The common conception of satellite reentry is that they just burn up and are "gone". But the total mass must still be somewhere, right? What forms does it all end up as, and where?
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5$\begingroup$ They end up as gases in the upper atmosphere. Some as dust. $\endgroup$– UweAug 25, 2017 at 13:58
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3$\begingroup$ When large satellites enter the atmosphere, big chunks (and probably smaller ones also) do occasionally hit the Earth. So you might want to limit your question to the smaller, modern satellites that are designed to safely and completely dissipate in the atmosphere. $\endgroup$– uhohAug 26, 2017 at 5:32
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1$\begingroup$ There's a reason decommissioned satellites are de-orbited with care if possible. Past examples including Skylab and at least one Russian satellite have dropped significant and/or hazardous chunks of debris on Earth's surface. Completely combust/vaporize they do not. $\endgroup$– Anthony XAug 26, 2017 at 22:50
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$\begingroup$ @AnthonyX: Admittedly, the only reason that Russian satellite was hazardous was because it had a large nuclear reactor on board when it deorbited. $\endgroup$– VikkiFeb 22, 2020 at 1:34
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2 Answers
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The products of combustion, apart from gases, are small solid particles, 'ash', that slowly floats down to the lower atmosphere and, eventually, to the surface.
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1$\begingroup$ I'm not sure that "combustion" is really the correct term for what happens to satellites during re-entry. Do they really burn - combine with oxygen? Or is much of the mass simply vaporized and the molecules are dissociated through various thermal and plasma chemistry processes? Are there also larger particles and pieces that can not really be described as "ash"? If you can add a link or two to your answer that supports the concept of satellite combustion, that would help. However while you are reading you may discover explanations that are different, and want to revise your answer a bit. $\endgroup$– uhohAug 26, 2017 at 5:25
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1$\begingroup$ There's oxygen present, and some of the vaporized matter will probably combine with it. Hence, 'combustion' is right, I believe... $\endgroup$– xxavierAug 26, 2017 at 7:02
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1$\begingroup$ There is nitrogen, oxygen, and various metals and other elements all in an extremely hot and partially ionized environment. The chemistry is probably quite complicated. For the purposes of writing a good stackexchange answer on this, it's better to do some reading about it than just hypothesize on your own. $\endgroup$– uhohAug 26, 2017 at 9:02
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1$\begingroup$ Refer to Skylab and Kosmos 954 for what can really happen when a satellite re-enters. $\endgroup$ Aug 26, 2017 at 22:53
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5$\begingroup$ "Nitrogen, oxygen, and ... other elements all in an extremely hot and partially ionized environment" is a pretty good description of ordinary terrestrial fire, is it not? $\endgroup$ Feb 15, 2019 at 18:04
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What is the destination for mass of a candle when you light it up? Same things will happen if some inbound spacecraft speed towards earth surface hitting air molecules. Only difference is the outer shell of spacecraft is too hard to burnout totally like a candle but remains mostly intact losing very few amount of mass by various forms including combustion.
During reentry on earth it collide with air molecules and heated up high enough to produce bright light visible. If same thing was not spacecraft but asteroid made by soft material it would burn out leaving gas and dust of various particles based on material it made on. Same thing would happen if any satellite fall from orbit and enter earth atmosphere.
Normally spacecraft do reentry process to return earth surface but satellite do not have such entry process in normal operation planing. Earth returning spacecraft designed with high temperature in mind and "gone" material during reentering process is relatively small which turned to gas if not fine dust. These materials ultimately falls on surface of earth if not light enough to float on air. But amount is really negligible to count.
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$\begingroup$ You might want to do some further reading about this first before you try to explain it. I'm not sure concepts like "combustion" and a burning candle can really apply to what is happening to a satellite upon re-entry. See if you can support some of these with verifiable links, so that it doesn't look like you are just hypothesizing on your own. $\endgroup$– uhohAug 26, 2017 at 5:30
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$\begingroup$ When people asked for answer with "burns and gone " in mind, I don't see any problem relating burning candles (as example of something reacting comically by fire) and spacecraft producing enough heat producing light. There's a lot of explanation about reentry on internet. If that's understandable by OP this question would never arise. By the way "Satellite" normally don't do reentry type things in lifetime but spacecraft do. Cheers. $\endgroup$ Aug 26, 2017 at 21:42
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$\begingroup$ For complying with your reference links to backup information posted here is summary for satellite life ending practice slate.com/articles/news_and_politics/explainer/2005/04/… it seems support my info "Satellite not necessarily reentered but spacecraft do frequently" $\endgroup$ Aug 26, 2017 at 21:56
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1$\begingroup$ @Bauani Re-entering spacecraft are usually designed to survive re-entry (think Apollo, Soyuz, Shuttle), where satellites are not. However, satellites, space stations, etc. have to be designed with end-of-life in mind. "Abandoned in place" means it will either contribute to space clutter, adding to risk of Kessler syndrome or eventually fall to Earth anyway. Whether re-entry is planned or not, some parts may survive, so where possible, re-entry is targeted to occur over uninhabited areas like open ocean where unlikely to do harm. $\endgroup$ Aug 27, 2017 at 0:27