On September 15, 2017, Cassini will plunge into Saturn’s atmosphere and be destroyed. It will be a sad day for science, but it’s one that we’ve known would be coming for a long time, and it’s decidedly not an accident.
Cassini, which has been orbiting Saturn since 2004, seven years after it left Earth in 1997, is a 19-year-old spacecraft, and it’s slowly running out of fuel to control its pointing and its orbit. It’s spent its time in the Saturnian system near Saturn’s major moons, and that’s a problem, because if it was left in an orbit like that, it would eventually crash into one of them. And if that moon was Titan or Enceladus, that could be a problem. See, Cassini wasn’t sterilized and certified for landing in the way that, for instance, the Curiosity rover is… and NASA doesn’t want to potentially contaminate other worlds with life from Earth.
NASA’s plan isn’t like the Prime Directive from Star Trek, though. It’s more about making sure that, should we ever find life elsewhere in the solar system, we can be sure that it didn’t accidentally come from Earth. The problem arose with the Apollo 12 mission from December 1969, which landed near Surveyor 3, an unmanned probe that had landed two years earlier. They took pieces of the probe back to Earth because NASA was curious to see what happened to it in the harsh atmosphereless radiation environment of the moon. What they found was Streptococcus Mitis bacteria that had apparently accidentally contaminated the launch vehicle, survived travel through the vacuum of space, and then two years being alternately blasted with solar radiation and frozen. (an alternative reading of the situation is that the pieces of Surveyor 3 got contaminated after Apollo 12 brought them back, but it’s hard to say conclusively). This was a huge problem, because it meant that bacteria could survive in the vacuum of space. (Life as we NOW know it is full of amazingly resilient creatures). And so NASA began to worry that it was going to contaminate other planets.
So NASA has to destroy Cassini before it runs out of fuel. That’s the reason Galileo plunged into Jupiter after eight years in orbit, and that’s why the new Juno spacecraft will itself one day take a fiery ride into the murky depths. (well, that, and Juno is swinging so close to Jupiter that it’s going to be super crispy fried* with radiation and its computers aren’t expected to work for very long – its demise is planned for February 2018)
Did it need to end this way? There were considerations that maybe Cassini could fire its rockets again and put itself in solar orbit, spending the remainder of its time as a distant telescope until its power source (a radiothermal generator) grew too weak… but that would have required a LOT of fuel, and would have had to be done years ago. This current plan means that Cassini gets to do the job it was meant to do for as long as possible. It’s already had two mission extensions over its planned four-year operation, and it’s now basically at the physical limits of the spacecraft’s capabilities.
And oh, what it’s done. It found geysers and a liquid water ocean on Enceladus, it’s found dozens of new moons, it’s mapped the surface of Titan, discovered liquid seas on it AND dropped a lander there, and it’s watched storms come and go in the atmosphere of Saturn.
Cassini has spent its entire mission to date in orbits that bring it close to the major moons. Starting in April of next year, Cassini will start firing its thrusters to elongate its orbit more and more – sending it into more daring orbits that plunge it closer and closer to Saturn on the near end and shooting it farther and farther above the planet on the backswing. It’ll see Saturn and the outer reaches of its system of moons from vantage points no one has ever seen it from before, and Saturn from closer than anyone has ever seen it before. And then, when the orbit tightens enough, it will hit the planet.
But it’ll give it one heck of a show in the process.
* Not the technical term.