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Solar Orbiter (SolO) is a planned Sun-observing satellite, under development by the European Space Agency (ESA). Due to its proximity to the Sun, it receives about 13 times more solar energy than we receive on Earth, causing the temperature of its sunward-facing side to soar to 600°C. So there must be a Thermal Protection System (Heat Shield) to prevent the spacecraft from melting. ESA's webpage gives the following explanation:

To protect the spacecraft and payload, a high-tech heat shield has been designed that is made of several protective layers of titanium, the outer of which is coated with a protective skin called “Solar Black", which has been developed specifically for Solar Orbiter.

Source: Solar Orbiter – structural and thermal model of heat shield

We know that any spacecraft gets heated by the Sun due to Thermal Radiation. White colour is a good reflector and a poor absorber of thermal radiation. On the other hand, black colour is a good absorber and a good emitter of thermal radiation. According to me, white is the best choice of heat shiels, as it is far better to reflect the radiation instead of absorbing it and emitting it. If you remember properly, the Parker Solar Probe, also had a heat shield, but its sunward-facing side was white in colour, and not black.

The following rendering shows that the heat shield is black in colour:

enter image description here

The following image shows only the heat shield ready to be tested in a Thermal Vacuum Chamber:

enter image description here

Image Source: Solar Orbiter sunshield

I don't understand why is the sunward-facing side of ESA's Solar Orbiter is black in colour, instead of white. Why did they choose black over white (on a thermodynamic basis)?

A few differences, which I could think of between Parker Solar Probe (PSP) and Solar Orbiter (SoIO), relevant to our analysis are as follows:

  • Distance from the centre of the Sun: PSP - 9.86 solar radii (6.9 million kilometres or 4.3 million miles); SoIO - ~60 solar radii

  • Material of the Heat Shield: Question/Answer regarding PSP

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    $\begingroup$ Answers to Why are RTGs different colors? explain that materials can have one absorbance/emittance at visible wavelengths (where heat is absorbed) and a very different one at thermal wavelengths (where heat is radiated), but in those cases the material had a lower absorbance in the visible. This is a real puzzler because it apparently has very high absorbance in visible light, where the Sun's output is maximum. Cool question! $\endgroup$
    – uhoh
    Oct 29, 2019 at 16:30
  • $\begingroup$ Paywalled paper that probably answers this: sciencedirect.com/science/article/abs/pii/S0094576509001702 $\endgroup$
    – Hobbes
    Oct 29, 2019 at 17:15

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Apparently, black was chosen despite its thermal properties. This 2005 paper was written at the end of the assessment phase where technical feasibility of the mission has been demonstrated.

It shows several options were studied, including a white shield.

Materials with a/e (absorption/emission constant, ed.) close to 1 could also be used. Typically this could be a Carbon-Carbon based shield. This material is also the baseline for NASA’s solar probe [4]. This alternative would have a much higher front layer temperature than some of the more white alternatives. However, the use of such materials could simplify the AIV/AIT (Assembly, Integration and Test/Verification. ed.) concept as there would be less need for testing degradation and the testing of the shield could be done with infrared lamps instead of a representative sun-flux.

Both using a black and a grey front layer have been shown as viable solutions for the heat shield, which could be implemented while keeping complexity to a minimum.

(emphasis mine). They chose black because it represented the easiest development path, and despite the higher temperatures the front shield would experience.

The white shield also mentioned in the study uses different materials:

This MLI (multilayer insulation, ed.) would use ABS (Alumina- Boria-Silica, ed.) as the front layer, which would have an a/ε of about 0.45 and high temperature resistant metallic foils such as titanium. The MLI would be supported by a sun shield support structure consisting of aluminium honeycomb.

This ABS MLI would be a new development, while carbon-carbon is well-known.

They investigated coatings, and found them unsuitable:

Only non-organic coatings would be possible to use due to the UV-flux. These coatings could potentially have a rather low a/ε value, although the End of Life (EOL) value would be uncertain. With a typical white coating it is assumed that the EOL a/ε could be as about 0.6. Nevertheless it would require extensive testing in a representative environment to make sure that this value is not exceeded, as the coating is likely to not sustain very high temperatures. Using coating on a metal will also have the problem that if some part of the metal gets exposed to direct sun light the metal could heat up drastically and thus the shield could fail.

This meant it wasn't feasible to take the carbon-carbon shield and just paint it white.

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  • $\begingroup$ Thanks for the answer. May I know how complexity increases due to changing the colour of the heat shield? One reason I can think of is PSP had no instrument peeking through tiny holes on the heat shield, but SoIO has. Am I right? $\endgroup$
    – Vishnu
    Oct 30, 2019 at 14:43
  • $\begingroup$ @Intellex added more info to answer $\endgroup$
    – Hobbes
    Oct 30, 2019 at 20:38
  • $\begingroup$ It looks great! Thanks :) $\endgroup$
    – Vishnu
    Oct 31, 2019 at 15:35

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