Hi Everyone! I'm the Italianmoose. You can call me Moose. I design spacecraft, real ones and fictional ones. Yes really, and it's a great way to kill a conversation. I also print the fictional ones.
In my day job I work in mission concepts, which is a fancy title for someone who works right at the start of a space mission, thinking up the initial concept. It's very different to most engineering jobs, I'm all "big picture" and less minutiae like what material bolts are made out of. However you do pick up some odd pieces of knowledge. So:
Why can't you paint it black?
Once a thermal engineer was having trouble keeping the modules in the satellite cool. Most satellites are made up of a number of big lumps of aluminium with circuitry inside, then solar panels glued to the outer surfaces or on big array wings. The aluminium is needed to reduce how much radiation the circuitry is exposed to, because it either degrades them over time or can short out sections wholesale if you've designed them wrong. Electronics generate heat when used, but in space you can't just duct in outside air to cool everything. In a vacuum suddenly you're left with the lousiest form of heat transfer, radiation to space. Heat gets moved from the core of the spacecraft to the outside via the physical connections between modules, and aluminium is a great conductor. If you need more then you can use an even more conductive material to move the heat outwards. Copper strips are common. For whatever reason the modules this engineer was concerned about needed a little extra. They settled on painting them black to give them a little boost from radiative heat.
Paint in space is hard. It does many unhelpful things. If you've ever accidentally knocked an old, peeling wooden beam and suddenly it's snowing, you will have encountered the first problem. The inside of a launcher - the big rocket that puts satellites into space - is not a nice place to be. Your satellite is sat on top of a large controlled explosion:
Your satellite is sat inside that tiny (actually IRL massive) cone-and-cylinder on top of the rocket called the fairing. The fairing takes the edge off the noise, heat, and air resistance you encounter as the rocket flings itself into space. It's still massively loud and the vibration is astonishing. That vibration will shake a badly-made satellite to pieces. So your paint has to survive been vibrated and buffeted for all of around 3 minutes from the launch pad to when the main engines shut off. Then there's another large shock as some explosive bolts are set off to cut the first stage, away (the first stage is most of the large cylinder in the picture. Your satellite is generally well insulated from this), and then the second stage starts. This is much quieter, but after the second stage cuts off and you're mostly out of the atmosphere there's another big bang as the fairing is split in two to let you out. Finally after some more second stage firings to get you to the right place, your separation system sets you free of the second stage and you're away. So you'd better hope your paint doesn't get flakes everywhere. Lens and mirror surfaces, and electronics, don't like having flakes of paint flying about.
Now you're on your own, and your paint is exposed to vacuum. Many materials when exposed to a vacuum sublimate, which is a posh word for turn to gas. Either that or the water they contain evapourates. The common term for this is "outgassing" for obvious reasons. This both messes up the properties of the original material - untreated carbon fibre is a particular culprit - and leaves you sat in a cloud of your own gases. These gasses get everywhere, and can condense or react with other spacecraft surfaces. So your extremely expensive finely polished mirror you have to take pictures of the Earth? Now that's got a thin layer of gunk over it and your pictures will be messed up. So you need a paint that can survive launch and also doesn't outgas.
Ever sat in the sun too long and got sunburn? Or had a colourful plastic chair out in the garden that, over time in the sun, faded? Both of these are examples of what ultraviolet light does to materials, even through the Earth's atmosphere. Take that away and it's so much worse! Plastics get brittle, paint fades or peels, and glass turns opaque under that heady mix of radiation the Sun pours out every day. So your paint needs to stay put and stay the right colour even after all that!
But wait, we've made the heady assumption that your satellite has made it to the launch intact! As anyone with an expensive paint job on their car will tell you, that stuff gets scratched. Modules in a satellite will get put together and taken apart again and also put through vibration, thermal, and shock tests to prove they can take it before you go to the expense of launch. At some point something will scratch that paint. Now you have the problem of fixing it.
You could paint over it? Trouble is, anyone will tell you that a patch can come off more easily. You also need a repeatable procedure to patch the paint and you'll need to assure your customer that it won't affect the thermal performance and has no risk of flaking.
You could glue the edges? That would stop it from flaking. But you've again got to prove it'll hold, and now you've got a space-rated glue. Which means COSHH forms as they're hazardous substances, you need to trace the mix and batch of the glue you use, and again a procedure to apply it. Because if something goes wrong on a satellite worth millions, you want to dashed well prove it wasn't your fault!
You could strip and repaint the module? The cleanest solution definitely, no new procedures for the painting. However this will now cost you a lot of time, not to mention a lot of money for a one-off. Time is very much money in space, and that module will be out of action for a while.
The best solution? Don't paint it black! Spend the time and money earlier on solving the problem instead of going for what appears to be a simple fix.
The only thing worse than painting it black? Painting it red. Red means remove before flight!
Further reading:
https://www.spaceflightnow.com/falcon9/001/f9guide.pdf - The Falcon 9 user guide. Contains all the information you need to design a satellite to launch aboard it. To an engineer this is a goldmine of information for mission and spacecraft design. To most people it's a very dry read.
https://www.microwavejournal.com/blogs/1-rog-blog/post/16667-what-is-outgassing-and-when-does-it-matter - What is Outgassing and When Does it Matter? Does what it says on the tin.
https://www.osti.gov/servlets/purl/10178461/ - The Effect of Ionising Radiation on Selected Optical Materials: An Overview. A fairly technical overview of ionising radiation on the materials commonly used for optics (lenses, mirrors, etc.) in space.
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