Minimize Parker Solar Probe imagery in the period 2019-18

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Parker Solar Probe imagery in the period 2019-18

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• 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. 1)

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Figure 1: 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. 2)

- 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 2: 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 3: 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 4: 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 5: 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 6: 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. 3)

- 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 7: 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 8: 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. 4)

- 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. 5)

- 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 9: 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. 6)

- 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 10: 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 11: 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 12: 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 13: 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. 7)

- 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 14: 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. 8)

- 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. 9)

Figure 15: 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 17 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 1: A long-held dream (Ref. 9)

• 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. 10)

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Figure 16: 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. 11)

- 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 17: 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. 12)

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Figure 18: 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 19: 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 20: 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 21: 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 22: 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 23: 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 24: 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. 13)

- 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.

- “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.”



1) Sarah Frazier, ”Revealing the Physics of the Sun with Parker Solar Probe: AGU 2019,” NASA, JHU/APL News, 11 December 2019, URL: http://parkersolarprobe.jhuapl.edu
/News-Center/Show-Article.php?articleID=134

2) Sarah Frazier, Rob Garner, ”NASA's Parker Solar Probe Sheds New Light on the Sun,” NASA, 4 December 2019, URL: https://www.nasa.gov/feature/
goddard/2019/nasas-parker-solar-probe-sheds-new-light-on-the-sun

3) Sarah Frazier, ”One Year, 2 Trips Around Sun for NASA's Parker Solar Probe,” NASA, 12 August 2019, URL: https://www.nasa.gov/feature/goddard/2019/one-year-2-trips-around-sun-for-nasas-parker-solar-probe

4) Sarah Frazier, ”Parker Solar Probe Completes Second Close Approach to the Sun,” NASA, 5 April 2019, URL: https://blogs.nasa.gov/parkersolarprobe/2019/
04/05/parker-solar-probe-completes-second-close-approach-to-the-sun/

5) Geoff Brown, ”All Systems Go As Parker Solar Probe Begins Second Sun Orbit,” JHU/APL, 22 January 2019, URL: http://parkersolarprobe.jhuapl.edu/News-Center/Show-Article.php?articleID=116

6) Sarah Frazier, ”Preparing for Discovery With NASA's Parker Solar Probe,” NASA, 12 December 2018, URL: https://www.nasa.gov/feature/goddard/2018/preparing-for-discovery-with-nasas-parker-solar-probe

7) Sarah Frazier, Geoff Brown, ”Parker Solar Probe Reports First Telemetry, Acquisition of Science Data Since Perihelion,” NASA, 20 November 2018, URL: https://blogs.nasa.gov/parkersolarprobe/2018
/11/20/parker-solar-probe-reports-first-telemetry-acquisition-of-science-data-since-perihelion/

8) Sarah Frazier, ”Parker Solar Probe Breaks Record, Becomes Closest Spacecraft to Sun,” NASA, 29 October 2018, URL: https://www.nasa.gov/feature/
parker-solar-probe-breaks-record-becomes-closest-spacecraft-to-sun

9) Sarah Frazier, Rob Garner, ”Parker Solar Probe Changed the Game Before it Even Launched,” NASA, 4 October 2018, URL: https://www.nasa.gov/feature/goddard/2018
/parker-solar-probe-changed-the-game-before-it-even-launched

10) Sarah Frazier, ”Parker Solar Probe Looks Back at Home,” NASA, 24 October 2018, URL: https://www.nasa.gov/feature/goddard/2018/parker-solar-probe-looks-back-at-home

11) Sarah Frazier,”Parker Solar Probe Changed the Game Before it Even Launched,” NASA, 4 October 2018, URL: https://www.nasa.gov/feature/goddard/2018
/parker-solar-probe-changed-the-game-before-it-even-launched

12) Sarah Frazier, Joy Ng. Aaron E. Lepsch, ”Parker Solar Probe First Light Data,” NASA/GSFC, 19 September 2018, URL: https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=13072&button=recent

13) ”Parker Solar Probe Marks First Mission Milestones on Voyage to Sun,”JHU/APL, 17 August 2018, URL: http://parkersolarprobe.jhuapl.edu/News-Center/Show-Article.php?articleID=95


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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