The temperature of objects in space varies massively. A comet travelling close to the Sun might experience millions of degrees centigrade as it hurtles around our star, but when it swings back out to the edges of the Solar System its surface will plummet to -220°C.
In general, reaction speed is closely linked to temperature, the rule of thumb being the rate of reaction halves for every 10°C drop. So a reaction that takes one second on that comet just as its ice surface melts might then take 1×1067 years out in the Oort Cloud. However, there are other sources of energy that can power reactions: high energy cosmic rays or ultraviolet light from a nearby star can give chemicals enough energy to react at less astronomical timescales.
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