# If solar radiation can heat up gas particles in the thermosphere, why will it not heat up a spacecraft?

NASA says that the reason why a spacecraft does not get hot in the thermosphere is because there are not enough gas particles to come into contact with the spacecraft to heat it up.

If you were to hang out in the thermosphere, though, you would be very cold because there aren't enough gas molecules to transfer the heat to you.

So the above statement would also apply to a spacecraft,

But if solar radiation is heating up the gas particles, it must also heat up the spacecraft. Is this the case? If so, then do spacecraft like Apollo 11 have coatings to reflect the solar radiation away?

• Can you provide a reference for where "NASA says" this? Otherwise this is an uninformed rant. – Organic Marble Jan 4 '20 at 23:55
• spaceplace.nasa.gov/thermosphere/en – Sean View Jan 4 '20 at 23:56
• You should read up on the thermodynamic definition of temperature. – Organic Marble Jan 5 '20 at 0:03
• This is an a bit poorly worded question in my view, which is not supported by the provided link. The sun will heat both, thermosphere and spacecraft. The spacecraft being dense, can reradiate heat and stays relatively coold. The hot thermosphere does however not heat the spacecraft, is that the question? – AtmosphericPrisonEscape Jan 5 '20 at 3:54
• You are misreading that NASA page.It does not say that the Sun does not heat the spacecraft. What it does say is that the thermosphere does not heat the spacecraft. – David Hammen Jan 5 '20 at 10:05

An interesting question!

Preface: I'm not nearly an expert on atmospheric heating. I'll try to analyze the problem from first principles - someone more experienced could do better.

There are not enough gas particles to come into contact with the spacecraft to heat it up.

This seems quite reasonable. Thin, diffuse gasses can be very hot without conducting much heat - particles in the glow discharge in a fluorescent tube lightbulb can exceed 10,000 K, and the glass doesn't even get hot to the touch.

solar radiation is heating up the gas particles, it must also heat up the spacecraft.

Perhaps not! If you think about how the optical properties of gases and spaceships could differ, you can probably come up with a bunch of options. Let's look at some ways:

• Spaceships are often shiny, or at the very least bright white.

For instance, the Apollo command module was coated in terrifically reflective aluminized mylar. Polished aluminum is so perfectly reflective that it's used to make telescope mirrors - it reflects >95% of all light across the spectrum. (Side note: the mylar backing was burned off during the searing heat of re-entry, so capsules in museums aren't very shiny).

You might argue 'ah, but gases are transparent! Surely they won't get hotter than a solid object!' True! But the atoms which comprise the gas are not transparent - and the definition of temperature refers only to the amount of energy that the atoms are absorbing.

Speaking of spectrum, modern thermal control engineering is really fantastic. There are materials with engineered spectral emissivity that reflect the sun's spectrum while emitting in every other wavelength - something like a photographer's notch filter. These can reach almost absolute zero totally passively, even in direct sunlight! (Youngquist et al, Cryogenic Selective Surfaces / "Solar White")

What if that's still not enough? Often a craft can be dunked in white paint and still overheat, especially if there's a lot of power being produced internally. Then we use Multi-Layer Insulation and huge radiators that emit infrared light. Radiators can be made essentially as big as required within the leeward shadow of the craft.

You can see how these are arranged on the ISS - they're almost always edge-on to the sun, and an ammonia loop dumps the ISS' ~300 Kw into them.

• Not all of the mylar backing burned off during reentry. For example, see this photo of Apollo 13 CM after recovery. Nearly half of the mylar survived reentry. I believe the remaining mylar was then stripped off (for research reasons, maybe?) before capsules went to museums. – Sergiy Lenzion Jan 5 '20 at 0:56
• This answer is so good that I'm going to retract my close vote on the question. – Organic Marble Jan 5 '20 at 1:57
• @OrganicMarble good point, I've done so as well. SE is primarily about good answers to on-topic questions and that's what happened. I'll keep my down vote in place of course. – uhoh Jan 5 '20 at 3:03
• @uhoh, agreed, but I'm still smarting from the 'con artist' stuff. – Organic Marble Jan 6 '20 at 23:42
• @OrganicMarble if the OP ever comes back to re-insert stuff, the edit then allows voting to be changed again. Right now the question is fine and brings readers to several high quality and informative answers. It's hard sometimes, but we should always vote on the post itself, not the user. – uhoh Jan 6 '20 at 23:46

You are misreading that NASA page. It does not say that the Sun does not heat spacecraft. What it does say is that the thermosphere does not heat spacecraft.

The thermosphere is much warmer than the Stefan-Boltzmann Law suggests. That law suggests a daytime temperature for a flat black plate face-on to the Sun on one side and face-on to empty space on the other side of about 57.8° C (136° F). So, why is the thermosphere so hot?

The answer lies in the nature and makeup of the atmosphere. Molecular dispersion dominates over turbulent mixing in the thermosphere. Unlike the lower portions of the atmosphere, the components of the thermosphere are not well mixed. The highest energy portions of sunlight split diatomic nitrogen and oxygen into monatomic components, with the highest energy components migrating upward. Those monatomic components might well absorb yet another high energy photon before colliding with another gas molecule. This makes the thermosphere increase in temperature as altitude increases, and do so well beyond the temperature suggested by the Stefan-Boltzmann Law.

The reason the thermosphere does not heat spacecraft is because of the extremely low density of the thermosphere. Even though the thermosphere is hot (by human standards), this low density limits the heat transfer rate from the thermosphere to spacecraft to very low values. A similar phenomenon occurs to spacecraft in interplanetary space. There the solar wind is extremely hot, over a million degrees. But once again the low density means that essentially no thermal energy is transferred from the solar wind to those interplanetary spacecraft.

Of course solar radiation heats up a spacecraft just as solar radiation heats up your body when you stand in sunlight on a clear day. During the coast period to the moon, the Apollo spacecraft had to rotate to avoid overheating one side of the ship. The side in shadow would radiate heat away as the side in sunlight heated up.

In fact the interesting question here is not why the spacecraft stays cool: it's why the thermosphere is so hot.

If you model the spacecraft as a perfect spherical black body heated only by the Sun (this is not a particularly accurate model but it will do to get a ballpack figure), then in the neighbourhood of Earth you'll get a temperature, in space, of about 278K: about 5 degrees above freezing. Not surprisingly this is not far from the average temperature of the Earth.

In fact things are worse than this: the spacecraft is generating heat because it's full of machinery and people, and it's also not perfectly conductive. This means that the part facing the Sun will get very much hotter and the part away from it will get very much colder, and the average temperature of the whole thing will end up significantly hotter than 278K. So to keep the temperature sane you need to spin the spacecraft, and also cover it with something which raises its albedo, ideally to visible light while keeping it low for IR. And you probably need to do various more aggressive cooling things as well to deal with heat generated in the spacecraft.

So that would deal with a spacecraft in a vacuum. But in fact it's not in a vacuum, quite: it's in the thermosphere: so then there are two questions:

• why is the spacecraft not heated by the thermosphere?
• why is the thermosphere so hot?

The answer to the first of these is easy: the thermosphere has an extremely low density. That means that the rate at which particles (atoms) of the thermosphere collide with the spacecraft & transfer energy (heat) to it is relatively tiny, so the rate at which energy being added to the spacecraft by this mechanism is tiny, which means the increase in temperature from this mechanism is tiny as the spacecraft only needs to be very slightly warmer to radiate all the heat away it is getting from the thermosphere. One way to see that the density must very low is simply to consider that the spacecraft is in orbit through the thermosphere: spacecraft do not orbit in atmosphere, or not for very long.

The answer to the second question is subtle, and I'm not sure I understand it. I think that David Hammen's answer provides much better information than I could.

• +1 for "...simply to consider that the spacecraft is in orbit through the thermosphere..." – uhoh Jan 6 '20 at 23:32