In October of 1997, nearly 20 years ago, NASA launched the last of its great probes to the outer planets. A joint mission with the European Space Agency, a single rocket sent Cassini and Huygens on a meandering path through the Solar System. Huygens plunged into the atmosphere of the moon Titan well over a decade ago, but the Cassini orbiter has been looping around Saturn for over 13 years. But in less than 24 hours, its time at the ringed planet will come to a close as Cassini plunges into Saturn's atmosphere.
This was the end that NASA had always planned for its hardware. Some of Saturn's moons are thought to be capable of harboring life. So, rather than risk contaminating those moons with life from Earth, Cassini and any microbes it harbors will burn up on entry into Saturn's atmosphere.
The decision to do this now is based on the dwindling supply of fuel for the probe's maneuvering engines, which will eliminate NASA's ability to make further adjustments in its orbit. With the chaotic gravitational interactions of a giant planet and multiple moons, there'd be no way to determine where Cassini would end up. So NASA is acting while it still manages the hardware's destiny.