Live every day as if it were your last and then some day you'll be right. ~H.H. "Breaker" Morant
Space.com has an interesting article on the ultimate fate of Cassini, the thusfar hugely successful probe circling Saturn and peeking through the clouds of Titan. The article brings up something we don't often think about: Sooner or later, our wonderful satellites that circle various planets and meander through the Solar System, these magnificent machines, wear out.
We're not talking about the ones that bought the farm due to human error or aliens shooting them down (those pesky Martians) but ones that complete their missions and just run down. Many that orbit the Earth just sink lower and lower until the atmospheric drag becomes too great, at which point they “nose in”, ending as flaming streaks in the sky. Occasionally, though, very large things come down, like Skylab and Mir, which require a more controlled approach. The usual intent is to drop them in the Pacific Ocean because, well, it's big and hard to miss.
Of course, Australians might point out that every time something large is brought down, pieces of it end up scattered across the Outback, but what's a few dead kangaroos amongst friends.
Some satellites were intended to come to a violent end. The Ranger series was designed to look for potential lunar landing sites. I vividly remember the pictures coming back as they descending rapidly to the Moon's surface before cratering. We had never seen the ground of a non-terrestrial place with such detail before. Even though the picture quality would be considered miserable today, it was great science then.
The Surveyors follow the Rangers, testing out landing methods that would ultimately be used by the LEMs. The Surveyors actually touched down softly on the Moon, where they sit to this day. The Apollo 12 mission, I think it was, landed near one and brought back a chunk from the satellite. This is the famous bit of jetsam that, when returned to Earth, was found to have bacteria on it that could be resuscitated, giving us the realization that microbes could conceivably hitch a ride inside a meteor and arrive on Earth.
Other probes were like that annoying little rabbit; they just keep going and going and going. Two of the Pioneer probes that sent back grainy pictures of Jupiter are chugging along at the edges of the Solar System. One of them was still transmitting weakly up to a couple of years ago. But the long distance record for both distance traveled and furtherest area code calling in belongs to Voyager 1 and 2. These puppies passed the Pioneers long ago, thanks to the gravity boosts received from the gas giants and are either headed for, are passing through, or are nearing the heliopause, depending on whose interpretation of data you believe. Thanks to their radioisotope thermoelectric generators (RTGs), they still have a trickle of power to run instruments and transmit back to Captain Kirk's ancestors.
Deep Impact, which shot a probe into a comet, is still chugging around the inner Solar System, fully functional and waiting for someone to come up with something for it to do. Ditto Deep Space One, which is putting along with its ion engine, patiently hoping for an assignment (I think there was a plan for a cometary flyby, but it's been a long time since I've heard anything about).
Occasionally, we've deliberately brought at least part of a satellite back. Stardust successfully landed in the desert, bearing interplanetary dust and comet grains. Genesis, as we painfully recall, had a little miscue in its circuitry that caused it – how shall I put it – to come a' tumblin' down and smack into the desert floor. There is an ongoing effort to recover the samples solar particles it gathered, but while the Genesis team is still assembling broken pieces, the Stardust team has so much data, they're letting the general public look for potentially interesting impacts in the aerogel.
Then there's NEAR, a mission to take an up-close-and-personal look at asteroid Eros. Seems the team had a dirty little secret they had kept since the outset of the mission. After accomplishing the mission goals, they took a device with no landing gear that was never designed to touch down anywhere and gently deposited it on the surface of the asteroid, a magnificent achievement.
Galileo, a warrior of a satellite, met a fitting end. On a mission to Jupiter, it had a malfunctioning high gain antenna that forced engineers to develop new data compression schemes. Then, despite going through intense radiation around the largest planet, it's mission was extended twice, and it sent back amazing images and data, kept on a tape drive that should have been fried several times over. It was the little-satellite-that-could.
It got to go out like a star, directed to dive into Jupiter's clouds, where it would burn up, becoming more atoms in the dense Jovian atmosphere. Galileo went out like a trooper, sending data to the last.
There are thoughts about doing the same for Cassini when it gets old in the tooth come 2012 or thereabouts. The trouble is that Saturn is surrounded by those beautiful rings, and any attempt to send Cassini into Saturn's clouds would have it passing through a dense portion of those rings. Once it started getting bounced around by the chunks of rock and ice in the rings, it's anyone's guess where it would end up. I'd have to believe, though. that more than a few scientists would like to take a shot at this option because so much could be learned about the makeup of Saturn's trademark rings.
Another possibility is to crash land on one of Saturn's moons. But, the RTGs would still be functioning and generating heat (there's almost zero risk they would burst open and contaminate the surface), and the heat could melt the surface ice, possibly doing localized harm to any potential ecosystem. Now, a functioning ecosystem on, say, Enceladus may seem like a long shot, but one of the overriding concerns of space exploration has been to not louse up anyplace we visit. Yes, we've been litterbugs on Mars and the Moon, but maybe we'll get to those places and clean up one day. Every attempt has been made, though, not to introduce anything that would alter even the immediate environment of probes to these planets.
The most likely possibility is to simply leave Saturn altogether. Cassini could be sent toward interstellar space, following the Voyagers and Pioneers; depending on the state of its much more advanced instrumentation, it could send back some interesting data. Alternatively, it could be sent inward, maybe to follow Galileo into the Jovian mists, or, with a gravity assist from Jupiter, it could head inward and possibly crash on Mercury or into the Sun.
Personally, I'd like to see it sent outward. It's just barely possible Cassini could send back data on Kuiper Belt objects and someday investigate the heliopause. Galileo, if I may be allowed a little more anthropomorphism, got to have its warrior's funeral, a blaze of glory no doubt. Cassini stands to still have some functionality, being more of a veteran explorer looking for one more mountain to climb or river to cross.
It's a big universe. Having one more human mission heading out into it seems like a fine idea.