By 2017, Cassini will have spent 13 years in orbit around Saturn, following seven years of “cruise” on its way outward from Earth. The spacecraft is beginning to run low on rocket fuel. If left unchecked, this situation would eventually prevent mission operators from controlling the course of the spacecraft.
Two moons of Saturn, Enceladus and Titan, have captured news headlines over the past decade as Cassini data revealed the moons’ potential to contain habitable – or at least "pre-biotic” – environments.
In order to avoid the unlikely possibility of Cassini someday colliding with one of these moons and contaminating them with any hardy Earth microbes that might have survived on the spacecraft, NASA has chosen to safely dispose of the spacecraft in the atmosphere of Saturn.