There are different kinds of decomposition.
The most rapid one is biological: bacteria and the
body's own enzymes both begin to break down
tissues within a few hours of death. These are
the processes that we generally call rotting, and
they produce the bad odors that we smell. Next
are scavengers such as insects and other animals,
but these probably won't be present in space,
unless they hitched a ride in the spacecraft.
Slower still would be heat degradation, where
proteins break down. Chemical exposure will
degrade what's left; bones will eventually decay
in weathering, and oxygen will react with proteins
and many other things. This won't happen in space
either. Finally, very slow processes such as
radiation would eventually break down molecules
over millions of years. The levels of radiation
in space are higher than on Earth but not so high
that tissues would degrade very quickly.
think there are three possibilities for
1. Unless there's a source of
heat nearby, the body will be quickly frozen, and
decomposition will take thousands or even millions
2. If there's heat, but no spacesuit
or spacecraft, the body will very quickly dry out,
because water evaporates extremely quickly in the
vacuum of space. This will almost completely stop
biological processes, and the lack of air will
prevent weathering and chemical degradation.
If the astronaut died while wearing their
spacesuit and the body stays warm, then
decomposing of tissue will be almost as fast as on
earth. But bones will last almost indefinitely.
Of course, if the body happens to fall in to a
planet, it will be destroyed by the extreme heat
of re-entry into the atmosphere, like a meteorite.
If he dies in his space suit, then yes, he
would decompose, because he still has bacteria and
such within his body that would decompose him. It
would use up all of the oxygen, however, so the
decomposition would become anaerobic very quickly,
going from respiration to fermentation.
Radiation would not significantly affect the
decomposition process. It would break up chemical
bonds and such, but our deceased astronaut is more
likely to collide with an object of some kind like
a meteor before that would really break him
If our astronaut is not in his suit,
then the vacuum would kill the bacteria
decomposing him in fairly short order.
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