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(As of July 17, Cassini had completed 13 such plunges.) Planetary protection works both ways: The probability of infecting another world with microbes from Earth cannot ever be zero.
Likewise, the possibility of bringing a non-native "bug" home to Earth on a returning spacecraft cannot be eliminated.
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(A full year on Saturn is nearly 29.5 "Earth years" long.) By watching both of the planet's hemispheres, the science community witnessed all four seasons in only half of Saturn's solar orbit.
That extended stay at Saturn meant Cassini would someday run out of rocket propellant and drift out of control.
The probe — a joint effort of NASA, the European Space Agency and the Italian Space Agency — will sacrifice itself to prevent an accidental crash that could contaminate one of the Saturnian moons where native life may exist, or may someday evolve.
Green = the prime mission (2004-2008), orange = the equinox mission (2008-2010), purple = the solstice mission (2010-2017, including the Grand Finale plunge into Saturn's atmosphere).
This clever blossom of gravity and inertia — termed the solstice mission — let researchers investigate the changes of Saturn's seasons, ultimately watching the planet change through half of its year.
[Latest Saturn Photos by Cassini] In 2009, with the primary mission complete and a completely healthy spacecraft well into a 27-month extended mission, Cassini managers looked at a variety of "what next? Trajectory modeling showed that Cassini could have been sent on to survey the "ice giant" planets, not visited by a spacecraft since Voyager 2's quick flybys of Uranus in the winter of 1986 and Neptune in the summer of 1989.
Alternatively, Cassini could have been directed back inward, to explore giant Jupiter and its many moons, rounding out science started on NASA's Galileo mission in 1995.Some scientists argue that material has jumped from planet to planet since the beginning of the solar system and so there's no need to worry.