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This is not nice perspective, but eventually it will happen. An astronaut falls out of spaceship because of damage caused by collision with other object, or because of suit decompression. The fluids from the body would evaporate, and if any bacteria would survive, than only as spores.
Does it mean the perfect mummification of the body? Or there will be some decay, caused by enzymes from damaged cells, for example?
It would seem that the ultra-cold vacuum would kill off most of the bacteria, etc in the body, as well as quickly boil off all of the water content. Here's a lovely description of the process from Focus magazine:
In space we can assume that there would be no external organisms such as insects and fungi to break down the body, but we still carry plenty of bacteria with us. Left unchecked, these would rapidly multiply and cause putrefaction of a corpse on board the shuttle or the ISS. Drifting exposed in the vacuum of space itself, however, this process would rapidly slow to a halt.
The low pressure would initially boil off most of the water and what was left would freeze, halting any biological processes. Depending on the trajectory of the corpse, there might be some warming from the side facing the Sun, but all this would do is accelerate the rate of water loss, leaving only a dried husk.
Other than decay caused on Earth by microorganisms, other factors for decay in space, where decay is defined as loss of pristine mummification of the original, freeze-dried corpse:
A body in orbit around the Sun would be impacted by the solar radiation. A quick search only found research on live tissue. Presumably, this would cause some disintegration of the mummified cells as well. It is unclear if orbital decay or solar wind damage would be the more significant erosion/decay vehicle.
A body in orbit around a planet would likely lose all cohesiveness to orbital decay prior to any significant loss due to other means.
A body drifting in open space, would be subject to micro meteor strikes as well as attraction to any other drifting bodies.
There are many factors; while the mummified body would not decay as fast as an Earth-bound body, it would not be eternally preserved.
Adding to the other answers, the body may decay very fast when colliding with space debris - depending on it's size. Probably more like 'exploding', as it would be brittle after freezing.
This assumes that the body is in an orbit where other debris exists - put that seems probable to me in general.
Answer: It depends.
You need to specify the situation.
Dr. Frank Poole found himself in a depressurize spacesuit, in a frigid orbit around Jupiter back in 2001. He likely froze solid before gut microbes had a chance to recycle him.
In 2013, Sandra Bullock found herself in Low Earth Orbit in a sealed suit, anticipating a warmer, but slower, death by asphyxia. Slowly rotating like a supermarket rotisserie chicken, her microbes would have plenty of time to keep her suit pressurized with tissue break-down gasses.
The end state of these two corpsicles would be quite different.
Even completely sterile tissue breaks down after death. Without oxygen-driven metabolism, compartmentalization inside the cells breaks down. Digestive enzymes are released and the cell auto-digests. This process is markedly slowed at low temperature, and essentially stops once frozen.
Similarly with micro organisms: their chemical machinery stops once frozen.
So the essential question is the rate of cooling. Sandra, who will be exposed to direct sunlight half her orbit and to the IR radiation of night-side Earth the rest of the time, will equilibrate to a temperature which may well be above freezing. Frank will largely be exposed to the background microwave temperature and get little warmth from the distant Sun.
