As the others said, because it is running out of fuel. They want to finish it in a controlled way instead of just letting it crash, which could end up with it crashing into some habitable region (Enceladus geysers say, or remote chance of hitting habitats elsewhere e.g. Titan cryovolcanism?).
They had the option to send it on to another planet, as it turns out that through flybys of Saturn's moons they can send it out to a point on the "interplanetary highway" that gives you access to just about the whole solar system. Indeed, through clever use of flybys, they could have sent it right out to the Kuiper belt - or right in to the asteroid belt given enough time like several decades. They did consider this.
But after looking at it carefully, they thought the chance of still having a functioning spacecraft able to do good science at the end wasn't good enough to take the risk. They could have sent it on a course to impact with Jupiter, but again, there's a decent chance they wouldn't get any good science return about Jupiter when they got there. It would be safe enough according to planetary protection, but just not likely to be very interesting.
They could have put it into a permanent orbit around Saturn far from any other moon, but the science return wouldn't have been that great without any ability to maneuver any more, in a circular orbit in the plane of Saturn's rings, and no longer doing close flybys of anything. They could have put it into a permanent orbit around Titan which would have a lot of science return, but there isn't enough fuel for that. At the end of the Cassini mission, could the probe be put into orbit around Titan to observe it on a permanent basis?
While impacting with Saturn means they can get some really close up images of both Saturn and its rings on the way in. It seems the best decision. And with enough fuel left, they can do some decent science on the way towards the crash as well.
They had the option to send it on to another planet, as it turns out that through flybys of Saturn's moons they can send it out to a point on the "interplanetary highway" that gives you access to just about the whole solar system. Indeed, through clever use of flybys, they could have sent it right out to the Kuiper belt - or right in to the asteroid belt given enough time like several decades. They did consider this.
But after looking at it carefully, they thought the chance of still having a functioning spacecraft able to do good science at the end wasn't good enough to take the risk. They could have sent it on a course to impact with Jupiter, but again, there's a decent chance they wouldn't get any good science return about Jupiter when they got there. It would be safe enough according to planetary protection, but just not likely to be very interesting.
They could have put it into a permanent orbit around Saturn far from any other moon, but the science return wouldn't have been that great without any ability to maneuver any more, in a circular orbit in the plane of Saturn's rings, and no longer doing close flybys of anything. They could have put it into a permanent orbit around Titan which would have a lot of science return, but there isn't enough fuel for that. At the end of the Cassini mission, could the probe be put into orbit around Titan to observe it on a permanent basis?
While impacting with Saturn means they can get some really close up images of both Saturn and its rings on the way in. It seems the best decision. And with enough fuel left, they can do some decent science on the way towards the crash as well.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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