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Solar Orbiter Mission

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Solar Orbiter is a satellite mission of ESA (in the footsteps of Helios, Ulysses, SOHO and the Cluster missions) to explore the inner regions of the sun and the heliosphere from a near-sun orbit. Solar Orbiter is part of the ESA's Science Program Cosmic Vision 2015-2025. The Solar Orbiter project was initially selected by ESA's Science Program Committee in Oct. 2000 and re-confirmed as part of the ESA program in 2003. The Solar Orbiter mission of ESA and the SPP (Solar Probe Plus) mission of NASA (launch scheduled for 2018) are part of the common GHO (Great Heliophysics Observatory) program.

In 2011, the Solar Orbiter mission has undergone extensive study over a period of more than 10 years, both internally in ESA and in industry. This has resulted in a mature, detailed design that satisfies the requirements placed on the mission by the science objectives and addresses the key risk areas. - ESA's Science Program Committee selected the Solar Orbiter mission for implementation on October 4, 2011 with a launch scheduled for 2017. 1) 2) 3) 4)

ESA-NASA collaboration: NASA and ESA have a mutual interest in exploring the near-Sun environment to improve the understanding of how the Sun determines the environment of the inner solar system and, more broadly, generates the heliosphere itself, and how fundamental plasma physical processes operate near the Sun. A NASA-ESA MOU (Memorandum of Understanding) for a Solar Orbiter mission cooperation was signed in March 2012. 5)

For Solar Orbiter, also referred to as SolO in the literature, ESA is providing the spacecraft bus, integration of the instruments onto the bus, mission operations, and overall science operations. NASA is providing an EELV (Evolved Expendable Launch Vehicle) that will place the Solar Orbiter spacecraft into an inner heliospheric orbit with perihelia ranging from 0.28 to 0.38 AU and aphelia from 0.73 to 0.92 AU. The SolO nominal science mission will begin with a series of perihelion passes where the spacecraft is nearly co-rotating with the Sun. It will then use multiple Venus gravity assist maneuvers to move its orbital inclination to progressively higher helio latitudes, reaching 25° by the end of the nominal prime mission phase and around 34° by the end of the extended mission.

The overall objective is to provide close-up views of the sun's high latitude regions - to study fundamental physical processes common to solar, astrophysical and laboratory plasmas. The Solar Orbiter will, through a novel orbital design and its state-of-the-art instruments, provide exactly the observations required. 6) 7) 8) 9) 10) 11) 12) 13) 14)

• During the nominal operational lifetime, the Solar Orbiter operational orbit shall have the following parameters:

- Minimum perihelion radius larger than 0.28 AU to maximize the reuse of BepiColombo technology

- Perihelion radius within 0.30 AU in order to guarantee multiple observations close to the Sun

- Inclination with respect to solar equator increasing to a minimum of 25º (with a goal of 35º in the extended operational phase).

• At minimum perihelion passage, the spacecraft shall maintain a relative angular motion with respect to the solar surface such that individual solar surface features can be tracked for periods approaching one solar rotation.

• The Solar Orbiter system lifetime shall be compatible with a launch delay of 19 months (launch window locked to the next Venus gravitational assist opportunity).

Scientific requirements: The overarching objective of the Solar Orbiter mission is to address the central question of heliophysics: How does the Sun create and control the heliosphere? Achieving this objective is the next critical step in an overall strategy to address one of the fundamental questions in the Cosmic Vision theme: How does the Solar System work? To this end, the Solar Orbiter will use a carefully selected combination of in-situ and remote-sensing instrumentation, a unique orbit and mission design, and a well-planned observational strategy to explore systematically the region where the solar wind is born and heliospheric structures are formed.

The broad question that defines the overarching objective of the Solar Orbiter mission is broken down into four interrelated scientific questions:

1) How and where do the solar wind plasma and magnetic field originate in the corona?

2) How do transients drive heliospheric variability?

3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?

4) How does the solar dynamo work and drive connections between the Sun and heliosphere?

Common to all of these questions is the requirement that Solar Orbiter make in-situ measurements of the solar wind plasma, fields, waves, and energetic particles close enough to the Sun that they are still relatively pristine and have not had their properties modified by dynamical evolution during their propagation. The Solar Orbiter must also relate these in-situ measurements back to their source regions and structures on the Sun through simultaneous, high-resolution imaging and spectroscopic observations both in and out of the ecliptic plane.

Basic mission requirements of Solar Orbiter:

- Total cruise phase duration < 3 years (goal) with valuable science during the cruise phase

- Orbital period in 3:2 resonance with Venus

- At least one orbit with perihelion radius < 0.25 AU and > 0.20 AU (science phase)

- Inclination with respect to solar equator increasing to a minimum of 30º

- During the extended operational lifetime, the Solar Orbiter operational orbit shall reach an inclination with respect to solar equator not lower than 35º (goal)

- Support a payload of 180 kg and 180 W (including 20% maturity margins) with a data rate of 100 kbit/s

- Provide onboard mass memory and communications with a single ESA deep-space ground station (New Norcia, Western Australia) in support of the science observations

- Fail-safe onboard autonomous operations during the perihelion passages (15 days without ground contact, in extremely harsh thermal environment).

The mission includes a nominal mission phase and a potential extended mission phase (corresponding to 6 solar orbits). The spacecraft consumables and radiation sensitive units shall be sized to meet the duration with extended phase: 9.5 years.

The Solar Orbiter mission has its origins in a proposal called “Messenger” that was submitted by Richter et al. in 1982 in response to an ESA call for mission ideas. At the meeting “Crossroads for European Solar and Heliospheric Physics” held on Tenerife in March 1998, the heliophysics community recommended to “Launch an ESA Solar Orbiter as ESA’s [next flexible] mission, with possible international participation, [for launch] around 2007.” The kick-off meeting for a pre-assessment study of the “ESA Solar Orbiter” concept was held at ESTEC on 25 March 1999.

Solar Orbiter was subsequently proposed in 2000 by E. Marsch et al., and was selected by the SPC (Science Program Committee) in October 2000 as a Flexi-mission for launch after BepiColombo (in the 2008-2013 timeframe). A number of internal and industrial studies were then carried out, including parallel system-level Assessment Studies performed in industry between April and December 2004. At its 107th meeting in June 2004, the SPC confirmed the place of Solar Orbiter in the Cosmic Vision program, with the goal of a launch in October 2013 and no later than May 2015.

Work continued on the mission and payload definition throughout 2005 and 2006, and at its meeting in February 2007, SPC asked the Executive to find ways to implement Solar Orbiter within a financial envelope of 300 M€ (at 2006 EC), while keeping a realistic contingency margin. In response to this request, a Joint Science and Technology Definition Team (JSTDT) comprising scientists and engineers appointed by ESA and NASA, studied the benefits to be gained by combining ESA’s Solar Orbiter mission and NASA's Solar Sentinels into a joint program. This led to the release of an ESA Announcement of Opportunity for the Solar Orbiter Payload on on Sept. 18 2007 and a NASA SMEX/FOSO (Small Explorer/Focused Opportunity for Solar Orbiter) AO that was issued on 22 October 2007.

A major change in the progress of Solar Orbiter occurred in November 2008, when the SPC decided to integrate the Solar Orbiter into the first planning cycle of Cosmic Vision 2015-25 as an M-mission candidate for the first launch opportunity in 2017. In addition, in view of NASA’s high prioritization of SPP (Solar Probe Plus) in its Living With a Star program (compared with Sentinels), and the strong science synergies between Solar Orbiter and SPP, ESA called for an independent review of the PRC’s (Payload Review Committee) recommended payload, now in the context of a joint Solar Orbiter-SPP scientific program. The joint ESA-NASA review panel confirmed the validity of the recommended payload in its report of March 2009. As a result, the instrument selections recommended by the PRC in 2008 were formally announced on 20 March 2009. In parallel, NASA announced the results of the FOSO selection, and selected 2 instruments and portions of 2 instruments to be included in the Solar Orbiter payload.

At its 128th meeting on Feb. 17-18, 2010, ESA’s Science Program Committee decided that the Solar Orbiter be one of the three M-class candidates to proceed into definition phase and made a further programmatic change by endorsing a “fast track” approach outlined in ESA/SPC(2010)3, rev. 1. This approach was based on the scientific acceptability of increasing the minimum perihelion to ~0.28 AU and making maximum reuse of Bepi-Colombo technologies. Furthermore, it called for the start of the spacecraft implementation program (Phase B2) in early 2011, with a view, if selected, to mission approval and adoption by the SPC in October 2011, leading to a launch in 2017. In line with this approach, the industrial Phase B2 activities were kicked-off in February 2011.

In March 2011, NASA informed ESA that it had decided to reduce its contribution to the payload to 1 full instrument and 1 sensor. Specifically, the Spectral Imaging of the Coronal Environment and Suprathermal Ion Spectrograph investigations would not be funded. Given the scientific importance of these investigations, the SPC decided that, should Solar Orbiter be selected, the SPICE and SIS measurement capabilities should be recovered through the inclusion of European-led instruments in the payload, procured under ESA’s responsibility.

Solar Orbiter was ultimately selected and adopted as the first M-class mission of ESA's Cosmic Vision program by the SPC on October 4, 2011.

Table 1: Historical overview of the Solar Orbiter program (Ref. 12)


As with all spacecraft, mass and volume are at a premium due to launch vehicle constraints; however, the Solar Orbiter main spacecraft body is further constrained due to the fact that a sizable portion of the budget is taken up by the heat-shield, along with the fact that the spacecraft must be optimized to fit behind the heat-shield with sufficient margin to cover off-pointing cases, e.g. due to spacecraft anomalies. The Solar Orbiter spacecraft main body is approximately 2 m3 (with stowed appendages). With 33 instrument units to accommodate on-board, the allowable volume of each instrument unit must be tightly controlled.

The Solar Orbiter spacecraft configuration is dominated by the presence of the heat shield located at the top of the spacecraft in order to protect the spacecraft from the intense direct solar flux when approaching perihelion. The heat shield is over-sized to provide the required protection to the spacecraft box and externally-mounted units, in combination with the attitude-enforcement function of the FDIR (Failure Detection, Isolation and Recovery). The mechanical platform revolves around a robust, reliable and conventional concept with a central cylinder, four shear walls and six external panels. This concept is inspired by Astrium’s Eurostar 3000 spacecraft platform. The design meets the Solar Orbiter’s mission requirements according to a low-risk and low-cost philosophy.

In April 2012, ESA awarded a contract to build its next-generation Sun explorer to Airbus DS (former Astrium UK, Stevenage). Astrium UK will lead a team of European companies who will supply various parts of the spacecraft. 15) 16) 17) 18)


Figure 1: Artist's view of a baseline spacecraft in solar orbit (image credit: ESA)

The spacecraft and mission PDR (Preliminary Design Review) was completed on March 7, 2012.Following contract negotiations with the prime contractor, Phase-B2/C/D is proceeding with subsystem-level procurement and several lower-tier procurements.

The spacecraft is three-axis stabilized and always sun-pointed. Given the extreme thermal conditions at 45 solar radii (or 0.22 AU), equivalent to about 20 solar constants or approximately 28 kW/m2, a phenomenal amount of power from which the majority of the spacecraft must be protected. The thermal design of the spacecraft has been considered in detail. Accordingly, the bulk of the spacecraft is protected from the sun by a local heatshield (also referred to as sunshield) on the +X panel face of the spacecraft, combined with a stringent maintenance of a sun-pointing attitude for the spacecraft at all times during periods close to the sun (below ~0.7 AU). 19) 20) 21)

The spacecraft configuration is based on a square structure housing a simple mono-propellant propulsion system with no main engine . Due to the stringent environment encountered on the heliocentric orbits, the spacecraft is always sun-pointed and protected from solar irradiation by a heatshield. This heatshield covers the spacecraft bus and some of the external components such as in-situ instruments. It contains aperture openings providing the required field of view (FOV) for the remote sensing instruments. 22)

The avionics architecture is based on segregated processing functions of the platform and the payload data. The OBMU (On-Board Management Unit) is in charge of the spacecraft command / control, running the DHS (Data Handling Subsystem), AOCS and mission software component, and housing the interfaces with the platform equipments and the payload support unit. The PDPU (Payload Data Processing Unit) supports all functions of the sensor complement. Onboard communications is based on one MIL-STD-1553B bus for the data communications between the OBMU and the platform units, and on a SpaceWire network. 23)

SpaceWire has been selected as the sole communication interface between each of the instruments and the spacecraft DHS (Data Handling Subsystem). It also provides a key interface within the DHS itself, between are the OBC (On-Board Computer ) and the SSMM (Solid State Mass Memory).


Figure 2: SpaceWire network architecture (image credit: Astrium, ESA)

AOCS (Attitude and Orbit Control Subsystem): The AOCS employs an autonomous star tracker, gyros, and sun sensors for attitude acquisition and safe mode sensing; actuation is provided by reaction wheels and thrusters. The AOCS baseline architecture also includes a hard-wired safe mode using a sun sensor and a coarse gyro aimed at recovering as fast as possible the sun-pointed attitude in case of contingency, which is essential for the spacecraft thermal safety.

The AOCS constitutes a suite of components that in close interaction with the rest of the spacecraft controls the orientation and stability of the spacecraft, and executes the ground requested velocity changes for adjustment of the otherwise ballistic trajectory. This function includes the monitoring of its own health, as well as the provision of a reference on selected data related to trajectory and orientation, in order to support control of mechanisms.

A set of primary requirements to the AOCS are:

- Maximum 6.5º off-pointing from the Sun, with maximum 50s off-pointing over 2.3º

- Capacity of fine pointing without star tracker measurements for at least 24 hours

- A fine pointing Absolute Pointing Error of 42 arcsec, with an Attitude Knowledge Error of 25 arcsec. The Pointing Drift Error is specified at 13 arcsec over 24 hours, using 10s integration windows. All figures are applicable to Line of Sight to the Sun, 95% confidence.

The AOCS consists of most of the classical elements found on interplanetary missions, but with the special feature that the onboard computer handles all tasks, such as data handling, thermal control, AOCS and FDIR (Failure Detection, Isolation and Recovery), on a single processing module. The equipment used are two pairs of Fine Sun Sensors, two Inertial Measurement Units, two Star Trackers, four Reaction Wheels, and a redundant bi-propellant propulsion system consisting of 9 thrusters per branch. The Inertial Measurement Units consist of one nominal branch featuring high performance rate measurements from four tetrahedron oriented gyroscopes and a contingency branch providing reduced rate measurement performance. The nominal branch also includes four tetrahedron oriented accelerometer channels. All units are communicated with via two MIL-1553B redundant busses. The units are synchronized to the onboard time reference at a minimum of 8Hz data acquisition, corresponding to the attitude control frequency. 24)

EPS (Electric Power Subsystem): The solar panel design relies on a carbon/carbon substrate with triple junction GaAs cells. The operational temperature of this new solar array technology is expected to be 230ºC. On SolO this can be achieved by implementing a large enough OSR (Optical Surface Reflector) ratio and solar array tilt angle such that the sun incidence angle is high enough to limit the incident solar flux. The EPS architecture employs a regulated power bus. One Li-ion battery is foreseen to cover mission needs during LEOP and Venus gravity assists. - The solar arrays can be rotated about their longitudinal axis to avoid overheating when close to the Sun.

The S/C dimensions are: 2.5 m x 3.0 m x 2.5 m. The pointing stability is better than 3 arcsec/15 min. The total spacecraft wet mass is about 1800 kg, the maximum power demand is ~ 1100 W. The payload suite mass budget is ~190 kg with a payload power consumption of 180 -250 W (depending on the mission phase).


3-axis stabilized platform, heat shield, two adjustable solar arrays, dimensions: 2.5 m x 3.0 m x 2.5 m(launch configuration)

Spacecraft orientation

Sun pointing

Telemetry band

Dual X-band

Data downlink

150 kbit/s (at 1 AU spacecraft-Earth distance)

Closest perihelion

0.28 AU (Astronomical Unit), corresponding to a Sun-spacecraft distance of ~ 42 million km.

Max. heliographic latitude

25º (nominal mission) / 34–36º (extended mission)

Launch date

January 2017 (March 2017 and September 2018 back-ups)

Nominal mission duration

7 years (including cruise phase)

Extended mission duration

3 years

Post-operations & archiving

2 years

Ground station

Malargüe (Argentina), 35 m antenna, 4 to 8 hours/day (effective)

Table 2: Overview of Solar Orbiter mission parameters 25)


Figure 3: Front view of the Solar Orbiter spacecraft configuration with the three RPW antennas, high-gain antenna, instrument boom and solar arrays deployed (image credit: ESA) 26)

Heatshield design:

The Solar Orbiter thermal control is based on using a sun pointed, flat heat shield to limit the sun flux on the spacecraft structure. By using this approach the elements behind the heat shield will be in a more benign thermal environment. 27)

All external components are shielded from direct solar illumination by the heat shield except for the instruments requiring direct view of the sun and the spacecraft appendages, i.e. the solar arrays, the RPW (Radio & Plasma Wave Analyzer) antennas and the HGA (High Gain Antenna). The heat shield is sized to prevent direct solar illumination on any of the shaded components during nominal pointing and for safe mode events of spacecraft off-pointing up to 6.5º from sun-center. However, the spacecraft must also withstand reflected solar flux and high IR flux from appendages outside of the heat shield shadow cone. In addition, the remote sensing instruments will all receive additional IR flux from the feedthroughs which allow them to view through the heat shield.

The design allocates the heatshield at the top of the spacecraft to free all four lateral walls for high efficiency radiators with good viewing factors towards cold space. A key strategy in the restriction of the Solar Orbiter mission cost is to reuse technology from other programs, primarily of course the BepiColombo program given the environmental similarities. The heatshield requirements call for:

• The heatshield must protect the majority of the spacecraft, including the payload, from the punishing incident solar flux (28 kW/m2 at perihelion)

• At the same time the heatshield must incorporate cut-outs to allow the RS (Remote Sensing)-instrumentation, and the sun sensors, access to the sun.

The definition of ‘protection’ is that the heatshield will:

• Limit the overall radiative heat flux to the spacecraft to no more than 30 W in total

• Limit the overall conductive heat flux at all attachment points to the spacecraft to no more than 15 W in total.

The technological challenges of the heatshield were addressed through parallel contracts awarded to TAS-I and Airbus DS (former EADS Astrium) with the goal of design and production of thermal breadboards to demonstrate the concepts.

The essential function of the heatshield was identical in both cases. Each heatshield presents a planar surface to the sun, and relies on using multiple layers with large gaps in-between to facilitate lateral heat rejection to cold space. However the two resulting breadboard concepts were different in a number of key aspects:

- Materials: The choice of material for the outer layer (sunshade) is obviously critical as this effectively sets the temperature of the outer layer and the subsequent performance of the entire heatshield. The TAS-I design employed Carbon-Carbon fabric with an additional Nickel light blocking layer; the Astrium design used Keplacoat© on a Titanium foil.

In the meantime, the initial choice – carbon-fiber fabric – was ruled out. Instead the sunshade team began looking for the answer outside the space business. They found it in the shape of Irish company Enbio and its CoBlast technique, originally developed to coat titanium medical implants. The process works for reactive metals like titanium, aluminum and stainless steel, which possess a surface oxide layer. The team sprays the metal surface with abrasive material to grit-blast this layer off; also included is a second ‘dopant’ material possessing whatever characteristics are needed. This simultaneously takes the place of the oxide layer being stripped out. The big advantage is that the new layer ends up bonded, rather than only painted or stuck on. It effectively becomes part of the metal.
The material Enbio will apply to the outermost titanium sheet of Solar Orbiter’s multi-layered heatshield is called ‘Solar Black’ – a type of black calcium phosphate processed from burnt bone charcoal. 28)

- Support Panel: The Astrium concept used a separate Aluminum support panel for the heatshield in addition to the +X spacecraft panel upon which it is mounted, which allows the heatshield to be treated as a separate item to the spacecraft (highly desirable for programmatic reasons); this is in contrast to the TAS-I design in which the support panel of the heatshield is part of the spacecraft primary structure.

- Number of gaps: The Astrium design utilized a single gap between the sunshade and the support panel, with an additional gap between the support panel and the +X panel of the spacecraft. In contrast the TAS-I design employed 3 equidistant space gaps between 4 layers.

- Layer support: The Astrium design relied on pretensioned lashes in order to provide stiffness to the sunshade layer and a high degree of planarity (this improves thermal performance). In contrast the TAS-I design favored loose support of the layers by rigid Star Brackets – although the planarity of the layers is reduced, the mechanical performance of this approach is considerably better.


Figure 4: Astrium heatshield design, incorporating 2 lateral layers separated by tensioned Titanium lashes to provide rigidity and a high degree of planarity (image credit: Airbus DS)


Figure 5: TAS-I heatshield design incorporating multiple lateral layers separated by Star Brackets which loosely hold the layers (image credit: TAS-I)

Feedthrough doors and mechanisms: A critical component of the overall heatshield design is the feedthrough and door arrangement that allows the RS-instruments to see through the heatshield. The generic design is applicable for all the RS-instrument feedthroughs: a cylindrical feedthrough with internal vanes to specify the FOV of the instrument. Each feedthrough is mechanically supported by an interface to the support panel of the heatshield, and in turn the feedthroughs provide local support to the sunshade (uppermost) layer of the heatshield through a second interface.

The doors are made of Titanium, with a ‘duck-foot’ design incorporating radial spars. The door does not provide any contamination control, it has only a light-blocking function, and consequently does not touch the structure underneath. Instead it is displaced above the feedthrough by ~1 mm, a sizing which ensures non-interaction of the door and feedthrough during launch. The accuracy of the door operation is not critical, as long as the door completely covers the aperture when it is required to do so. A launch lock is present at the door to constrain rotation during launch.

RF communications: The subsystem consists of a redundant set of transponders using X-band for the uplink, and X-band and Ka-band for the downlink. Depending on the mission phases, the transponders can be routed via RF switches to different antennas. The telecommunication subsystem provides hot redundancy for the receiving function and cold redundancy for the transmitting function. One steerable HGA (High Gain Antenna) is being used to support the X-band services for engineering data, and the Ka-band for the science data transmissions.

The X-DST (X-band Deep Space Transponder) is designed and developed by TAS-I (Thales Alenia Space, Italy). The digital platform (whosedesign is inspired by the software-defined radio concept) features a system-on-chip based DSP core, implementing on the same chip all the X-DST signal processing algorithms. 29)


Figure 6: Block diagram of the RF communications system (image credit: TAS-I)

The operations concept is such that the instrument data will be stored in a SSMM (Solid State Mass Memory), for later downlink during daily ground station passes of 8 hours. The science data is downlinked in X-band via the high gain antenna. During the 10 day science windows, the allocation for the nominal average data generation rate of the full payload is 120 kbit/s. This is also controlled via an allocation of the average per instrument. For the remote sensing instruments in particular, their allocation is insufficient to downlink the full raw data and therefore their designs are such as to allow pre-processing, data reduction, selection and associated internal data storage in order to ensure that optimum use is made of the TM bandwidth to downlink the best data. This is not only important for each instrument individually, but for the mission as a whole, as the overriding science objectives rely on combining observations of the same phenomenon from different instruments.


Figure 7: Photo of the EM (Engineering Model) deep space transponder (image credit: ESA, TAS) 30)

Thermal architecture of spacecraft:

The TCS (Thermal Control Subsystem) of the spacecraft represents the main design challenge, a critical element for spacecraft integrity and performance for a large proportion of the mission duration. The fundamental Solar Orbiter thermal requirement stipulates that the TCS will support payload and spacecraft subsystems such that it is designed to withstand all thermal environments encountered during the entire life of the mission. The selected approach is to rely on a sun-pointed spacecraft with the spacecraft protected from solar flux by the heatshield, and on specific technologies for the remaining exposed parts, such as the solar panels, communication antennas, and the heatshield. 31)

The heat rejection efficiency of the heatshield permits a quasi-decoupling of the spacecraft body from the direct sun irradiation (flux density of up to 28 kW/m2 at 0.2 AU).The heatshield is made with a highly reflecting/emissive external layer to dissipate the incident flux as much as possible radiatively.
Note: A number of technology developments required by the Solar Orbiter are common to BepiColombo, an ESA mission to the hot planet Mercury with a planned launch in 2013.


Figure 8: Schematic of thermal architecture (image credit: EADS Astrium)

Several payload instrument apertures are implemented through the heatshield to let the remote sensing instruments observe the sun through baffles, and acquire the incident rays on their sensitive detectors. The instruments are either mounted directly on spacecraft lateral walls (in-situ instruments), and use dissipation transferred from the base plate of the unit to the external radiator, or mounted on the spacecraft shear walls (remote-sensing instruments) and use a conductive link from the instruments to the radiators viewing cold space, mounted on external walls, or use dedicated fluid loop pipes. Other radiators accommodated on the lateral walls of the spacecraft are used to cool down internal equipment that dissipate heat or receive solar flux (Figure 9).

The heatshield itself is an innovative and the most sophisticated piece of hardware on SolO. A flat heatshield design is selected and accommodated on top of the spacecraft whose side is always facing to the sun. The heatshield is supported by a structure decoupled from the spacecraft. This structure carries the remote sensing instrument baffles. The baffles cannot be supported by the spacecraft wall since they contain high temperature points or regions. The load-carrying structure is thermally decoupled from the spacecraft wall to minimize conduction loads. The mechanically autonomous heatshield design with respect to the spacecraft is very user-friendly to all AIV (Assembly, Integration and Verification) activities.

The preliminary design of the heatshield outside reflecting layer consists of a white ceramics coating on a titanium (Ti) plate, with an α/ε (absorption/emission) ratio as low as 0.4 - 0.6 at EOL. A multi-layer concept made of polished Ti foils and VDA/VDA (Vapor Deposited Aluminum) kapton foils is proposed for the next layer of insulation to efficiently dissipate the heat and maintain the spacecraft wall at room temperature.

The different layers are held through regularly spaced Ti stand-offs made with limited conductivity towards an Al honeycomb structure to which they are attached. This plate acts as the support structure of the heatshield and is mounted onto the spacecraft wall through a few stand-offs, with a classical MLI (kapton + Dacron) in-between for insulation.


Figure 9: Conceptual design of the heatshield structure (image credit: EADS Astrium)

The interfaces between the remote sensing instruments and the heatshield mainly comprise the baffles and instrument shutters aimed at protecting them from contamination and solar flux when they are not operated. The baseline concept is to thermally decouple the baffle from the instrument by attaching it to the heatshield support structure, and to dissipate their heat by conductive coupling through a radiator installed at the edge of the heatshield. Baffles of optical instruments are assumed to be in SiC, while baffles of particle detection instruments (SWA) could be in the same material as the heatshield first layer in order to lower their temperature.


Figure 10: Overview of the TCS (image credit: Astrium Ltd., ESA)


Figure 11: Illustration of the deployed Solar Orbiter spacecraft (image credit: Airbus DS, ESA)

Launch: ESA's Solar Orbiter spacecraft was launched on 10 February 2020 (04:03 GMT) by a NASA-provided Atlas-V 411 vehicle of ULA, designated AV-87, from KSC (Kennedy Space Center) SLC-41 (Space Launch Complex), Cape Canaveral, FL, USA. 32) 33) 34)

- Solar Orbiter, an ESA-led mission with strong NASA participation, will provide the first views of the Sun's uncharted polar regions from high-latitudes, giving unprecedented insight into how our parent star works. This important mission will also investigate the Sun-Earth connection, helping us to better understand and predict periods of stormy space weather.

- Signals from the spacecraft were received at New Norcia ground station in Australia at 06:00 CET (Central European Time), following separation from the launcher upper stage in low Earth orbit.

Figure 12: Replay of the launch of the Solar Orbiter spacecraft (video credit: ESA)


Figure 13: The ESA Solar Orbiter team, along with ESA Director of Science Günther Hasinger (eighth from the right), at the launch pad at NASA's Kennedy Space Center in Florida, US, on Sunday 9 February 2020. In the background, the Atlas V 411 rocket that would lift the spacecraft into space several hours later(photo credit: ESA, P. Olivier)

Orbit: The Solar Orbiter will use Venus gravity assists to obtain the high inclinations reaching 35º with respect to the sun's equator (inclined ecliptic orbit) at the end of the cruise phase mission (the cruise phase will last about 3.4 years).

Using SEPM (Solar Electric Propulsion Module) in conjunction with multiple planetary swing-by maneuvers, it will take the Solar Orbiter only two years to reach a perihelion of 45 solar radii with an orbital period of 149 days. Within the nominal 5 year mission phase, the Solar Orbiter will perform several swing-by maneuvers at Venus, in order to increase the inclination of the orbital plane to 30º with respect to the solar equator. During an extended mission phase of about two years, the inclination will be further increased to 38º.

Operating orbit:

- Elliptical orbit around the Sun with a perihelion as low as 0.28 AU and with increasing inclination up to more than 30º with respect to the solar equator.

- Aphelion between 0.8 AU and 0.9 AU

- Co-rotation pass: duration 10 days, with a maximum drift of 50º

- Period about 150 days

- Inclination evolving from 0º-30º (with respect to solar equator), 34º in the extended mission.

During the initial cruise phase, which lasts until November 2021, Solar Orbiter will perform two gravity-assist maneuvers around Venus and one around Earth to alter the spacecraft’s trajectory, guiding it towards the innermost regions of the Solar System. At the same time, Solar Orbiter will acquire in situ data and characterize and calibrate its remote-sensing instruments. The first close solar pass will take place in 2022 at around a third of Earth’s distance from the Sun. 35)

Figure 14: Animation showing the trajectory of Solar Orbiter around the Sun, highlighting the gravity assist maneuvers that will enable the spacecraft to change inclination to observe the Sun from different perspectives (video credit: ESA/ATG medialab)

The spacecraft’s orbit has been chosen to be ‘in resonance’ with Venus, which means that it will return to the planet’s vicinity every few orbits and can again use the planet’s gravity to alter or tilt its orbit. Initially Solar Orbiter will be confined to the same plane as the planets, but each encounter of Venus will increase its orbital inclination. For example, after the 2025 Venus encounter it will make its first solar pass at 17º inclination, increasing to 33º during a proposed mission extension phase, bringing even more of the polar regions into direct view.


Figure 15: Solar Orbiter trajectory to orbit the Sun (image credit: ESA)


Figure 16: January 2017 launch: solar distance (image credit: ESA) 36)


Figure 17: January 2017 launch: solar latitude (image credit: ESA)

From its launch early in 2017, the Solar Orbiter will reach the nominal orbit around the Sun in 2020, operating in its near-Sun environment for at least 6 years, including the extended mission phase. During this period, the spacecraft will carry the science payload through 14 perihelion passages. At the same time, the heliocentric latitude will be gradually increased through repeated Venus gravity assist maneuvers, providing information about the behavior of the Sun at high latitudes.

Mission status

• September 30, 2020: ESA has released its first Solar Orbiter data to the scientific community and the wider public. The instruments contributing to this data release come from the suite of in-situ instruments that measure the conditions surrounding the spacecraft (see Figure 28). 37)

- As soon as a mission is in space, the teams work to commission the instruments and start collecting science data. As soon as the information starts to flow in, interest builds towards the first data release.

- In the case of many space missions, the first data release usually comes after six months or a year, to reward the teams that have built the instruments with an exclusive first look at the data. Long before launch, however, it was agreed that Solar Orbiter would be different.

- “We want Solar Orbiter to be one of the most open space missions. This means open to the whole world, not only to the teams who have built the instruments,” says Yannis Zouganelis, Solar Orbiter Deputy Project Scientist for ESA.

- Based on the successful approach taken by previous solar physics missions, it was decided that the time between the data being received on Earth and it being released to the world would be at most 90 days. During this period, the instrument teams calibrate the data taken by Solar Orbiter from its ever-changing distance to the Sun. That would be a fast turn-around at the best of times; with the unprecedented challenges of the pandemic this year, it is a double achievement to hit the deadline.

- “To do this in COVID-19 times was very challenging,” says Yannis, “But we are ready to deliver the data to the scientific community according to the plan, so that they can do science with it.”

Figure 18: Solar Orbiter facing the Sun. This visualization begins by showing small sliding doors in Solar Orbiter’s heatshield opening to allow the internally-mounted remote-sensing instruments to observe the Sun. Special windows block out heat to protect the instruments during operations. The doors are closed when the instruments are not observing. The animation continues with an artistic impression of Solar Orbiter facing the Sun. At its closest approach, Solar Orbiter will operate from within the orbit of Mercury (video credit: ESA/ATG medialab)

- The work begins long before launch, with the various instrument teams getting ready to receive and process their data. The teams themselves are composed of dozens of people, often in many different countries.

- Once the instruments are taking data, the mission enters a calibration phase in which a lot of work is put in by each instrument team to understand how their instrument is working in space, whether the data coming back is as expected, and which instrumental and spacecraft effects need to be corrected for. For example, instrument readouts depend on the temperature of the detector, but often the thermometers are by necessity located at a certain distance away. Thus, the data must be calibrated with the actual ‘in-orbit’ thermal behavior of the spacecraft.

- Once the working instrument is understood, the teams process the data and send them to ESA’s European Space Astronomy Centre (ESAC), near Madrid. There, the data is archived at the ESAC Science Data Centre and made accessible to the public.

- “It’s a coordinated effort involving dozens of people from many different teams, many different countries, and all parties have to work together, like an orchestra, to make sure that everything is ready at the right time,” says Yannis.

- This orchestra is directed by ESA’s Solar Orbiter archive scientist Pedro Osuna in concert with the ESAC Science Data Centre. The effort requires dedicated hands-on commitment from all instrument teams to transform the raw data into calibrated products for scientific analysis.

- “When the data is received on the ground it is raw data, just ones and zeros,” says Javier Rodríguez-Pacheco, University of Alcalá, and the Principal Investigator of EPD (Energetic Particles Detector). “This is sent to us and we transform it into physical units that can be used for scientific purposes.”

- For this first data release, Javier says that most of the data was cleaned and calibrated by hand but in future, once they fully understand the responses of the different EPD sensors, they are looking to create a data pipeline that will be able to largely automate the process. Although, someone will always supervise the process.

- The data being released from RPW (Radio and Plasma Waves) comes from readings that were taken after 15 June because the period before was the commissioning phase rather than the science phase. “In the commissioning period, we do all sorts of strange things with the instrument,” says Jan Soucek, Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague and lead co-investigator of RPW.

- In this mode, readings are collected in a number of different ways and this makes them poorly suited for science. “If you want to look at statistics, you need to make sure that you are measuring things in a consistent way, so if we play with the instrument all the time, it's not very consistent,” says Jan.

- But like the other instruments, once its behavior is known, the data can be processed relatively easily and quickly.

Figure 19: Solar Orbiter boom deployment and first magnetic field measurements (image credit: ESA; Spacecraft: ESA/ATG Medialab; Data: ESA/Solar Orbiter/MAG)

- In the case of MAG, the job was to learn about all the small magnetic fields that the spacecraft itself generates when its various circuits and equipment are switched on and off. Tim Horbury, Imperial College, and Principal Investigator of MAG, says that the fact the data is ready on time is testament to the hard work of the engineering team at Imperial College.

- “They have worked incredibly hard over the last few months. It’s been an immense amount of work,” he says. “There's a lot of it that we're releasing that nobody's really looked at in great detail yet. So I am sure there will also be a whole extra set of wonders – we just don't know what they are yet. There's an enormous amount for people to do, and I really hope that people will dive in.”

- The fourth in-situ instrument, SWA (Solar Wind Analyzer), is still working on its data processing and calibration. “We have had a number of teething challenges operating safely with the high-voltages that are an integral part of all three of our sensors,” says Christopher Owen, Mullard Space Science Laboratory, University College London, and SWA’s principal investigator. “As a consequence we have not been able to take as much data, or to spend as much time on understanding performance as we would have liked."

- However, Chris is optimistic. “The sensors themselves are fundamentally healthy, and we can see from the data we do have that they are capable of delivering great science and fulfilling the important roles they have in delivering the unique mission science goals,” he says.

- In the meantime, there is more than enough data from the other instruments for the scientific community to begin work with. In tandem with the data release, a special issue of the journal Astronomy and Astrophysics is being published that contains mission and instrument descriptions.

- “Now any scientist from any country can get the data and do science with it. In fact, there are already hundreds of scientists working together to make sense out of this unique data,” says Yannis.


Figure 20: ESA's Solar Orbiter mission will face the Sun from within the orbit of Mercury at its closest approach (image credit: ESA/ATG medialab)

• July 16, 2020: The first images from ESA’s Solar Orbiter are already exceeding expectations and revealing interesting new phenomena on the Sun. 38) 39)

Figure 21: This animation combines a series of views captured with several remote-sensing instruments on Solar Orbiter between 30 May and 21 June 2020, when the spacecraft was roughly halfway between the Earth and the Sun – closer to the Sun than any other solar telescope has ever been before. The red and yellow images were taken with the EUI (Extreme Ultraviolet Imager) in the extreme ultraviolet region of the electromagnetic spectrum, at wavelengths of 30 and 17 nm, respectively (video credit: Solar Orbiter/EUI Team; PHI Team; METIS Team; SoloHI Team /ESA & NASA)

- The close-up views by EUI show the upper atmosphere of the Sun, or corona, with a temperature of around 1 million degrees. With the power to see features in the solar corona of only 400 km across, these images reveal a multitude of small flaring loops, erupting bright spots and dark, moving fibrils. A ubiquitous feature of the solar surface, uncovered for the first time by these images, have been called ‘campfires’. They are omnipresent miniature eruptions that could be contributing to the high temperatures of the solar corona and the origin of the solar wind.

- The EUI images are followed by three views based on data from the PHI (Polarimetric and Helioseismic Imager) instrument. The blue and red view is a ‘tachogram’ of the Sun, showing the line of sight velocity of the Sun, with the blue side turning to us and the red side turning away. The following view is a magnetogram, or a map of magnetic propertied for the whole Sun, featuring a large magnetically active region in the lower right-hand quadrant of the Sun. The yellow-orange view is a visible light image and represents what we would see with the naked eye: there are no sunspots visible because the Sun is displaying only low levels of magnetic activity at the moment.

- On larger scales, the METIS (Multi-Element Telescope for Imaging and Spectroscopy) coronagraph blocks out the dazzling light from the solar surface, bringing the fainter corona into view. METIS observes the corona simultaneously in visible light (shown in green) and ultraviolet light (shown in red) for the first time with unprecedented temporal coverage and spatial resolution. These images reveal the two bright equatorial streamers and fainter polar regions that are characteristic of the solar corona during times of minimal magnetic activity.

- On even grander scales, the Heliospheric Imager (SoloHI) telescope takes images of the solar wind – the stream of charged particles constantly released by the Sun into outer space – by capturing the light scattered by electrons in the wind. The first-light image from SoloHI is shown at the end, as a mosaic of four separate images from the instrument’s four separate detectors. In this view, the Sun is located to the right of the frame, and its light is blocked by a series of baffles; the last baffle is in the field of view on the right-hand side and is illuminated by reflections from the solar array. The partial ellipse visible on the right is the zodiacal light, created by sunlight reflecting off the dust particles that are orbiting the Sun. The signal from the solar wind outflow is faint compared to the much brighter zodiacal light signal, but the SoloHI team has developed techniques to reveal it. Planet Mercury is also visible as a small bright dot near the lower edge of the upper left tile.

- Solar Orbiter is a space mission of international collaboration between ESA and NASA.

• July 16, 2020: The first images from ESA’s Solar Orbiter, captured around the spacecraft’s first close pass of the Sun, some 77 million kilometers from its surface, are already exceeding expectations revealing interesting new phenomena on our parent star. 40)

Figure 22: This animation shows a series of close-up views captured by the Extreme Ultraviolet Imager (EUI) at wavelengths of 17 nm, showing the upper atmosphere of the Sun, or corona, with a temperature of around 1 million degrees. These images reveal a multitude of small flaring loops, erupting bright spots and dark, moving fibrils. A ubiquitous feature of the solar surface, uncovered for the first time by these images, have been called ‘campfires’. They are omnipresent miniature eruptions that could be contributing to the high temperatures of the solar corona and the origin of the solar wind (video credit: Solar Orbiter/EUI Team (ESA & NASA); CSL, IAS, MPS, PMOD/WRC, ROB, UCL/MSSL)

- Captured on 30 May 2020, when Solar Orbiter was roughly halfway between the Earth and the Sun, these are the closest views of the Sun ever taken, allowing EUI to see features in the solar corona of only 400 km across. As the mission continues, Solar Orbiter will go closer to the Sun and this will increase the instrument’s resolving power by a factor of two at closest approach.

- The color on this image has been artificially added because the original wavelength detected by the instrument is invisible to the human eye.

Figure 23: Solar Orbiter first images revealed (video credit: ESA)

• June 29, 2020: ESA’s Solar Orbiter has successfully completed four months of painstaking technical verification, known as commissioning. Despite the challenges imposed by the COVID-19 pandemic, the spacecraft is now ready to begin performing science as it continues its cruise towards the Sun. 41)

- When Solar Orbiter blasted into space on an Atlas V rocket from Cape Canaveral, Florida, on 10 February 2020, the teams behind the €1.5 billion mission did not anticipate that within weeks, the spread of COVID-19 would evict them from their high-tech control rooms, making the challenging process of commissioning the spacecraft’s instruments even harder.

- In normal circumstances, many of the project’s scientists and engineers would have gathered at the European Space Operations Center (ESOC) in Darmstadt, Germany. Together, they would have worked in close cooperation with the spacecraft operators, to bring the spacecraft and its instruments to life.

- This happened more or less as usual during the most challenging early weeks of Solar Orbiter’s in-orbit existence, but when the instrument teams were invited to ESOC in March, the situation in Europe was rapidly changing.

- Each of the ten instrument teams needed many representatives on site. Two or three from each team were allowed in a dedicated Solar Orbiter control room. “The other representatives worked from a dedicated support area,” says Sylvain Lodiot, ESA’s Solar Orbiter Spacecraft Operations Manager. It was not unusual to have 15 or more people in the main control room working too. But within a week, it became clear that European countries were heading into lockdown and so the external teams were asked to return home. The situation became even more serious when several workers at ESOC tested positive for the virus, and the site effectively closed.

The COVID lockdown

- “We had to protect the people,” says Sylvain, whose last task before going home was to switch off all the instruments on Solar Orbiter. “It felt horrible because I didn’t know when those instruments were coming back online,” he says.

- One of the instrument teams most affected was the Solar Wind Analyzer (SWA) team. The solar wind, which is constantly released from the Sun, is composed of a mixture of electrically charged particles called ions, and electrons. The SWA instrument comprises three different sensors to measure the fluxes and composition of these various particle populations. Each sensor operates as a kind of ‘electrical periscope’ that uses high voltages, up to 30 kilovolts in one case, to divert the solar wind particles into the detector.

- To operate those high voltages safely, the team had planned not to turn on the instrument until at least a month after launch. This was intended so no traces of Earth’s atmosphere would remain within the SWA sensors. If there were, these high voltages could cause arcing and damage the sensors.

- The switch on process for each of the SWA detectors is long because each high-voltage subsystem must be powered up in steps of just 20 or 50 volts at a time. After each increase, the instrument is checked to make sure nothing untoward has happened.

- When SWA’s principal investigator Christopher Owen, of the Mullard Space Science Laboratory, University College London (MSSL/UCL), had left Germany, he and his team had begun to make plans to commission the instrument from their lab in the UK. But then the UK lockdown was announced, meaning a move to working from the home office for almost everybody.

- “When I left the lab, I grabbed a couple of laptops and four screens, and brought them home. I then evicted my two-year-old from his nursery and set everything up in there,” says Christopher. And from this temporary control center, once ESOC had returned to work, he worked remotely with the rest of the SWA team and the skeleton staff in Darmstadt to get the instrument commissioned.

- “We had serious doubts about whether we could work like this,” says Sylvain about the process in general, “but we adapted and in the end, it worked very well because the team all knew each other.”


Figure 24: Christopher Owen, of the University College London's Mullard Space Science Laboratory (UCL/MSSL), the principal investigator of the Solar Wind Plasma Analyzer (SWA), led the commissioning of the instrument from his two-year-old son’s playroom. Security concerns required ingenious solutions such as viewing the ESOC control screens, which showed the data from the spacecraft, via a webcam. The main drawback of the ‘home-office’ set-up was the added delay between the action being performed by the spacecraft and the data about the status of the operation reaching the team. The laptop on the right in this image shows the webcam footage from ESOC with live data from SWA. The center laptop hosts a meeting between the UK, US, French, Italian and Czech teams behind the SWA instrument.“The two stand-alone screens show plots of ‘as-close-as-one-can-get-to-real-time’ data downloaded from ESOC to MSSL, plotted on the fly and updated every minute, so we can view the response of the sensor to commands before moving to the next step of the high-voltage ramp up,” says Chris. “The only person who could issue the actual commands to the spacecraft was our colleague at ESOC, Daniel Lakey, with whom we communicated via Webex.” (image credit: ESA, Christopher Owen)

- The other instrument teams also successfully finished their commissioning. “This is undoubtedly the first mission whose instruments were completely commissioned from people's homes,” says David Berghmans, from the Royal Observatory of Belgium, Brussels, Belgium, and principal investigator of the Extreme Ultraviolet Imager (EUI).

- Not only did the job get done, but they made up for lost time and managed to complete their commissioning on the original timeline. “Even in a normal world I would be very happy with where we are now,” says Daniel Müller, Solar Orbiter Project Scientist at ESA, “I never expected that almost everything would work flawlessly out of the box.”

- That’s testimony to the expertise with which the spacecraft was made by the prime contractor Airbus DS (UK) and its instruments were made by the various instrument teams. On 25 June, the Solar Orbiter Review Board endorsed this achievement by declaring the Mission Commissioning Results Review successful.

- For César García Marirrodriga, ESA’s Solar Orbiter’s Project Manager, it was a big moment because with commissioning over, his job is done and he hands over the spacecraft to the mission operations manager. “I’m very happy to hand it over because I know it is going in the right direction,” says César.

- And for Daniel, it is a big moment too because now the mission is ready to perform science. “In these four months since launch, the 10 instruments onboard have been carefully checked and calibrated one by one, like tuning individual musical instruments. And now it is time for them to perform together,” he says.

- This month’s 'remote-sensing checkout window’ from 17 to 22 June presented the first opportunity to have all the instruments play together. Receiving the recordings from the spacecraft, which is currently more than 160 million km away, will be completed in the next few days.

- “We’re very excited about this first ‘concert’. For the first time, we will be able to put together the images from all our telescopes and see how they take complementary data of the various parts of the Sun including the surface; the outer atmosphere, or corona; and the wider heliosphere around it. This is what the mission was built for,” says Daniel. These first light images will be released to the public in mid-July.

- Other instruments are already collecting data too. In the case of the Magnetometer (MAG), this was first switched on just a day after launch. “We got just under 100 days’ worth of data through the commissioning period, and it's wonderful data,” says Helen O’Brien, from Imperial College and MAG’s chief engineer.

- MAG was switched on early so that it could take readings as it was carried away from the spacecraft as its boom arm was deployed. “The instrument behaved beautifully. It was wonderful to see the field decay as we moved away from the spacecraft,” says Helen.

- That data will allow the team to understand the magnetic field being generated by the spacecraft itself, so that they can now remove it from their science data to leave just the magnetic field being carried into space away from the Sun. And there is plenty of data already. The team already has more than two billion scientific measurements to analyze. “The data is outstanding, really, really good, so we’re very happy,” says Tim Horbury, Imperial College, UK, and principal investigator for the instrument.

- The mission now continues on course to the Sun. During this cruise phase, the spacecraft's in-situ instruments will gather scientific data about the environment around the spacecraft, while the remote-sensing instruments will be fine-tuned by the teams in preparation for science operations in closer vicinity of the Sun. The cruise phase lasts until November 2021, after which Solar Orbiter will begin the science phase of its mission.


Figure 25: Timothy Horbury and his team from Imperial College London connected via zoom to run experiments on Solar Orbiter’s magnetometer amid the COVID-19 pandemic. Solar Orbiter was launched on 10 February 2020, only a few weeks before the pandemic struck Europe, enforcing strict measures including temporary closures of non-essential facilities (image credit: ESA, Tim Horbury)

• June 15, 2020: ESA’s Sun-exploring mission Solar Orbiter has made its first close approach to the star on June 15, getting as close as 77 million km to its surface, about half the distance between the Sun and Earth. 42)

- In the week following this first perihelion, the point in the orbit closest to the Sun, the mission scientists will test the spacecraft’s ten science instruments, including the six telescopes on-board, which will acquire close-up images of the Sun in unison for the first time. According to ESA’s Solar Orbiter Project Scientist Daniel Müller, the images, to be released in mid-July, will be the closest images of the Sun ever captured.

- “We have never taken pictures of the Sun from a closer distance than this,” Daniel says. “There have been higher resolution close-ups, e.g. taken by the four-meter Daniel K. Inouye Solar Telescope in Hawaii earlier this year. But from Earth, with the atmosphere between the telescope and the Sun, you can only see a small part of the solar spectrum that you can see from space.”

- NASA’s Parker Solar Probe, launched in 2018, makes closer approaches. The spacecraft, however, doesn’t carry telescopes capable of looking directly at the Sun.

- “Our ultraviolet imaging telescopes have the same spatial resolution as those of NASA’s Solar Dynamic Observatory (SDO), which takes high-resolution images of the Sun from an orbit close to Earth. Because we are currently at half the distance to the Sun, our images have twice SDO’s resolution during this perihelion,” says Daniel.

- The primary objective of these early observations is to prove that Solar Orbiter’s telescopes are ready for future scientific observations.

- “For the first time, we will be able to put together the images from all our telescopes and see how they take complementary data of the various parts of the Sun including the surface, the outer atmosphere, or corona, and the wider heliosphere around it,” says Daniel.

Figure 26: On June 15 2020, ESA’s new Sun-explorer Solar Orbiter reached its first perihelion, the point on its orbit closest to the Sun. The first perihelion also marked the end of Solar Orbiter’s commissioning phase and the beginning of the cruise phase, which will last until late 2021. During the science phase that follows, the spacecraft will get as close as 42 million km to the Sun’s surface, closer than the planet Mercury (video credit: ESA/MediaLab)

- The scientists will also analyze data from the four in-situ instruments that measure properties of the environment around the spacecraft, such as the magnetic field and the particles making up the solar wind.

- “This is the first time that our in-situ instruments operate at such a close distance to the Sun, providing us with a unique insight into the structure and composition of the solar wind,” says Yannis Zouganelis, ESA’s Solar Orbiter Deputy Project Scientist. “For the in-situ instruments, this is not just a test, we are expecting new and exciting results.”

- Solar Orbiter, launched on 10 February this year, is completing its commissioning phase on 15 June and will commence its cruise phase, which will last until November 2021. During the main science phase that follows, the spacecraft will get as close as 42 million km to the Sun’s surface, which is closer than the planet Mercury.

- The spacecraft will reach its next perihelion in early 2021. During the first close approach of the main science phase, in early 2022, it will get as close as 48 million km.

Figure 27: Animation showing the trajectory of Solar Orbiter around the Sun, highlighting the gravity assist maneuvers that will enable the spacecraft to change inclination to observe the Sun from different perspectives. The spacecraft’s orbit has been chosen to be ‘in resonance’ with Venus, which means that it will return to the planet’s vicinity every few orbits and can again use the planet’s gravity to alter or tilt its orbit. Initially Solar Orbiter will be confined to the same plane as the planets, but each encounter of Venus will increase its orbital inclination. For example, after the 2025 Venus encounter it will make its first solar pass at 17º inclination, increasing to 33º during a proposed mission extension phase, bringing even more of the polar regions into direct view (video credit: ESA/ATG medialab)

- Since the spacecraft is currently 134 million km from Earth, it will take about a week for all perihelion images to be downloaded via ESA’s 35-m deep-space antenna in Malargüe, Argentina. The science teams will then process the images before releasing them to the public in mid-July. The data from the in-situ instruments will become public later this year after a careful calibration of all individual sensors.

- “We have a nine-hour download window every day but we are already very far from Earth so the data rate is much lower than it was in the early weeks of the mission when we were still very close to Earth,” says Daniel. “In the later phases of the mission, it will occasionally take up to several months to download all the data because Solar Orbiter really is a deep space mission. Unlike near-Earth missions, we can store a lot of data on-board and downlink it when we are closer to home again and the data connection is much better.”

• May 29, 2020: ESA’s Solar Orbiter will cross through the tails of Comet ATLAS during the next few days. Although the recently launched spacecraft was not due to be taking science data at this time, mission experts have worked to ensure that the four most relevant instruments will be switched on during the unique encounter. 43)

- Solar Orbiter was launched on 10 February 2020. Since then, and with the exception of a brief shutdown due to the coronavirus pandemic, scientists and engineers have been conducting a series of tests and set-up routines known as commissioning.

- The completion date for this phase was set at 15 June, so that the spacecraft could be fully functional for its first close pass of the Sun, or perihelion, in mid-June. However, the discovery of the chance encounter with the comet made things more urgent.

- Serendipitously flying through a comet’s tail is a rare event for a space mission, something scientists know to have happened only six times before for missions that were not specifically chasing comets. All such encounters have been discovered in the spacecraft data after the event. Solar Orbiter’s upcoming crossing is the first to be predicted in advance.

- It was noticed by Geraint Jones of the UCL Mullard Space Science Laboratory, UK, who has a 20-year history of investigating such encounters. He discovered the first accidental tail crossing in 2000, while investigating a strange disturbance in data recorded by the ESA/NASA Ulysses Sun-studying spacecraft in 1996. This study revealed that the spacecraft had passed through the tail of Comet Hyakutake, also known as ‘The Great Comet of 1996’. Soon after the announcement, Ulysses crossed the tail of another comet, and then a third one in 2007.

- Earlier this month, realizing that Solar Orbiter was going to be 44 million kilometers downstream of Comet C/2019 Y4 (ATLAS) in just a matter of weeks, Geraint immediately alerted the ESA team.

Bonus science

- Solar Orbiter is equipped with a suite of 10 in-situ and remote-sensing instruments to investigate the Sun and the flow of charged particles it releases into space – the solar wind. Fortuitously, the four in-situ instruments are also perfect for detecting the comet’s tails because they measure the conditions around the spacecraft, and so they could return data about the dust grains and the electrically charged particles given off by the comet. These emissions create the comet’s two tails: the dust tail that is left behind in the comet’s orbit and the ion tail that points straight away from the Sun.

- Solar Orbiter will cross the ion tail of Comet ATLAS on 31 May–1 June, and the dust tail on 6 June. If the ion tail is dense enough, Solar Orbiter’s magnetometer (MAG) might detect the variation of the interplanetary magnetic field because of its interaction with ions in the comet’s tail, while the Solar Wind Analyzer (SWA) could directly capture some of the tail particles.

- When Solar Orbiter crosses the dust tail, depending on its density – which is extremely difficult to predict – it is possible that one or more tiny dust grains may hit the spacecraft at speeds of tens of kilometers per second. While there is no significant risk to the spacecraft from this, the dust grains themselves will be vaporised on impact, forming tiny clouds of electrically charged gas, or plasma, which could be detected by the Radio and Plasma Waves (RPW) instrument.

- “An unexpected encounter like this provides a mission with unique opportunities and challenges, but that’s good! Chances like this are all part of the adventure of science,” says Günther Hasinger, ESA Director of Science.

- One of those challenges was that the instruments seemed unlikely to all be ready in time because of the commissioning. Now, thanks to a special effort by the instrument teams and ESA’s mission operations team, all four in-situ instruments will be on and collecting data, even though at certain times the instruments will need to be switched back into commissioning mode to ensure that the 15 June deadline is met.

- “With these caveats, we are ready for whatever Comet ATLAS has to tell us,” says Daniel Müller, ESA Project Scientist for Solar Orbiter.


Figure 28: Solar Orbiter's suite of ten science instruments that will study the Sun. There are two types: in situ and remote sensing. The in situ instruments measure the conditions around the spacecraft itself. The remote-sensing instruments measure what is happening at large distances away. Together, both sets of data can be used to piece together a more complete picture of what is happening in the Sun’s corona and the solar wind (image credit: ESA-S.Poletti)


Figure 29: Anatomy of a comet - Infographic. The main components of a comet – nucleus, coma, hydrogen envelope, dust and plasma tails – indicating their composition, relative sizes and location. The diagram is representative and not to scale (image credit: ESA)

Expect the unexpected


Figure 30: The NASA/ESA Hubble Space Telescope has provided astronomers with the sharpest view yet of the breakup of Comet C/2019 Y4 (ATLAS). The telescope resolved roughly 30 fragments of the comet on 20 April and 25 pieces on 23 April. The comet was first discovered in December 2019 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) and its fragmentation was confirmed in April 2020 (image credit: NASA, ESA, D. Jewitt (UCLA), Q. Ye (University of Maryland); CC BY 4.0)

- Another challenge entails the comet’s behavior. Comet ATLAS was discovered on 28 December 2019. During the next few months, it brightened so much that astronomers wondered whether it would become visible to the naked eye in May.

- Unfortunately, in early April the comet fragmented. As a result, its brightness dropped significantly too, robbing sky watchers of the view. A further fragmentation in mid-May has diminished the comet even more, making it less likely to be detectable by Solar Orbiter.

- Although the chances of detection have reduced, the effort is still worth making according to Geraint.

- “With each encounter with a comet, we learn more about these intriguing objects. If Solar Orbiter detects Comet ATLAS's presence, then we'll learn more about how comets interact with the solar wind, and we can check, for example, whether our expectations of dust tail behavior agree with our models,” he explains. “All missions that encounter comets provide pieces of the jigsaw puzzle.”

- Geraint is the principal investigator of ESA’s future Comet Interceptor mission, which consists of three spacecraft and is scheduled for launch in 2028. It will make a much closer flyby of an as yet unknown comet that will be selected from the newly discovered comets nearer the time of launch (or even after that).

Grazing the Sun


Figure 31: Solar Orbiter: journey around the Sun (image credit: ESA)

- Solar Orbiter is currently circling our parent star between the orbits of Venus and Mercury, with its first perihelion to take place on 15 June, around 77 million kilometers from the Sun. In coming years, it will get much closer, within the orbit of Mercury, around 42 million kilometers from the solar surface. Meanwhile, Comet ATLAS is already there, approaching its own perihelion, which is expected on 31 May, around 37 million kilometers from the Sun.

- “This tail crossing is also exciting because it will happen for the first time at such close distances from the Sun, with the comet nucleus being inside the orbit of Mercury,” says Yannis Zouganelis, ESA Deputy Project Scientist for Solar Orbiter.

- Understanding the dust environment in the innermost region of the Solar System is one of Solar Orbiter’s scientific objectives.

- “Near-Sun comets like Comet ATLAS are sources of dust in the inner heliosphere and so this study will not only help us understand the comet, but also the dust environment of our star,” adds Yannis.

- Looking at an icy object rather than the scorching Sun is certainly an exciting – and unexpected – way for Solar Orbiter to start its scientific mission, but that’s the nature of science.

- Scientific discovery is built on good planning and serendipity. In the three months since launch, the Solar Orbiter team has already proved that it’s ready for both,” says Daniel.

• February 17, 2020: First measurements by a Solar Orbiter science instrument reached the ground on Thursday 13 February providing a confirmation to the international science teams that the magnetometer on board is in good health following a successful deployment of the spacecraft’s instrument boom. 44)


Figure 32: Data collected with the Magnetometer (MAG) instrument during the deployment of the instrument boom of ESA's Solar Orbiter spacecraft show how the magnetic field decreases from the vicinity of the spacecraft to where the instruments are actually deployed (image credit: ESA)

- Ground controllers at ESOC in Darmstadt, Germany, switched on the magnetometer’s two sensors (one near the end of the boom and the other close to the spacecraft) about 21 hours after liftoff. The instrument recorded data before, during and after the boom’s deployment, allowing the scientists to understand the influence of the spacecraft on measurements in the space environment.

- “We measure magnetic fields thousands of times smaller than those we are familiar with on Earth,” says Tim Horbury of Imperial College London, Principal Investigator for the Magnetometer instrument (MAG). “Even currents in electrical wires make magnetic fields far larger than what we need to measure. That’s why our sensors are on a boom, to keep them away from all the electrical activity inside the spacecraft.”

- “The data we received shows how the magnetic field decreases from the vicinity of the spacecraft to where the instruments are actually deployed,” adds Tim. “This is an independent confirmation that the boom actually deployed and that the instruments will, indeed, provide accurate scientific measurements in the future.”

- As the titanium/carbon-fibre boom stretched out over an overall 30-minute period on Wednesday, almost three days after liftoff, the scientists could observe the level of the magnetic field decrease by about one order of magnitude. While at the beginning they saw mostly the magnetic field of the spacecraft, at the end of the procedure, they got the first glimpse of the significantly weaker magnetic field in the surrounding environment.

- “Measuring before, during, and after the boom deployment helps us to identify and characterize signals that are not linked to the solar wind, such as perturbations coming from the spacecraft platform and other instruments,” says Matthieu Kretzschmar, of Laboratoire de Physique et Chimie de l'Environnement et de l'Espace in Orleans, France, Lead Co-investigator behind another sensor located on the boom, the high frequency magnetometer of the Radio and Plasma Waves instrument (RPW) instrument.

- “The spacecraft underwent extensive testing on ground to measure its magnetic properties in a special simulation facility, but we couldn’t fully test this aspect until now, in space, because the test equipment usually prevents us from reaching the needed very low level of magnetic field fluctuations,” he adds.

• February 14, 2020: At 16:00 CET on Thursday, 13 February, the critical first 83 hours of Solar Orbiter’s unique mission to study our star came to an end. 45)

- “This early phase is like the birth of a child,” says Operations Director Andrea Accomazzo. “Engineers want to be sure that it can survive on its own in its new environment.”

- “In the case of a spacecraft, they need it to be powered by its solar arrays, able to communicate with Earth, and able to control its orientation in space.”

- After a thunderous launch at 05:03 CET on Monday, Solar Orbiter headed south east from Cape Canaveral Air Force Station in Florida, flying over the South Atlantic before being released far above the shores of Western Australia. In Darmstadt, ESA’s mission control team soon took control of the spacecraft and began several days of round-the-clock flight control activities.

- “We planned for years and trained for months for this and I’m proud of all the teams that worked through this critical period,” says Andrea. “The first stage of Solar Orbiter’s mission was a success, but it certainly threw us a few challenging moments!”

Sunny side up

- The solar arrays were deployed at 06:24 CET and unfolded from the spacecraft’s body like wings. They soon began to supply power to the craft, relieving the burden on the batteries.

- With communication established and the necessary tests carried out, the first command was sent to the craft and the mission officially began.

- Until then, Solar Orbiter had still been in its strobing phase – rotating to maximize the chance of pointing one of its two communication antennas at ground stations on Earth. With this command, the spacecraft fired its thrusters and stabilized its orientation.

Figure 33: Visualization showing the deployment of various boom/antennas on the Solar Orbiter spacecraft. Initially, the first Radio and Plasma Waves (RPW) antenna is deployed. Then the boom hosting a suite of scientific instruments is deployed (MAG, RPW, and SWA to measure the magnetic and electric fields, and solar wind around the spacecraft). Subsequently, the remaining two RPW antennas are deployed. Finally, the high gain antenna dish is unfurled. In reality this sequence is spaced out over a 24 hour period (image credit: ESA)

Rise and shine

- The mission control team then began to wake up vital parts of the craft. They switched on the ‘reaction wheels’ – used to more precisely control the orientation of the spacecraft than can be achieved with thrusters – and the ‘Mass Memory’ to store the science data collected by Solar Orbiter’s science instruments.

- “Following successful tests, Solar Orbiter was brought into its Nominal Control Mode,” says Andrea.

- “Then came the window for the deployment of the ‘Radio and Plasma Waves’ (RPW) antennas, the instrument boom, and the high-gain antenna. The antennas and the boom are used to move sensors away from the spacecraft body to prevent disturbances to their measurements, while the high-gain antenna is used to communicate with the spacecraft across astronomical distances.”

Cool under pressure

- Before each antenna or boom could be deployed, Solar Orbiter was tilted to point it towards the Sun and warm it up. After the deployment of the first RPW antenna, the instrument boom was due next.

- But as the team tilted the craft to warm up the instrument boom, they noticed something unexpected.

- “We saw that the pins that hold the doors of the remote observation instruments safely in place during launch were cooling down more rapidly than expected as we tilted them away from the Sun,” says Andrea.

- “If their temperature had fallen to below -40°C, they could have undergone ‘cold welding’, sticking them in place and preventing the doors of the remote sensing instruments from opening.”

- “The pins had already gotten too cold to move, and were in danger of sticking. To prevent this, we sent the command for Solar Orbiter to enter its ‘safe mode’, resetting its orientation and pointing the pins back towards the Sun.”

- “Every launch comes with a unique set of challenges. But our teams train for events like this and were able to quickly respond to this situation and ensure a safe start to Solar Orbiter’s mission.”

Out with the boom

- Once warm enough, the pins were safely moved (it had now proven necessary to move them at an earlier stage than originally planned) and the team continued with the deployment of the boom.

- Following the deployment of the second and third RPW antennas and the high-gain antenna, operations began to wind down as the radio communication link between Solar Orbiter and ESA’s ground stations was switched to the more powerful high-gain antenna.

- “We were thrilled to see that the instrument boom and all three electric antennas were correctly deployed,” says Yannis Zouganelis, ESA’s deputy project scientist for the Solar Orbiter mission.

- “These appendages will enable us to probe the solar wind and, together with the remote-sensing instruments, reveal our Sun and its behavior in unprecedented detail. We can’t wait to start taking our measurements.”

- Solar Orbiter will now spend approximately three months in its commissioning phase, during which the mission control teams will conduct a test maneuver and check that the spacecraft and its 10 scientific instruments are working as intended to achieve the mission’s ambitious goals.

- In roughly two years, Solar Orbiter will reach its primary science orbit where it will study our star’s polar regions like no spacecraft has before.

• February 11, 2020: Solar Orbiter is ESA’s latest mission to study the Sun up close. Launched in the early hours of 10 February from Cape Canaveral, Florida, the spacecraft is due to arrive at its fiery destination in approximately two years. 46)


Figure 34: At this very moment, a spacecraft is headed toward the brightly burning Sun, photographed here on an Antarctic summer day by ESA sponsored medical doctor Stijn Thoolen at Concordia research station (photo credit: ESA/IPEV/PNRA–S. Thoolen)

- Solar Orbiter will face the Sun from within the orbit of Mercury, approximately 42 million kilometers from the solar surface. This is an ideal distance: from here Solar Orbiter can take remote images and measurements that will provide the first views of the Sun’s uncharted polar regions.

- At the southern poles on Earth, in Antarctica, the Sun has an exceptional presence on people living at the remote Concordia research station. During the Antarctic summer, the sun shines 24 hours a day. It would be perfect for sunbathing, except for the fact that the average summer temperature is only –30ºC.

- Consequently, in the winter the Sun does not appear above the horizon for over three months and the crew stationed in Concordia live with outside temperatures of –80ºC in complete darkness.

- While Solar Orbiter is en route to observing the Sun up close, the crew in Concordia are preparing for life without and enjoying the last rays of sunlight while they can. This picture shows a halo that can occur when sunlight is refracted off ice crystals in the atmosphere.

- The mission will investigate how intense radiation and energetic particles being blasted out from the Sun and carried by the solar wind through the Solar System impact our home planet, to better understand and predict periods of stormy ‘space weather’.

- While this results in beautiful aurora seen in the Arctic and Antarctic circles, stormy space weather can be disastrous. Solar storms have the potential to knock out power grids, disrupt air traffic and telecommunications, and endanger space-walking astronauts, for example.

- A better understanding of how our parent star works is critical to our preparedness for these scenarios on Earth.

Sensor complement:

The instruments have been selected jointly by ESA and NASA as part of the collaboration to provide the in situ and remote observations. The sensor complement consists of six remote sensing instruments operating at wavelength ranges from visible to X-ray, as well as four in-situ instruments covering all attributes of the interplanetary medium. It will acquire simultaneous spectra and images of the photosphere and corona; images of the photospheric magnetic field and gas velocity as well as measurements of the magnetic field and in-situ plasma properties at the location of the spacecraft. 47)

The challenging nature of the Solar Orbiter mission, along with tough constraints in the area of volume, mass and data rates, have led to a range of innovative design solutions for the payload, involving new technologies never previously used in space. In the thermal and optical domains, heat rejecting windows, limiting the bulk of the solar flux while allowing the wavelength(s) of interest to pass through, are being developed for two instruments. Several multi-layer coatings are also being designed for internal lenses and mirrors. For the three deployable antennas, the harsh thermal environment has had an important influence on the design of the deployment mechanism. For the purpose of polarization measurements, newly space qualified Liquid Crystal Variable Retarder technology is being applied and several instruments are also making advances in detector design, including newly designed CdTe X-ray detectors and back-illuminated Extreme UV CMOS detectors.

The remote sensing instruments opto-mechanical assemblies are mounted at the periphery of the main structure, by means of isostatic mounts thermally insulating the instruments from the rest of the spacecraft. This technique permits a direct FOV for the instruments and their baffle to cold space behind the heatshield, while keeping the main structure unaffected by the thermal control of the instruments; hence, insuring the very stable thermoelastic behavior needed for the line of sight (LOS) co-alignment.


Pointing direction & FOV

LOS pointing stability (RPE)

Instrument accommodation

Institution and PI (Principal Investigator)

No of units

Remote sensing instruments


Sun pointing
FOV=2.7º FDT, 17º HRT

0.02 arcsec in 10 s

Located behind shield, 2 apertures (12.5 and 1.5 cm diameter) with door and heat rejection filters

MPS (Max Planck Institute for Solar System Research), Lindau, Germany, PI: Sami Solanki

2(+2 windows)


Sun pointing
FOV=34º x 1.0º slit

1.0 arcsec in 10 s

Located behind shield, 1 aperture (7 cm diameter) with door

RAL Space, UK (ESA funded)



Sun pointing
FOV=16.7º HRI, 5.4º FSI

1.0 arcsec in 10 s

Located behind shield, 4 apertures (2 cm dia.) with doors and baffles, small Al filters

CSL (Centre Spatial de Liège), Belgium, PI: Pierre Rochus



Sun pointing

2 arcsec over few s

Located behind shield, 1 aperture (17 cm dia.) with door and occulter (8 cm diameter)

INAF (Astronomical Observatory of Turin), Italy, PI: Ester Antonucci



Sun pointing

2 arcsec over few s

Located behind shield, 1 apertures (12 x 12 cm2) with door (TBC) and filters

Institute of Astronomy, ETH Zürich, Switzerland, PI: Arnold O. Benz



Sun pointing



NRL (Naval Research Laboratory), Washington, D.C., PI: Russell A. Howard


In-situ instruments


EAS: FOV= 4º sr,
PAS: Sun pointed, FOV = 60ºx10º
HIS: Sun pointed, FOV = 60ºx10º


PAS and HIS are S/C body mounted with small (10 cm2) aperture through the heatshield, EAS sensors are on the S/C behind the shield

MSSL (Mullard Space Science Laboratory), UK, PI: Christoper Owen



3-axis sensing


3 x antenna on S/C under direct sun light, magnetometer loop and 3 x search coils on the boom

Observatoire de Paris, France, PI: Milan Maksimovic



3-axis sensing


2 x sensors located on boom (in the shadow)

ICL (Imperial College London),PI: Tim Horbury



Several directions wrt orbital plane Typical FOV of order 60º x 60º


5 x sensors located on the spacecraft corners behind the heatshield

University of Alcala, Spain, PI: Javier Rodríguez-Pacheco


Table 3: Summary of payload pointing and accommodation requirements


Figure 35: Payload accommodation onboard the Solar Orbiter (image credit: ESA) 48)


Figure 36: Mounting locations of some instruments on the SolO spacecraft (SolO consortium)

Design innovations:

Although the Solar Orbiter payload is based on heritage from previous solar science missions, the very nature of this mission, along with its constraints, has triggered considerable innovation covering the full range of technical domains (Ref. 47).

ESA spearheaded the investigation of promising new technologies which were considered to hold benefits for the Solar Orbiter payload via several activities in the early phase of the mission (Phase A/B). These included TDAs (Technology Development Activities) for the concept of the PHI HREW (Heat Rejecting Entrance Window), developed under contract with Selex Galileo, and the LCVRs, developed under contract with INTA. Further examples include early UV detector radiation testing and the DRPM (Dynamically Reconfigurable Processing Module) study, which demonstrated the feasibility of the concept of routine partial reconfiguration of the Virtex-4 FPGAs as foreseen in the PHI design.

The instrument teams were involved from the early stages of the requirements specifications for these contracts and were able to follow the progress throughout. Following the end of the activity, responsibility for further development of these items was handed over to the instrument team with reduced risk.

Common procurement: Although the instruments are diverse, wherever possible a common approach to new technologies has been encouraged and facilitated. This is exemplified by the establishment of a CPPA (Central Parts Procurement Agency), which was established by ESA on behalf of the instrument teams for procurement of EEE (Electronic, Electrical and Electromechanical) parts. The service provided to the instrument teams includes the qualification of the EEE components themselves. This provides the opportunity for harmonization and reduces the cost to the instruments. It also lowers the schedule risk associated to qualification at mission level by coordinating procurement milestones and providing visibility to ESA of the qualification progress.

A) Entrance windows and filters:

Due to the high incident flux, the remote sensing instruments, which require a direct view of the Sun, must reject the majority of the incident energy, while allowing the wavelength range of interest to pass through. This is done in a variety of ways. PHI and STIX have windows located in the spacecraft heat-shield, EUI has entrance filters inside the instrument, while SPICE and the chronographs METIS and SoLOHI have internal mirror systems.

Optical Heat Rejecting Entrance Windows (PHI): Of these the most technologically challenging is the PHI Heat Rejecting Entrance Window (HREW), due to the very narrow requirement for the PHI science wavelength (617.3 nm ± 1.5Å), the large aperture (140 mm for the HRT) and the need to minimize the thermal flux entering the instrument. Although a filtergraph inside the instrument filters the science wavelength to the required level, it is a delicate component which cannot tolerate the full solar load and thus the GREW is the first stage in this filtering process and restricts the incident light to a ~20nm band, allowing only 4% of the total incident energy through.

The concept of the PHI GREW consists of a series of coatings on a glass substrate (Sprawl 300), which filter the wavelength of interest in stages (see Figure 3). In addition to the strict requirements on wavelength pass band, as PHI is an optical polarimeter, there are strict requirements on the polarization, uniformity and wavefront distortion induced by the HREW.

These coatings must be shown to maintain their properties steadily throughout the lifetime of the mission under all operational conditions. This has been validated via a series of environment tests, both at sample and prototype level, involving thermal cycling and radiation tests. A further challenge has been to design a suitable mount for the HREW, which will withstand the vibrational loads while protecting the glass and ensuring that the mount does not disrupt the optical properties of the HREW. The current design has been qualified to 275°C and shown to survive and maintain its optical and thermal performance in all tests, with the exception of a simultaneous radiation test at high temperature in vacuum, which is yet to be performed. In sample level tests, the coated glass has been shown to survive up to a temperature of 350°C without degradation.


Figure 37: Schematic of PHI Heat Rejecting Entrance Window and its functional concept (image credit: ESA)

X-ray Windows (STIX): Although it’s requirements are not as stringent as those of PHI, STIX needs a window which will be transparent to X-rays (above 4 keV), while minimizing the transmission of lower wavelength ranges (being opaque at wavelengths >300 nm). Due to the nature of the STIX instrument, which involves the generation of Moire shadowgrams on the detectors, the uniformity of the X-ray transmission is crucial, with a requirement of <4% rms variation in the transmission over the aperture (~200 mm diameter). These requirements lead to the need for a Be window with a thickness accuracy of 25 µm. The design involves two windows, one at the front and one at the back of the feed-through in the spacecraft heat-shield.

In order to lower the overall temperature of the window, a protective thermal coating of Al-SiOx is being qualified for use on the sun facing side of each window (Figure 38). For the STIX windows, along with the qualification of the coating, the processes and facilities for manufacture of the window are critical due to the nature of Be dust as a toxic material. Therefore, additional safety precautions are taken in planning and executing qualification tests, such as vibration testing, to ensure sufficient margins of safety against breaking the window.


Figure 38: STIX X-ray window coating (image credit: ESA)

Heat rejecting mirror system (SPICE): The SPICE optics unit (Figure 39) is composed of two parts: the telescope section and the spectrometer section. The primary purpose of the telescope is to reflect and focus as much as possible of the EUV solar radiation in the entire spectral range of SPICE onto the spectrograph entrance slits, while rejecting the unused solar flux (UV/visible/IR).


Figure 39: Illustration of the SPICE optics unit (image credit: ESA)

This is accomplished in several steps:

1) The “Solar Transparent” Primary Mirror (PM) is designed with a thin (~10 nm) boron carbide (B4C) coating applied on a fused silica substrate. This provides a VUV (Vacuum Ultraviolet) reflectance of >0.27 between 40 nm and 200 nm. This allows to reflect the EUV radiation of interest for science towards the spectrograph entrance slits.

2) Most of the solar visible and near-infrared radiation is transmitted with little absorption by the PM to the back of the instrument where it is reflected by the HRM (Heat Rejection Mirror) towards outer space. The HRM is a highly reflective vacuum deposited silver fold mirror accommodated inside a CFRP structure (“chimney”). The other internal surfaces of the chimney are uncoated and provide some radiative cooling due to their view to space.

3) The undesired the solar radiation reflected by the PM is reflected to a single heat dump by another set of mirrors in front of the slit. The flux is routed from this heat dump to S/C radiators. These mirrors are configured so that only the required science beam passes through to the slit. Baffles also intercept solar radiation that either diverges as it comes into the instrument or is off-axis due to spacecraft pointing.

Out of the 31.8 W solar load that enters the instrument (end-of-life hot operational case), 22.7 W are rejected by the HRM while only 9.1 W are absorbed internally by the instrument and by the heat dump, as illustrated in Figure 6.


Figure 40: SPICE thermal inputs and outputs (SDM: SPICE Door Mechanism; PM: Primary Mirror; HRM: Heat Rejection Mirror), image credit: ESA

EUV Entrance Filters (EUI): In order to filter the EUV science wavelength of interest for EUI, which is in a narrow passband of 15-31nm, entrance filters for each channel are located at the front of the instrument. These consist of 150 nm thick aluminum filters. The filters receive almost the full bulk of the solar flux (the 47.4 mm High Resolution Imager EUV filter will receive 17.44 kW/m2). While commercial space-qualified filters are available, these have not been qualified to the temperatures and incident solar flux levels which will be experienced by EUI. In addition, due to the very thin nature of these filters, a supporting mesh structure is required to survive the launch loads, however the mesh support in the standard commercial filters do not evacuate the IR load sufficiently.

Developments have therefore been undertaken to customize the filter design, introducing different Ni mesh patterns and also encapsulating the filter in a ribbed frame (Figure 41). Several permutations of the mesh and frame patterns and materials have been trialled to find the optimum combination of transmission performance, thermal conductivity and response to thermal cycling under high temperature and in vacuum. CSL have also investigated a prototype in which the supporting grid and frame are grown simultaneously on the Al filter (“grid-on-filter”). These investigations have led to a convergence on a custom design involving a spiral Al frame/rib pattern matching that of the underlying Ni mesh (Figure 41). This has been tested successfully to 13 SC.


Figure 41: Illustration of the EUV entrance filter (image credit: ESA)

B) Polarizers and other optics:

LCVR (Liquid Crystal Variable Retarder) technology introduction, (PHI and METIS): The polarization measurements in the PHI and METIS instruments are performed by means of LCVRs, a technology which has considerable heritage in ground- and balloon-borne instruments but has not yet flown in space. These polarizers consist of LC (Liquid Crystal) cells, which are electrooptical polarization modulators. By adjusting the voltage applied to the cells, the orientation of the LC molecules is changed and this changes the optical retardance (Figure 42). In the case of PHI, two LCVR cells will be oriented with their fast axes at 45° to each other as part of the PMP (Polarization Modulator Package), to enable differential retardance of the different polarization states. 49)

The advantages of this technology for space applications derive from the low resource demands (power, mass and volume) as well as the fact that it avoids the use of mechanisms and is easy to control and to synchronize with the detector readout.

The qualification activities performed for these cells included the investigation of several material types as well as the design of a mount assembly. The optimized design successfully passed mechanical, thermal and radiation tests such that TRL (Technology Readiness Level) 5 was reached in mid-2011.

The PMP for PHI consists of two APAN (Anti-Parallel Nematic) LCVRs oriented with their fast axes at 45° with respect to each other followed by a linear polarizer (the polarizing beam-splitter, the polarization analyzer) aligned with the fast axis of the first LCVR. The PMP generates four modulations of the polarization state in order to extract the Stokes parameters of the solar incoming light.

For METIS, only one LCVR is necessary in the PMP with a quarter-waveplate (not included in the PMP, but placed previously) in order to analyze the linear polarization. Nevertheless, an additional LCVR has been included with its fast axis parallel to the first cell, but with the pretilt angles of the liquid crystal molecules in opposite direction to obtain an extended and wider acceptance angle.

Obviously, in both instruments, a linear polarizer is included before the detector to be able to analyze the signal as change in the detected intensity.


Figure 42: LCVRs variable retardance with applied voltage (image credit: ESA)

Etalon (PHI): The fine filtering of the PHI science wavelength to the required narrow passband (optimum bandwidth in the range 80-120 mÅ) is performed in a Filtergraph consisting of Lithium Niobate (LiNbO3) electro-optic etalon. This is a single crystal with a clear aperture of 50 mm and fabrication finesse of ≥ 30. The etalon needs to be tuned in order to scan the spectral line as well as to compensate for the spacecraft radial velocity and this is performed by adjusting an applied high voltage. The temperature sensitivity of such an etalon is 2.9 pm/K, which means that the etalon must be kept in a temperature controlled oven, set to a temperature which can be maintained throughout the operational orbit.

As in the case of the LCVRs, the etalon wafers are commercially available and have been used on balloon-borne instruments, however they have not as yet flown on a space mission. A qualification for the Solar Orbiter is being undertaken, of which vibration testing, vacuum testing and proton irradiation testing up to 60 krad on a structurally representative sample have so far been performed successfully.

C) Detectors and Electronics:

Back-illuminated CMOS detectors: The detectors for each of the three channels of the EUI (Extreme UV Imager) instrument (Table 3) are based on CMOS APS technology. Prototype 1 k x 1 k front-thinned, double-gain detectors (APSOLUTE) have been developed for EUI, in parallel to similar 2 k x 2 k single-gain prototype development for PHI. These have successfully undergone testing to demonstrate suitability for the Solar Orbiter environment.

However further improvements on these designs are needed in order to fulfil the EUI science objectives:

- While front-thinned detectors are suitable for the EUI Lyman-α channel, the EUV channels require a new development of back-thinned detectors. This is needed in order to allow back-illumination of the detectors to achieve the required sensitivity in the EUV 10-40 nm range (Figure 43)

- Double gain, as developed in the APSOLUTE prototypes is needed in order to minimize the read-out noise and increase the speed and dynamic range.

- For the Full Sun Imager, the array size has to be increased to 3 k x 3 k in order to achieve the required FOV and angular resolution.

Each of these new developments presents its own challenges and full qualification of the new design for Solar Orbiter must be performed. This activity is currently underway.


Figure 43: Back-thinning of APS detectors (image credit: ESA)

IDeF-X ASIC (STIX and EPD): The IDeF-X (Imaging Detector Front-end) ASIC is a new development in Front End Electronics which may be flown for the first time on Solar Orbiter in the STIX and EPD (Energetic Particle Detector) instruments. STIX will use the IDeF-X HD (High-Dynamic) version, while the IDeF-X BD (Bi-directional) version will be used in EPD-STEP.

For STIX, the ASIC is integrated in the Caliste-SO detectors, which are hybrid components integrating a pixelated 10 mm x 10 mm CdTe sensor with its HV power supply, the analog front end read-out electronics in the form of the IDeF-X HD, passive components and a 20 pin surface mount interface (SOP interface). The µPCBs are stacked perpendicular to the CdTe sensor in the Caliste-SO body (Figure 44).

The IDef-X performs the read-out of up to 32 channels in terms of pixel location, energy and rise time. The design involves a CSA (Charge Sensitive Amplifier) stage followed by a pulse shaper, with several tunable parameters, including CSA bias, peaking time and low level discriminator. Different (partial) channel read-out modes can be configured in order to optimize power consumption.

The advantages of this ASIC are primarily in terms of savings on volume, mass and power. The power consumption per channel is 800 µW. As the detectors have to be cooled in order to minimize the noise and leakage current, savings in power consumption and thus heat dissipation are very beneficial.

The characterization of the Caliste-SO prototype has shown its ability to meet the STIX performance requirements and full qualification of this device is underway. 50)


Figure 44: Illustration of the STIX Caliste-SO hybrid (image credit: ESA)

Virtex-4 FPGA (PHI): The full data processing chain of PHI requires to perform an inversion of the RTE (Radiative Transfer Equation) to derive the magnetic vectors and line of sight velocity. Due to the limited telemetry bandwidth on Solar Orbiter, this must be performed onboard as there is insufficient bandwidth to downlink the raw PHI data. Such onboard processing can only be performed by an FPGA such as the Xilinx Virtex-4, while remaining within the instrument power budget. PHI are thus planning to use two Virtex-4 CF1140 devices (1140 pins) in their DPU (Data Processing Unit).

While the Virtex-4 component itself has been qualified for use on space missions, there is currently no assembly house in Europe which is space-qualified for soldering the CF1140 package of this component. Thus, the PHI project is undertaking a dedicated qualification program for the assembly of this device, involving the manufacture of dedicated verification boards which will undergo vibration and thermal cycling tests to simulate the mechanical strain to the solder joints which may be experienced during the mission.

D) On-board data processing:

Routine FPGA reconfiguration (PHI): In addition to the processing required for the RTE inversion, PHI require considerable on-board processing capability for instrument functionality such as control of the Image Stabilization System and data pre-processing.

To optimize mass, volume and power, the PHI project has developed a design making use of the ability to re-program the Virtex-4 FPGAs to routinely switch the FPGA code in order to perform different functions at different points in the orbit, as follows (Figure 45):

- In one configuration, FPGA1 performs ISS (Image Stabilization System) control and FPGA2 performs the data accumulation.

- In the second configuration, FPGA1 performs the RTE inversion and FPGA2 performs the data pre-processing.

While reprogrammable FPGA technology has been flown in space on several missions, the PHI architecture involving routine changes of the FPGA code throughout the mission is novel and relies on the robustness of not only of the FPGA code protection but also that of the SoCWire network between the components. This is also under verification as part of the ESA DRPM (Dynamically Reconfigurable Processing Module) study.


Figure 45: PHI use of routine FPGA reconfiguration (image credit: ESA)

E) Deployment Mechanisms:

Antenna deployment mechanism (RPW): The RPW (Radio and Plasma Wave) antenna (ANT) system consists of three identical antennas which are oriented at angles of roughly 120º in the operational configuration. Each antenna comprises a 5 m long sensor and an almost 1 m long boom that keeps the sensor at a distance from the spacecraft to reduce disturbances and supports the preamplifier close to the sensor. Due to the antenna length it is obvious that the antennas need to be launched in stowed configuration and deployed in orbit. As most of the length of the antennas sticks out from the heat shield, material selection must take into account direct sun illumination.

The general antenna design is based on several pipe segments connected by “Maeva” elastic hinges, the deployment of which is controlled by SMA (Shape Memory Alloy) components. In stowed configuration each antenna is fixed by three HDRMs (Hold Down and Release Mechanisms) which are opened in sequence to release the antenna. For each HDRM, two stowing flanges are guided by journal bearings and driven to their open position by springs, once a Frangibolt non-pyrotechnical device (SMA based) is actuated. The stowed antenna assemblies are protected by MLI shells in order to keep the temperature of the SMAs in the Frangibolts and hinges below the transformation temperature, thus avoiding premature and uncontrolled deployment. The MLI shells are mounted on carbon tubes that are attached to the HRDM flanges.


Figure 46: RPW antenna overview in stowed configuration (image credit: ESA)

The Maeva hinges, controlled by the SMA components, were developed by CNES. The Maeva hinges constitute Carpentier elastic blades that ensure both deployment and locking of the hinges. To control the deployment, each hinge is equipped with two SMA strings around which Cerafil heater wires are wound. For each antenna, all SMA heaters are electrically connected in series.


Figure 47: Deployment hinges (image credit: ESA)

Mechanical, thermal and functional (in particular deployment) tests are being performed on full antenna models and subsets. High temperature materials are also being qualified separately.

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