In Wikipedia they mention that the Cassini spaceship will be destroyed in by sending it into Saturn's atmosphere:

However, due to the spacecraft's dwindling fuel resources for further orbital corrections, it is currently planned to be destroyed by diving into the planet's(Saturn) atmosphere in September 2017. This method of disposal was chosen to avoid potential biological contamination of Saturn's moons.

This is to make sure that it will never crash into one of the moons (like Titan) where biological material would contaminate.

But how can a spaceship be a source of potential biological contamination?

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    $\begingroup$ A fun google search on the topic is "Planetary Protection." The efforts taken to not contaminate other planets with life such as bacteria are extraordinarily fascinating. As it turns out, its actually really difficult to eradicate all life completely, even on something so small as a rover! $\endgroup$ – Cort Ammon Apr 14 '17 at 2:51

Currently all spacecraft start out on Earth and while fairly stringent measures are taken during manufacture to keep them clean it is pretty much impossible to guarantee that no bacteria, viruses, spores or other biological material get in somewhere. Certainly anything which is exposed to untreated air at any point is quite likely to be contaminated with something.

Although it is pretty unlikely it's still a big deal: if the craft was allowed to crash on a moon and evidence of life was subsequently found there there would always be some doubt about where it came from.

Obviously there are cases like landers where things are different but if you are planning for a vehicle to come into contact with a planet or moon you can take steps to mitigate the risk.

Equally as the craft is coming to the end of its life anyway it may well be just as easy to dispose of it in a controlled way while you still can than just leave it without knowing exactly where it will end up, potentially posing a hazard to other missions.

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    $\begingroup$ I totally vote we contaminate an area of the moon and see what happens $\endgroup$ – Mooing Duck Apr 13 '17 at 23:53
  • $\begingroup$ Good answer except for the last sentence. Space is big, and the chance of a future Saturn mission colliding with an abandoned Cassini probe is effctively zero. You'll note that no similar precautions are taken with Mars orbiters, even though Martian orbit is a much smaller volume of space than Saturn orbit. $\endgroup$ – Mike Scott Apr 14 '17 at 5:22
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    $\begingroup$ @MikeScott Space is big, but certain places are not. If a later trajectory happened to match a prior one, for whatever reason, then all the vastness of space means nothing to you. $\endgroup$ – can-ned_food Apr 14 '17 at 6:18
  • $\begingroup$ If we don't seed some life onto some other planets asap, there's a chance our current economies could collapse, another world war, global warming etc which will close the doors on interplanetary travel, perhaps for ever $\endgroup$ – Innovine Apr 14 '17 at 11:26
  • $\begingroup$ @MikeScott: Mars orbits are more stable than Saturn orbits (fewer moons to mess things up). $\endgroup$ – Hobbes Apr 14 '17 at 14:32

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