Minimize Solar Orbiter Mission

Solar Orbiter Mission

Spacecraft  Development Status    Launch    Sensor Complement  References

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)

Development status:

• November 21, 2019: ESA’s mission to the Sun has been unpacked following its arrival in Florida earlier this month, ready to begin pre-launch testing and checks. 32)

- The mission is currently scheduled to lift off from Cape Canaveral launch complex late in the evening of 5 February U.S. time (early morning 6 February central European time) on an unprecedented mission to study our star up-close.

- An Antonov cargo plane transported the spacecraft and essential ground support equipment from Munich, Germany, to Florida, landing at the Shuttle Landing Facility at Kennedy Space Center on 1 November. From there the satellite and equipment travelled by road to the AstroTech Space Operations facility. The first weeks were dedicated to setting up the equipment that will be needed to perform the upcoming checks and tests on the spacecraft. This will include repeated simplified tests of the spacecraft and science instruments so that the functioning of the various systems is confirmed as it was before the long flight, and checking of the propellant pressurization system pressure before eventually fueling the spacecraft.

- In the new year attention will shift to mating the spacecraft with the launch adapter and encapsulating the spacecraft inside the fairing. In the final stages of preparation, the spacecraft will be mounted atop the Atlas V 411 rocket and moved to the launch pad ready for liftoff.


Figure 12: ESA’s mission to the Sun has been unpacked following its arrival in Florida earlier this month, ready to begin pre-launch testing and checks (image credit: Airbus DS Ltd)

- Once in space, and over the course of several years, the spacecraft will repeatedly use the gravity of Venus and Earth to raise its orbit above the poles of the Sun, providing new perspectives on our star, including the first images of the Sun’s polar regions. Its complementary suite of instruments means it will be able to study the plasma environment locally around the spacecraft and collect data from the Sun from afar, connecting the dots between the Sun’s activity and the space environment in the inner Solar System, which is essential to understand the effects of space weather at Earth.

- Solar Orbiter is an ESA mission with strong NASA participation. The prime contractor is Airbus Defence and Space in Stevenage, UK. The mission will provide complementary datasets to NASA’s Parker Solar Probe that will allow more science to be distilled from the two missions than either could achieve on their own.

• October 18, 2019: ESA’s Solar Orbiter mission has completed its test campaign in Europe and is now being packed ready for its journey to Cape Canaveral at the end of this month, ahead of launch in February 2020. 33)


Figure 13: Solar Orbiter at IABG in Ottobrunn, Germany (image credit: ESA, S. Corvaja)

- The spacecraft was on display today for the final time in Europe, at the IABG test center. It was built at Airbus Stevenage, UK, and has spent the last year at IABG undergoing essential testing such as checking deployment mechanisms, and that it can withstand the vibrations of launch, and the thermal extremes and vacuum of space. It has now been declared ready for shipment to the launch site and will travel an Antonov cargo plane on 31 October.

- Once launched it will follow an elliptical path around the Sun, at its closest bringing it within the orbit of Mercury, just 42 million kilometers from the Sun. As such, Sun-facing parts of the spacecraft have to withstand temperatures of more than 500ºC – due to solar radiation thirteen times more intense than for Earth-orbiting satellites – while other parts remain in shadow at -180ºC.

- The mission is essential to learn more about the Sun-Earth connection. We live inside a giant bubble of plasma generated by the Sun that surrounds the entire Solar System, within which we are prey to space weather. Solar Orbiter will provide a deeper understanding as to how activity on the Sun is linked to these solar storms, which can disrupt electrical systems, satellite communications, GPS, and create higher doses of radiation for polar flights and astronauts.

- “Solar Orbiter is set for answering some of the biggest scientific questions about our star, and its data will help us to better protect our planet from the global challenges of space weather,” says Günther Hasinger, ESA Director of Science.

• October 17, 2019: 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. 34)

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.

• September 23, 2019: An important stage in the development of ESA's Solar Orbiter mission was completed between May and June, when a series of tests to validate the electromagnetic compatibility and magnetic properties was carried out on the spacecraft's flight model. 35)

- After shipment from prime contractor Airbus Defence and Space in Stevenage, UK, last year, the Solar Orbiter spacecraft has been undergoing a series of tests at the premises of IABG in Ottobrunn, Germany, testing its thermal and mechanical properties, the deployment of several elements, and most recently its electromagnetic compatibility (EMC) and magnetic behavior.

- The EMC testing took place in an anechoic chamber at IABG, where the spacecraft was isolated from external electromagnetic interference. The chamber walls are covered with thousands of pointy pyramids that fully absorb reflections of electromagnetic waves, muting also any sound echoes and creating an eerie silence. Performed between 8 and 22 May, these tests were needed to verify that the spacecraft's electrical equipment will be fully electromagnetically compatible throughout all phases of the mission.

- Electromagnetic compatibility is a key aspect of any space mission, because electrical and electronic equipment can be a source of electromagnetic emissions and/or sensitive to such emissions, so the compatibility between the various onboard emission sources and their susceptibilities needs to be verified before launch.

- One part of the EMC tests was designed to check that the electromagnetic emissions from various parts of the spacecraft – particularly the various radio antennas onboard – did not interfere with other spacecraft subsystems. During this phase of the tests, the spacecraft's high gain, medium gain and low gain antennas that will provide telemetry, tracking and communication (TT&C) for the mission were turned on one at a time, checking that all other systems were still operating properly.

- The engineers also had to verify the compatibility of the spacecraft's systems with sensitive external equipment. The Solar Orbiter EMC tests verified that no emissions from the spacecraft would interfere with the launch vehicle's radio receivers and transponders or with nearby radio antennas during the launch preparations and lift-off from the Cape Canaveral launch site in Florida, USA.

- In addition, some specific compatibility tests were performed to determine whether the Radio and Plasma Waves (RPW) instrument would be affected by the spacecraft's electromagnetic emissions. This instrument is sensitive to electric and magnetic signals, so it was important to characterize the fields produced by the spacecraft in order to be discriminated later on from the actual measurements of the fields in space.

- Once in space, Solar Orbiter will deploy three 7-m long monopole antennas that are part of the RPW instrument. However, these are too long to be deployed in the EMC chamber, so the tests were carried out using shorter placeholder antennas plugged into the instrument, together with an external antenna linked to a receiver.

- Another phase of the EMC tests checked the compatibility of another one of the RPW elements – the search coil magnetometer (SCM) – with the low frequency alternating current (AC) magnetic fields emitted by the spacecraft. The flight version of Solar Orbiter will carry the SCM in the middle of the instrument boom, but deployment of the actual instrument boom during the EMC test was not possible, so an electrically representative boom mock-up was prepared and used with the SCM qualification model connected to the spacecraft.

- Analysis of the test data showed that the spacecraft meets the EMC requirements with respect to interactions with the TT&C subsystems onboard and with external equipment during launch. Analysis of the EMC test for the RPW instrument yielded results that were consistent with prior expectation from unit level testing and analysis, but further characterization will be necessary during in-flight and in-orbit commissioning.


Figure 15: The Solar Orbiter spacecraft, ready to start electromagnetic compatibility (EMC) testing in a special anechoic chamber in May 2019 at the IABG facility in Ottobrunn, Germany. These tests verified that the spacecraft's electrical equipment will be fully electromagnetically compatible throughout all phases of the mission (image credit: Airbus Defence and Space)

- While most spacecraft undergo only EMC testing, missions that involve measuring magnetic fields in space with exquisite accuracy – such as Solar Orbiter, which will measure the magnetic field of the solar wind with its Magnetometer (MAG) and with the SCM component of the RPW instrument, or magnetospheric plasma missions like ESA's Cluster and Swarm – require an additional set of tests to fully characterize their magnetic properties.


Figure 16: The Solar Orbiter spacecraft in the IABG magnetic field simulation facility in Ottobrunn, Germany. The facility is located just outside the premises in a nearby forest to avoid interference with human-generated magnetic fields, consists completely of non-magnetic materials like wood, and contains twelve 15 m coils – nearly as large as the building – to create a homogeneous magnetic environment that compensates Earth's own magnetic field, simulating outer space conditions. Tests performed here in June 2019 verified the magnetic behavior of the unpowered spacecraft, to make sure the magnetic field of Solar Orbiter is low enough so that the Magnetometer (MAG) instrument can operate at its most sensitive range (image credit: ESA–S. Corvaja)

- These tests were conducted in a unique facility, the magnetic field simulation facility (MFSA), located just outside the IABG premises in a nearby forest to avoid interference with human-generated magnetic fields. In addition to that, the facility consists completely of non-magnetic materials like wood, and contains twelve 15-m coils – nearly as large as the building – to create a homogeneous magnetic environment that compensates Earth's own magnetic field, simulating outer space conditions.

- Performed between 18 and 25 June 2019, the testing first verified the magnetic behavior of the unpowered spacecraft, to make sure the magnetic field of Solar Orbiter is low enough so that the MAG instrument can operate at its most sensitive range. Later, the tests proceeded with electric current powering the spacecraft – first looking at direct current (DC) and finally alternating current (AC).

- The last scenario is especially important because it measured possible variations in the spacecraft's magnetic field during specific operations, for example with extra currents generated by the motors or by mobile elements such as the changeable optical filters used by the cameras on board to take images of the Sun.

- The analysis of the magnetic tests indicates that the mission requirements were met within the limits of the testing facility. After launch, in the even quieter environment of space, further measurements during the commissioning phase will complement the results of these tests to fully characterize the magnetic properties of the spacecraft.

- The spacecraft electromagnetic compatibility and magnetic cleanliness are especially important aspects for the science that Solar Orbiter will perform, not only for the RPW (EMC aspects) and MAG (magnetic properties) instruments. They are also key elements for the two particle detector instruments on board – the Solar Wind Plasma Analyzer (SWA) and the Energetic Particles Detector (EPD) – in order to reconstruct the path of incoming particles that might be deflected by any residual spacecraft electric or magnetic field.

- Throughout the mission development, a specific EMC working group, involving representatives of industry, ESA and all instrument teams, has looked into these aspects, setting stringent requirements for the overall spacecraft design. With this phase of the test campaign now completed, the teams are pleased to see that these choices eventually paid off, as the satellite checks off another important milestone on the way to launch.

• August 5, 2019: The image (Figure 17) was captured in 2015 from the site of the European Space Astronomy Center (ESAC) in Madrid, Spain, using a Solarmax 90 H-alpha telescope (9 cm in diameter) and a QHY5-II monochromatic camera. A grayscale 283-second video was initially created of the solar surface, and the best 30% of these 8222 frames were then combined and colored to produce this image. 36)


Figure 17: This image shows a snippet of the Sun up close, revealing a golden surface marked by a number of dark, blotchy sunspots, curving filaments, and lighter patches known as ‘plages’ – brighter regions often found near sunspots. The width of the image would cover roughly a third of the diameter of the solar disc (image credit: ESA/ESAC/CESAR, A. de Burgos)

- The part of the Sun shown here is known as the chromosphere (literally ‘sphere of color’), one of the three main layers comprising our star. This layer sits just above the photosphere, the visible surface of the Sun with which we are most familiar. When viewed using a H-alpha telescope, as seen here, the chromosphere can reveal myriad intriguing features decorating the whole solar disc.

- Sunspots are not permanent fixtures on the Sun. They exist for days or weeks at a time, and come about as intense magnetic fields become twisted and concentrated in a given place, stifling the flow of energy from the Sun’s interior to the surface. This leaves sunspots cooler than their surroundings, causing their darker appearance, while gas continues to flow both beneath and around these areas of magnetic disruption.

- The ESA/NASA Solar and Heliospheric Observatory (SOHO) mission, launched in 1995, has probed deeper into these features, characterized the flows in and around the spots themselves, and found that they form as magnetic fields break through the visible surface of the Sun. The work of missions such as SOHO will be continued by ESA’s upcoming Solar Orbiter, the first medium-class mission selected for ESA's Cosmic Vision 2015-2025 Program.

- Solar Orbiter will explore how the Sun creates and manipulates a patch of space known as the heliosphere – a bubble blown by the solar wind, an ongoing stream of charged particles heading out from the Sun into the Solar System. The mission will also clearly image the solar poles for the first time, and track magnetic activity as it builds up and gives rise to powerful flares and eruptions. Planned for launch in February 2020, Solar Orbiter will make significant breakthroughs in our understanding of how our host star works.

• July 1, 2019: ESA’s Solar Orbiter mission is being put through its paces to prepare it for facing the Sun following launch in February 2020. 37)


Figure 18: This image captures the scene part way through a solar array deployment test at the IABG facilities in Ottobrunn, Germany, earlier this year. Fully extended, the tip of the array stretches 8.2 m from the spacecraft body. The panels are suspended from above to simulate the weightlessness of space (image credit: ESA, S. Corvaja)

- The spacecraft is being tested to withstand the vibrations of launch, the vacuum of space, and the extreme temperature ranges and magnetic environment that it will experience as it journeys from Earth to within the orbit of the innermost planet, Mercury. The deployment mechanisms of instrument booms, antennas and solar arrays are also checked out.

- The solar arrays have to provide the required power throughout the mission over a wide range of distances from the Sun. Close to the Sun, the spacecraft will endure around 13 times the amount of solar heating that Earth-orbiting satellites experience, with temperatures in excess of 500ºC, so the solar arrays can also be rotated to avoid overheating when closest to the Sun.

- Solar Orbiter's mission is to provide new views of our star, in particular providing the first close-up observations of the Sun’s poles. Its unique orbit will allow scientists to study the Sun and its outer atmosphere, the ‘corona’, in much more detail than previously possible.

- We cannot usually see the corona because it is overwhelmed by the bright light of the Sun’s surface itself. During a total eclipse however, when the Moon passes between Earth and the Sun, that light is blocked, revealing the beautiful white glowing corona around the Sun, its structures shaped by the Sun’s magnetic field. This rare sight will be much sought after by astronomers in parts of South America on 2 July, who are getting ready to watch a total solar eclipse.

- We essentially live in the extended atmosphere of the Sun. The corona continuously expands and spreads into space, developing as the solar wind that interacts with the planets and beyond, sometimes leading to aurora and other space weather effects observed at Earth.

- Solar Orbiter will measure the solar wind and magnetic fields in the vicinity of the spacecraft while simultaneously taking high-resolution images of features on the Sun, linking the two together. This will give us unprecedented insight into how the Sun creates and controls its dynamic atmosphere, and how it interacts with the planets. Studying the Sun-Earth connection is fundamentally important to understanding how our Solar System works in its entirety.

- In addition to delivering ground-breaking science in its own right, Solar Orbiter also has important synergies with NASA’s Parker Solar Probe. Coordinated observations will contribute greatly to our understanding of the Sun and its environment.

• April 4, 2019: While the Solar Orbiter spacecraft is at Ottobrunn, the Solar Orbiter mission control team located at ESA’s ESOC mission control center in Darmstadt, Germany, is getting ready to establish data links to the satellite. 38)


Figure 19: In this image, ESA’s new Solar Orbiter spacecraft is seen during preparations for a vibration test campaign at the IABG facility in Ottobrunn, Germany, in March 2019 (image credit: ESA, S. Corvaja)

- The live links, dubbed ‘system validation tests’, will see the flight team connect their mission control system to the spacecraft, as they will in future when the control systems on ground ‘talk’ to the spacecraft in orbit via radio signals transmitted by a ground station antenna.

- “The prime objective of the system validation tests for any spacecraft is to validate that the mission control system can correctly send and receive telecommands to the satellite,” says ESA’s Jose-Luis Pellon-Bailon. “The tests also confirm that the spacecraft launch configuration is as expected by the post-launch Flight Control Procedures.”

- An initial series of system validation tests were run last summer, when the Solar Orbiter was still at its manufacturer, Airbus Defence & Space UK, in Stevenage.

- “Since then, it has moved to Ottobrunn where we will run the next series of tests in early May and early August, lasting nine days in total and running around the clock,” says Jose-Luis.

- “Solar Orbiter will then move to the US for launch from Cape Canaveral, where we will run a final series of connection tests at the end of November.”

- Solar Orbiter will be launched in 2020 to study how the Sun creates and controls the heliosphere, the vast bubble of charged particles blown by the solar wind into the interstellar medium.

• April 1, 2019: Selected in 2011 as the first medium-class mission in ESA's Cosmic Vision program, Solar Orbiter was designed to perform unprecedented close-up observations of the Sun. The spacecraft carries a suite of 10 state-of-the-art instruments to observe the turbulent, sometimes violent, surface of the Sun and study the changes that take place in the solar wind that flows outward at high speed from our nearest star. 39)

- Having successfully completed its thermal-vacuum tests in December, Solar Orbiter has been subjected to a new series of arduous environmental tests at the IABG facility in Ottobrunn, Germany, including intense shaking of the spacecraft to ensure that it will survive the stress of launch.

Figure 20: The Solar Orbiter spacecraft is undergoing important pre-launch tests at the IABG National Space Center in Ottobrunn, Germany, ahead of its launch, scheduled for February 2020. Solar Orbiter is an ESA-led mission with strong NASA participation. The spacecraft was built and is being tested by Airbus. This film contains interviews with César García, ESA Solar Orbiter Project Manager, and Ian Walters, Solar Orbiter Project Manager at Airbus Defence and Space (video credit: ESA, Published 2 April 2019)


Figure 21: An infrared view of the Solar Orbiter spacecraft, which is currently undergoing a series of tests at the IABG facility in Ottobrunn, Germany, ahead of its launch, scheduled for February 2020 (image credit: Airbus Defence and Space/IABG)

- Solar Orbiter’s unique orbit will allow scientists to study our parent star and its corona in much more detail than previously possible, and to observe specific features for longer periods than can ever be reached by any spacecraft circling the Earth. In addition, it will measure the solar wind close to the Sun, in an almost pristine state, and provide high-resolution images of the uncharted polar regions of the Sun.

- After the preliminary definition and design phase, the mission started its integration and qualification in 2016, including environmental testing of the spacecraft as well as validation of all mission systems and subsystems.

- The first phase of Solar Orbiter’s environmental testing campaign was conducted in IABG’s special thermal-vacuum chamber in December 2018. Inside the chamber, powerful lamps are used to produce a ‘solar beam’ that simulates the Sun's radiation to demonstrate that the spacecraft can sustain the extreme temperatures it will encounter in the Sun's vicinity.

- The image of Figure 21 was taken with an infrared camera, and the coloring indicates the temperatures of the spacecraft surface, corresponding to the range indicated in the color bar on the right-hand side. During this thermal-vacuum test on the spacecraft, the solar beam was used at its maximum flux of about 1800 W/m2, reaching temperatures up to 107,6 ºC. An additional thermal-vacuum test was conducted on the heat shield that protects the entire platform from direct solar radiation: during this test, which used infrared plates to simulate the Sun’s heat, the heat shield reached higher temperatures, up to 520 ºC, similar to what it will experience during operations.

- In this view, the spacecraft panel that will face the Sun is visible on the left, covered with the heat shield. The dark elements visible in the upper part of the panel are sliding doors that will open the path for sunlight to reach the remote-sensing instruments during science operations. Some of the thrusters that will be used to control the spacecraft orbit and to perform maneuvers are hosted on the panel that is visible on the right in this view.

Figure 22: This video shows the Solar Orbiter spacecraft during tests conducted in December 2018 in the thermal-vacuum chamber at the IABG facility in Ottobrunn, Germany (video credit: Airbus Defence and Space/IABG)

Legend to Figure 22: Filmed with an infrared camera, the video shows the rotation of the spacecraft. It starts by displaying the +Z panel, on which the medium-gain antenna and some of the thrusters are located, slowly revealing the Sun-facing panel of the spacecraft, covered with a heat shield to protect the entire platform from direct solar radiation. Sliding doors on the heat shield, visible in the upper part of the Sun-facing panel in this view, cover the feed-throughs of most of the remote-sensing instruments. - Inside the chamber, powerful lamps simulate the Sun's radiation to demonstrate that the spacecraft can sustain the extreme temperatures it will encounter in the Sun's vicinity.

- After completing the thermal-vacuum tests, Solar Orbiter also successfully concluded the mechanical testing phase, including intense vibration tests, shaking the spacecraft to ensure that it will survive the stress of launch.

• January 30, 2019: The spacecraft flight model has been readied by prime contractor Airbus in the UK. Due to launch in 2020, Solar Orbiter will observe the Sun and measure the solar wind from a minimum of 42 million km away, or less than one-third Earth’s distance. As a result, the spacecraft will be subjected to around 13 times the amount of solar heating that Earth-orbiting satellites experience, and to temperatures in excess of 500°C. 40)


Figure 23: A side view of ESA’s Solar Orbiter as it entered a vacuum chamber for thermal vacuum testing at the IABG test facility in Ottobrunn, Germany, last month (image credit: Airbus DS)

- Solar Orbiter’s main body will be protected from direct sunlight by a Sun-facing multi-layer titanium heat shield. The 1.1-m diameter high-gain antenna seen here will be deployed from the body of the spacecraft to, transmit science data back to Earth in high-bandwidth X-band.

- The antenna’s black color is unusual. It is covered with the same kind of protective, high temperature coating as the front of Solar Orbiter’s heatshield, based on burnt-bone charcoal. Developed by the Irish company ENBIO, this ‘Solar Black’ coating was selected because it can maintain the same color and surface properties despite years of exposure to unfiltered sunlight and ultraviolet radiation.

- The high-gain antenna is placed at the end of a maneuverable 1-m long boom allowing Solar Orbiter to maintain a reliable, high-bandwidth link with Earth throughout its science-gathering phase.

- This test campaign – using powerful lamps to simulate the Sun’s radiation – began by simulating the conditions the spacecraft will undergo as it maneuvers to its operational orbit through flybys of Earth and Venus.

- "During 99% of the mission operations time, the heatshield will protect Solar Orbiter, but there will be more than a dozen maneuvers when one of the side panels will be exposed to sunlight," explained Claudio Damasio, ESA's Solar Orbiter project thermal engineer. "Therefore, we need to know how the Proto Flight Model responds when the exterior of the insulation on these panels reaches a temperature of about 120–150 degrees Celsius."

- For practical reasons, some elements, such as the solar arrays and the instrument boom, were not integrated with the spacecraft during the test. They were integrated this month on the spacecraft, which will next undergo a series of mechanical and electromagnetic compatibility tests.

• September 20 , 2018: After several years of development, the main communications dish on ESA's Solar Orbiter is now ready to be integrated with the pioneering spacecraft. 41)


Figure 24: Solar Orbiter High-Gain Antenna during vibration testing (image credit: SENER Aerospace / European Test Services)

- Solar Orbiter's HGA is located on the end of a 1-m long, movable boom. The boom can extend above or below the spacecraft's heat shield – which provides protection from the high levels of solar flux near perihelion – to achieve a reliable link with ground stations on Earth and transmit the huge volumes of data generated by the mission's scientific payload.

- Besides the challenging thermal environment, the antenna must also be made of conducting material to avoid a build-up of electrostatic potential that would have a negative impact on the payload's capability to perform solar wind measurements.

- To this aim, the 1.1-m diameter HGA reflector is made of titanium alloy, covered with a new type of high-temperature coating that is based on burnt-bone charcoal. Developed by Irish company EnBio, this Solar Black paint was chosen because it can maintain the same 'thermo-optical' properties even after years of exposure to unfiltered sunlight and ultraviolet radiation.

- Due to the mission's unique orbit, the throughput of the data downlink is highly variable – from tens of bits per second up to 1 Mbit/s. Most of the time, data will, therefore, initially be stored in Solar Orbiter's onboard memory before it is sent back to Earth at the earliest possible opportunity.

• March 28, 2018: Solar Orbiter will be used to examine how the Sun creates and controls the heliosphere, the vast bubble of charged particles blown by the solar wind into the interstellar medium. The spacecraft will combine in situ and remote sensing observations to gain new information about the solar wind, the heliospheric magnetic field, solar energetic particles, transient interplanetary disturbances and the Sun's magnetic field. 42)

- Scheduled for launch in 2020, the mission will provide close-up, high-latitude observations of the Sun. Solar Orbiter will have a highly elliptic orbit – between 0.9AU at aphelion and 0.28AU at perihelion. It will reach its operational orbit three-and-a-half years after launch by using GAMs (Gravity Assist Maneuvers) at Earth and Venus. Subsequent GAMs at Venus will increase its inclination to the solar equator over time, reaching up to 25° at the end of the nominal mission (approximately 7 years after launch) and up to 34° in the extended mission phase.

- Being close to the Sun allows for observations of solar surface features and their connection to the heliosphere for much longer periods than from near-Earth vantage points. The view of the solar poles will help us to understand how dynamo processes generate the Sun's magnetic field.

- The science payload of Solar Orbiter comprises both remote-sensing and in situ instruments. The in situ instruments will operate continuously. During each orbit, the complete instrument suite will be operated around closest approach, and at the minimum and maximum heliographic latitudes – the segments of the orbit where Solar Orbiter will be farthest below and above the solar equator. Since the orbital characteristics will change in the course of the mission, individual orbits will be dedicated to specific science questions.

• October 2, 2017: Now being fitted with its state-of-the-art instruments, ESA’s Solar Orbiter is set to provide new views of our star, in particular providing close-up observations of the Sun’s poles. 43)

- Following its launch in February 2019 and three-year journey using gravity swingbys at Earth and Venus, Solar Orbiter will operate from an elliptical orbit around the Sun. At its closest it will approach our star within 42 million km, closer than planet Mercury.


Figure 25: An artist’s impression of Solar Orbiter in front of the stormy Sun is depicted here. The image of the Sun is based on one taken by NASA’s Solar Dynamics Observatory. It captures the beginning of a solar eruption that took place on 7 June 2011. At lower right, dark filaments of plasma arc away from the Sun. During this particular event, it watched the plasma lift off, then rain back down to create ‘hot spots’ that glowed in ultraviolet light [image credit: ESA/ATG medialab; Sun: NASA/SDO/ P. Testa (CfA)]

- Solar Orbiter’s over-arching mission goals are to examine how the Sun creates and controls the heliosphere, the extended atmosphere of the Sun in which we reside, and the effects of solar activity on it. The spacecraft will combine in situ and remote sensing observations close to the Sun to gain new information about solar activity and how eruptions produce energetic particles, what drives the solar wind and the coronal magnetic field, and how the Sun’s internal dynamo works.

- Its 10 scientific instruments are in the final stages of being added to the spacecraft before extensive tests to prepare it for the 2019 launch from Cape Canaveral, USA. Solar Orbiter is an ESA-led mission with NASA participation.

• September 2017: The manufacture and delivery of spacecraft FM units and subsystems is approaching completion, in particular all AOCS (Attitude and Orbital Control System) equipment has been delivered. The spacecraft FM integration is ongoing. Functional testing continues on the two spacecraft Engineering Test Benches and on the FM avionics already installed on the spacecraft. 44)

- Of the remaining spacecraft units in production, the FM heat shield is ready for environmental tests; the solar generator QM wing completed its environmental testing; the first FM wing is under testing, and the second FM wing is being integrated. The payload radiator panel is being fitted with its thermal management equipment. Solutions for straylight protection, for surface conductivity and for cleanliness and contamination control are being manufactured.


Figure 26: Solar Orbiter’s Flight Model heat shield showing the colder set of multi-layer insulation, with 28 layers of embossed aluminised material, the star-shaped supports that will hold the 18 layers of titanium insulation on the hotter side looking towards the Sun, and the feed-through tubes for the instruments behind the shield (image credit: TAS)

• On April 28, 2017, TAS (Thales Alenia Space) announced two milestones in the Solar Orbiter mission to investigate the Sun and the heliosphere. It has completed construction of the heat shield on behalf of Airbus Defence and Space for ESA’s Solar Orbiter spacecraft, and delivered to the Italian space agency ASI the scientific instrument METIS (Multi-Element Telescope for Imaging and Spectroscopy). 45)

- The heat shield, designed and produced by TAS in Turin, Italy for Airbus DS, has been shipped to the IABG facility in Ottobrunn, Germany for final thermal-mechanical acceptance tests. The heat shield was a daunting technological challenge because it will have to protect the satellite from the intense solar radiation present at such close range. It is sized to keep the entire satellite in the shade, radiating the accumulated heat into deep space. The outermost layer is made of titanium, and can withstand temperatures up to 600ºC.

- With these two latest milestones in the Solar Orbiter program, Thales Alenia Space continues to confirm its technological expertise for leading-edge space exploration endeavors. The company has already developed advanced scientific instruments for the Planck and GOCE missions, and will provide sophisticated radio instruments for the upcoming BepiColombo mission to explore Mercury and the Euclid satellite designed to map the geometry of the “dark universe”.

• April 26, 2017: In late Fbruary 2017, CSL (Centre Spatial de Liège, Belgium) reported already on the EUI (Extreme Ultraviolet Imager) assembly. Two months later, not only is this instrument fully assembled, it has also been aligned, thermal-vacuum tested, and has undergone an extensive vibration testing ("Shaken, not stirred"). Last week, it has been shipped to PTB (Physikalisch-technische Bundesanstalt) in Germany for final calibration at a synchrotron source (particle accelerator). The final issues are being addressed and the instrument is ready for its scheduled delivery to ESA in May. 46)

• March 16, 2015: The Structural and Thermal Model (STM) for the Solar Orbiter mission left the Airbus Defence and Space premises in Stevenage (UK) at the end of March for mechanical testing at IABG in Ottobrunn, Germany. This test is a crucial stage in the development of the Solar Orbiter. At the end of this three-month test campaign, the STM will be shipped back to Stevenage for further building before being shipped again to IABG for thermal testing. 47) 48) 49)


Figure 27: Photo of the STM during testing at IABG in Ottobrunn (image credit: IABG)

• On March 13, 2015, the Solar Orbiter passed a major milestone when the heat shield was attached to the engineering model (Figure 28, left). MSSL (Mullard Space Science Laboratory) of UCL (University College London) has a significant involvement in the Solar Orbiter mission on both the remote sensing and in-situ sides. On the remote sensing side, the electronics box for the EUI (Extreme Ultraviolet Imager) is being built at MSSL, while on the in-situ side, MSSL is the PI (Principle Investigator) institute for the SWA (Solar Wind Analyzer) suite of instruments. 50)


Figure 28: The photo shows the heat shield on the left and on the right, Prof. Louise Harra (EUI Co-PI) and Prof. Chris Owen (SWA PI) at Airbus DS in Stevenage to see the Solar Orbiter heat shield being attached (image credit: UCL/MSSL)

• Fall 2014: Six instruments have passed CDRs: SolOHI, MAG, SPICE, EUI, PHI and EPD. The STIX and SWA CDRs were performed and awaiting outcome. Two CDRs (RPW, METIS) will follow. Various instrument STM (Structural/Thermal Model) items have been delivered to prime contractor facilities and integrated on the STM spacecraft. 51)

• June 3, 2014: ESA’s Solar Orbiter mission has undergone its latest major test: its protective shield has been subjected to concentrated sunlight to prove it can cope with the fierce temperatures close in to our parent star. The outcome ensures it will balance solar illumination, the cold of deep space and internal heat sources to maintain the perfect operating temperature. 52)

• May 2014: Members of ESA’s Solar Orbiter team watch expectantly as an essential part of the spacecraft is lowered into Europe’s largest vacuum chamber: the multi-layered shield (of size 3.1 m x 2.4 m) that will protect their probe from the Sun’s remorseless glare. 53)

- ENBIO, a surface treatment technology enterprise based on the Belfield Campus in Nova UCD (University College Dublin, Ireland), was awarded the contract to coat the main heatshield for ESA’s Solar Orbiter mission. After undergoing and completing an extreme test process at the ESA/ESTEC in the Netherlands, the ENBIO solution was approved for use on flight hardware. The solution combines some old and new thinking: a pigment used in 30,000-year-old cave paintings and ENBIO’s patented CoBlast process. 54)

- To provide such a system, ENBIO has been collaborating with ESA and Airbus Defence & Space since 2011, to develop a novel protective CoBlast Skin, called SolarBlack. It is critical that the Skin maintains its thermo-optical properties, despite years of exposure to extreme infrared and ultraviolet radiation, while not shedding material or outgassing vapor, which would risk contaminating Solar Orbiter’s highly sensitive instruments. Additionally, the Skin needs to be conductive to avoid the build-up of static charge which might threaten a disruptive or destructive discharge to the craft. 55)

- SolarBlack is a CoBlast Skin of black calcium phosphate, which will be applied to the outermost titanium sheet of Solar Orbiter’s multi-layered heatshield. It will be deployed via ENBIO’s patented CoBlast process, which replaces, in one process step, a metal’s natural oxide surface layer with a desired functional Skin – in this case SolarBlack. What makes CoBlast unique is the direct bond produced between the desired Skin and the underlying metal, without a troublesome oxide layer, providing the durability and adhesion required for skin integrity under such extreme conditions. CoBlast is also an environmentally friendly process, requiring no chemical, vacuum or thermal inputs. - SolarBlack has been qualified to meet the demands of this mission and is being specified on an increasingly wide variety of additional applications including sensor internals and heatshields.


Figure 29: On May 2, 2014, the engineering model of the sunshield, sandwiched together from multiple layers of titanium and outermost carbon coating, was placed into the 15 m-high and 10 m-diameter Large Space Simulator at ESA/ESTEC (image credit: ESA)

Legend to Figure 29: The Large Space Simulator is Europe's largest vacuum chamber for the sunshield's trial by sunlight. As its crucial test begins, all air will be extracted to produce space-quality vacuum, while the chamber walls are pumped with –190°C liquid nitrogen to mimic the extreme cold of deep space. - Then the light from 19 xenon lamps, each consuming 25 kW, will be tightly focused by mirrors into a concentrated beam of artificial sunlight upon the sunshield for a number of days.

• The CDR (Critical Design Review) for the spacecraft and the payload started in September 2013.

Launch: The Solar Orbiter spacecraft is projected to be launched in February 2020 by a NASA-provided EELV (Evolved Expendable Launch Vehicle) from KSC (Kennedy Space Center), Cape Canaveral, FL, USA (Ref. 42). 56)

- The launch of Solar Orbiter is now planned to take place in October 2018. The launch was previously targeted for July 2017. The decision to postpone the launch was taken in order to ensure that all of the spacecraft’s scientific goals will be achieved, with all the system’s components adequately tested prior to sending the spacecraft to the launch site. 57)

- In March 2014, NASA selected ULS (United Launch Services) LLC of Centennial, CO, to launch the Solar Orbiter Collaboration (SOC) mission to study the sun in July 2017. The Solar Orbiter will launch on an Atlas V 411 rocket from Space Launch Complex 41 at Cape Canaveral Air Force Station, Florida. 58)

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.


Figure 30: Solar Orbiter trajectory to orbit the Sun (image credit: ESA)


Figure 31: January 2017 launch: solar distance (image credit: ESA) 59)


Figure 32: 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.