change direction — like Voyager 2, which performed a gravity assist after its final planetary flyby at Neptune to bank toward its moon, Triton.

The idea for a solar probe gravity assist was a little different. In the original orbit plans, the primary functions of the Jupiter gravity assist were to slow the spacecraft's speed to almost nothing and fling it upwards, out of the nearly-flat plane that contains all of the known planets of the solar system, called the ecliptic plane. This would put the solar probe on a path to get a rare and better-than-ever look at the Sun's polar regions, which are difficult to image, but important scientifically as they produce some of the Sun's high-speed solar wind. Nearly all of our solar observatories have orbited in the ecliptic plane, with the exception of Ulysses, which used a Jupiter gravity assist to achieve polar passes more than 200 million miles from the Sun.

But sending a spacecraft out to Jupiter and bringing it back into the inner solar system is hard. First, no matter how you plan the journey, it's a long mission, with a minimum of nearly half a decade between meaningful events. Most of the time would be spent cruising in deep space.

Second, traveling that far from the Sun means you have to get creative with power. Near Jupiter, the sunlight is about 25 times dimmer than what we experience at Earth, so the only options are huge solar panels to make the most of the sparse sunlight, or some other source of power, like nuclear. Large solar panels pose a problem for a solar probe, though, because the panels would need to be shielded during solar encounters to avoid overheating. The size of a solar panel required to power the spacecraft out near Jupiter is too big to effectively stow near the Sun, so they'd have to be jettisoned at perihelion — and that limits you to just one solar pass, once you've lost your source of power. With nuclear power — RTG (Radioisotope Thermoelectric Generator), the same source that powers deep-space missions like Cassini and New Horizons — performing a Jupiter gravity assist is a viable option.

Changing the mission paradigm: But the mission design was soon to change. David McComas, chair of the definition committee, remembers a call from Andy Dantzler, then project manager for the Solar Probe mission at APL. Dantzler passed away in 2011 at age 49; the Delta IV Heavy rocket that carried Parker Solar Probe to space was dedicated to him.

"Andy asked if there was any way the committee might go for a mission where you stay in the ecliptic plane but have lots of passes by the Sun and slowly reduce the perihelion," said McComas, who is now the principal investigator of the mission's Integrated Science Investigation of the Sun, or ISOIS, suite and a professor of astrophysical sciences at Princeton University in New Jersey.

This was an entirely new paradigm for the mission. A hallmark of the original plan was passing over the Sun's poles, the source of the Sun's fast solar wind but a region of relative mystery to scientists. Additionally, staying in the ecliptic plane would almost certainly mean ending up farther from the Sun than had previously been anticipated.

"If you're trading perihelion distance, you have to swap it for something that will give you opportunities to fill in the science in some other way," said McComas.

Subsequently, two developments supported the choice to make these changes to the orbit and create the Parker Solar Probe mission we know today.

The first was new research published in 2009 by Thomas Zurbuchen — then a scientist at the University of Michigan and now the associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. This research showed that the solar wind that could be measured from the ecliptic plane was actually from a diverse mix of sources. It was not only the slower solar wind known to be more common near the Sun's equator, but also the high-speed solar wind that often originates closer to the Sun's poles. By sampling the solar wind from the ecliptic plane over a period of years, scientists could learn about this fast solar wind in ways they hadn't previously anticipated.

The second development was the shift that made such sampling possible: the design of Parker Solar Probe's current trajectory.

"When starting, I had no clue if I could find a solution," said Guo, the mission trajectory designer. "Everybody thought Jupiter was the only practical way you could get closer to the Sun, within 10 solar radii."

In 2007, she came up with five alternative options that would keep the spacecraft near the ecliptic plane and would not require traveling out to Jupiter. These trajectory options used some combination of Earth and Venus gravity assists to gradually draw the spacecraft closer to the Sun over the course of a number of years. One fulfilled all the requirements for the Solar Probe mission — a total mission duration under 10 years, with a final close approach clocking in under 10 solar radii (equivalent to 4.3 million miles). This was chosen as the trajectory of the current mission, now called Parker Solar Probe after Dr. Eugene Parker, including seven Venus gravity assists that spiral the orbit in closer and closer to the Sun over the mission's seven-year lifetime. — See Figure 58 for the final orbit of the Parker Solar Probe mission.

The biggest hurdle to overcome for a trajectory with such repeated gravity assists is phasing. Of course, Venus is in constant motion around the Sun, so every time the spacecraft passes the planet and swings around our star, Venus is in a completely different place. But Guo's design solves that problem, with multiple opportunities for launch. This trajectory design carries the spacecraft through 24 orbits around the Sun. The seven Venus gravity assists happen at different points in the spacecraft's orbit, to account for the phasing problem — some, like the one on Oct. 3, happen as the spacecraft heads towards the Sun, while the others happen as Parker Solar Probe speeds away from the Sun.

This orbit is decidedly different than the original single-Jupiter-gravity-assist concept. Rather than two passes over the Sun's poles, coming within 1.23 million miles of the surface, this version of the mission provides 24 passes around the Sun near its equator, coming within 3.83 million miles of the Sun's surface.

Though Parker Solar Probe doesn't get as close to the Sun, this version of the trajectory provides the spacecraft with more than 900 hours in this critical inner region of the Sun's corona, within 20 solar radii (about 8.65 million miles). In comparison, earlier designs using Jupiter gravity assists provided less than 100 hours in this region.

"Here was this technical solution that was safer and cheaper and a better scientific mission because of all the samples we’d be getting," said McComas. "The Sun isn't a stable object — it's variable — so this would let us do a better scientific job."

This change to the mission also solved the power problem. Instead of requiring an RTG or unmanageably-large solar panels, Parker Solar Probe is powered by a pair of articulated solar panels that are slowly drawn into the shadow of the heat shield as the spacecraft approaches the Sun. At closest approach, only a small area remains exposed to generate the needed power for the spacecraft, cooled by the mission's first-of-its-kind solar array cooling system.

But though it solved a major problem, rethinking the mission in this way also required a complete rethinking of the spacecraft itself.

"The whole spacecraft design changed dramatically," said Nicola Fox, formerly the mission's project scientist at APL. Fox is now the director of the heliophysics division at NASA Headquarters. "With the earlier trajectory, the heat shield was the spacecraft. It was like a cone, with the pointed end facing the Sun, because when you're doing such a fast polar orbit it's tough to keep a shield oriented correctly."

"We aren't going in as far with the new trajectory, so we could go to a simpler shape for the heat shield, because it's possible to keep the heat shield oriented between the spacecraft and the Sun at all times. The whole thing looks really different."

The mission team credits Andy Dantzler with guiding them through this fundamental change in the mission's design that led to the mission we know today.

"When Andy called and asked if the definition team would go for it, I really didn't know the answer," said McComas. "As our definition team worked through the science, I became convinced that it wasn't just an equivalent mission, but actually a better scientific mission, because we get so much more time close to the Sun and so many more samples at different times."

The first flyby: During the Oct. 3 gravity assist, Parker Solar Probe came within about 1,500 miles of Venus' surface, reaching this closest point at about 4:45 a.m. EDT.

Venus is an interesting case for heliophysicists, who study not only the Sun, but also its effects on planets. Unlike Earth, Venus doesn't have an internal magnetic field — instead, a weak magnetic field is induced over the surface by the constant barrage of solar charged particles flowing over the planet and interacting with its very dense atmosphere.

This first flyby offered a unique opportunity for calibration, as Parker Solar Probe flew through the trailing end of Venus' magnetic field, called the magnetotail. Three of Parker Solar Probe's four instrument suites — SWEAP, ISOIS and FIELDS — gathered data during the flyby on particles and fields in this region.

Though the data is still making its way back to Earth, the science team hopes to analyze it before they set their sights on Parker Solar Probe's next major celestial encounter: its first close approach to the Sun. Parker Solar Probe's first solar encounter will happen Oct. 31 – Nov. 11, with the closest approach happening on Nov. 5 at a distance of 15 million miles from the Sun. The science data from this encounter will start downlinking to Earth in early December.

Parker Solar Probe is part of NASA’s Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living with a Star program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. APL designed, built and operates the spacecraft.

Table 4: A long-held dream (Ref. 53)

• Parker Solar Probe captured a view of Earth on 25 September 2018 as it sped toward the first Venus gravity assist of the mission. Earth is the bright, round object visible in the right side of the image. 54)

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Figure 57: The view from Parker Solar Probe’s WISPR instrument on 25 Sept. 2018, shows Earth, the bright sphere near the middle of the right-hand panel. The elongated mark toward the bottom of the panel is a lens reflection from the WISPR instrument (image credit: NASA/Naval Research Laboratory/Parker Solar Probe)

- The WISPR (Wide-field Imager for Solar Probe) instrument is the only imaging instrument on board Parker Solar Probe. During science phases, WISPR sees structures within the Sun's atmosphere, the corona, before they pass over the spacecraft. The two panels of WISPR's image come from the instrument’s two telescopes, which point in slightly different directions and have different fields of view. The inner telescope produced the left-hand image, while the outer telescope produced the image on the right.

- Zooming in on Earth reveals a slight bulge on the right side: that is the Moon, just peeking out from behind Earth. At the time the image was taken, Parker Solar Probe was about 27 million miles from Earth.

- The hemispherical shaped feature in the middle of the right-hand image is a lens flare, a common feature when imaging bright sources, which is caused by reflections within the lens system. In this case, the flare is due to the very bright Earthshine. Close passes by Venus and Mercury may occasionally create similar patterns in the future, but these are limited cases and do not affect the science operations of the instrument.

- Some of the visible objects in the image — like Pleiades to the low-left of Earth in the right-hand image and the two bright objects, Betelgeuse and Bellatrix, near the bottom of the left-hand image — appear elongated because of reflections on the edge of the detector.

• On 3 October 2018, Parker Solar Probe performed the first significant celestial maneuver of its seven-year mission. As the orbits of the spacecraft and Venus converged toward the same point, Parker Solar Probe slipped in front of the planet, allowing Venus' gravity — relatively small by celestial standards — to twist its path and change its speed. This maneuver, called a gravity assist, reduced Parker's speed relative to the Sun by 10 percent — amounting to 7,000 miles per hour — drawing the closest point of its orbit, called perihelion, nearer to the star by 4 million miles. 55)

- Performed six more times over the course of the seven-year mission, these gravity assists will eventually bring Parker Solar Probe's closest approach to a record 3.83 million miles from the Sun's surface — about a seventh the distance of the current record-holder, Helios 2, which achieved a pass of 27 million miles from the Sun in 1976. Even before its closest approach, Parker Solar Probe is expected to overtake this record and become the closest human-made object to the Sun in late October 2018.

- Venus is an interesting case for heliophysicists, who study not only the Sun, but also its effects on planets. Unlike Earth, Venus doesn't have an internal magnetic field — instead, a weak magnetic field is induced over the surface by the constant barrage of solar charged particles flowing over the planet and interacting with its very dense atmosphere.

- This first flyby offered a unique opportunity for calibration, as Parker Solar Probe flew through the trailing end of Venus' magnetic field, called the magnetotail. Three of Parker Solar Probe's four instrument suites — SWEAP, ISOIS (ISIS-EPI) and FIELDS — gathered data during the flyby on particles and fields in this region.

- Though the data is still making its way back to Earth, the science team hopes to analyze it before they set their sights on Parker Solar Probe's next major celestial encounter: its first close approach to the Sun. Parker Solar Probe's first solar encounter will happen Oct. 31 – Nov. 11, with the closest approach happening on Nov. 5 at a distance of 15 million miles from the Sun. The science data from this encounter will start downlinking to Earth in early December.

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Figure 58: The final orbit for the Parker Solar Probe mission uses seven Venus gravity assists to rack up more than 900 hours close to the Sun. The original mission concept, using a single Jupiter gravity assist, got the spacecraft closer to the Sun, but gave scientists less than 100 hours in key areas (image credit: NASA's Goddard Space Flight Center/Mary Pat Hrybyk-Keith)

• September 19, 2018: Just over a month into its mission, Parker Solar Probe has returned first-light data from each of its four instrument suites. These early observations – while not yet examples of the key science observations Parker Solar Probe will take closer to the Sun – show that each of the instruments is working well. The instruments work in tandem to measure the Sun's electric and magnetic fields, particles from the Sun and the solar wind, and capture images of the environment around the spacecraft. The mission’s first close approach to the Sun will be in November 2018, but even now, the instruments are able to gather measurements of what’s happening in the solar wind closer to Earth. 56)

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Figure 59: First light data from Parker Solar Probe's WISPR (Wide-field Imager for Solar Probe) instrument suite. The right side of this image — from WISPR's inner telescope — has a 40º FOV, with its right edge 58.5º from the Sun's center. The bright object slightly to the right of the image's center is Jupiter. - The left side of the image is from WISPR’s outer telescope, which has a 58º FOV and extends to about 160º from the Sun. It shows the Milky Way, looking at the galactic center. There is a parallax of about 13º in the apparent position of the Sun as viewed from Earth and from Parker Solar Probe (image credit: NASA/Naval Research Laboratory/Parker Solar Probe)

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Figure 60: First light data from EPI-Lo (the lower-energy Energetic Particle Instrument), part of the ISIS (Integrated Science Investigation of the Sun) suite aboard Parker Solar Probe (image credit: NASA/Princeton University/Parker Solar Probe)

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Figure 61: First light data from EPI-Hi (the higher-energy Energetic Particle Instrument), part of the ISIS (Integrated Science Investigation of the Sun) suite aboard Parker Solar Probe (image credit: NASA/Princeton University/Parker Solar Probe)

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Figure 62: Data gathered during the FIELDS suite's boom deployment, measuring the magnetic field as the boom swung away from Parker Solar Probe. The early data is the magnetic field of the spacecraft itself, and the instruments measured a sharp drop in the magnetic field as the boom extended away from the spacecraft. Post-deployment, the instruments are measuring the magnetic field in the solar wind (image credit: NASA/UC Berkeley/Parker Solar Probe)

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Figure 63: Early data from Parker Solar Probe's FIELDS instrument suite (center and bottom) showing a radio burst from a solar flare, with data from NASA's Wind mission (top) for comparison (image credit: NASA/UC Berkeley/Parker Solar Probe/Wind)

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Figure 64: Early data from the Solar Probe Cup, part of the SWEAP (Solar Wind Electrons Alphas and Protons) instrument suite aboard Parker Solar Probe, showing a gust of solar wind (the red streak), image credit: NASA/University of Michigan/Parker Solar Probe

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Figure 65: First light data from the SPAN-A (Solar Probe Analyzer Ahead) instrument aboard Parker Solar Probe, which is part of the SWEAP (Solar Wind Electrons Alphas and Protons) instrument suite. This data shows measurements of solar wind ions (top) and solar wind electrons (bottom), image credit: NASA/University of Michigan/Parker Solar Probe

• August 17, 2018: Just two days after launch on Aug. 12, 2018, from Cape Canaveral Air Force Station in Florida, NASA’s Parker Solar Probe achieved several planned milestones toward full commissioning and operations, announced mission controllers at the JHU/APL (Johns Hopkins University/Applied Physics Laboratory) in Laurel, Maryland. 57)

- On Aug. 13, the high-gain antenna, which Parker Solar Probe uses to communicate high-rate science data to Earth, was released from locks which held it stable during launch. Controllers have also been monitoring the spacecraft as it autonomously uses its thrusters to remove (or “dump”) momentum, which is part of the flight operations of the spacecraft. Managing momentum helps the spacecraft remain in a stable and optimal flight profile.

- As of 12:00 p.m. EDT on 16 August, Parker Solar Probe was 2.9 million miles from Earth, traveling at 39,000 mph, and heading toward its first Venus flyby scheduled for Oct. 3, 2018, at 4:44 a.m. EDT. The spacecraft will use Venus to slightly slow itself and adjust its trajectory for an optimal path toward first perihelion of the Sun on 6 November 2018, at 03:27 UTC.

- “Parker Solar Probe is operating as designed, and we are progressing through our commissioning activities,” said project manager Andy Driesman of APL. “The team – which is monitoring the spacecraft 24 hours a day, seven days a week – is observing nominal data from the systems as we bring them on-line and prepare Parker Solar Probe for its upcoming initial Venus gravity assist.”

• August 12, 2018: During the first week of its journey, the spacecraft will deploy its high-gain antenna and magnetometer boom. It also will perform the first of a two-part deployment of its electric field antennas. Instrument testing will begin in early September and last approximately four weeks, after which Parker Solar Probe can begin science operations (Ref. 38).

- “Today’s launch was the culmination of six decades of scientific study and millions of hours of effort,” said project manager Andy Driesman, of the JHU/APL (Johns Hopkins University/Applied Physics Laboratory) in Laurel, Maryland. “Now, Parker Solar Probe is operating normally and on its way to begin a seven-year mission of extreme science.”

- Over the next two months, Parker Solar Probe will fly towards Venus, performing its first Venus gravity assist in early October – a maneuver a bit like a handbrake turn – that whips the spacecraft around the planet, using Venus’s gravity to trim the spacecraft’s orbit tighter around the Sun. This first flyby will place Parker Solar Probe in position in early November to fly as close as 15 million miles from the Sun – within the blazing solar atmosphere, known as the corona – closer than anything made by humanity has ever gone before.

- Throughout its seven-year mission, Parker Solar Probe will make six more Venus flybys and 24 total passes by the Sun, journeying steadily closer to the Sun until it makes its closest approach at 3.8 million miles. At this point, the probe will be moving at roughly 430,000 miles per hour, setting the record for the fastest-moving object made by humanity.

- Parker Solar Probe will set its sights on the corona to solve long-standing, foundational mysteries of our Sun. What is the secret of the scorching corona, which is more than 300 times hotter than the Sun’s surface, thousands of miles below? What drives the supersonic solar wind – the constant stream of solar material that blows through the entire solar system? And finally, what accelerates solar energetic particles, which can reach speeds up to more than half the speed of light as they rocket away from the Sun?

- Scientists have sought these answers for more than 60 years, but the investigation requires sending a probe right through the unrelenting heat of the corona. Today, this is finally possible with cutting-edge thermal engineering advances that can protect the mission on its daring journey.

- “Exploring the Sun’s corona with a spacecraft has been one of the hardest challenges for space exploration,” said Nicola Fox, project scientist at APL. “We’re finally going to be able to answer questions about the corona and solar wind raised by Gene Parker in 1958 – using a spacecraft that bears his name – and I can’t wait to find out what discoveries we make. The science will be remarkable.”