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Planetary protection is a guiding principle in the design and planning of planetary probes: avoiding carrying life forms from Earth to other planets and moons (or between such bodies). The design and operation of the probes and missions adhere to this. It is motivation for sterilizing the spacecraft prior to launch, and terminating missions with the destruction of the spacecraft in such manner as to kill life forms aboard, such as crashing Cassini straight into Saturn where the harsh atmosphere and the heat would destroy it rather than leaving it in an orbit that could collide with a moon at some point.
Note that the term planetary protection is also sometimes meant to include future defense of the Earth from impacts, e.g., from near-Earth objects.