There's a
lot of wrong answers to your question. It's frustrating to read all these people mouthing off about how planetary protection is a waste of time and this is BS when they don't know jack shit about what's going on. The Cassini mission planners did an amazing job of picking out a final mission plan that maximizes scientific payout from a probe that is on its last legs.
The real reason is that Cassini is almost dead. It needs hydrazine to power its maneuvering thrusters. Without them, we have no control over where it goes. The hydrazine reserves are basically down to fumes at this point. There was even some concern that they wouldn't even be able to maneuver into the Grand Finale trajectory because of the low fuel status.
Without the thrusters, we lose all control over where Cassini goes. Eventually, the reaction wheels will become saturated with momentum and we'll even lose the ability to point Cassini and permanently lose contact with it as it starts an uncontrollable tumble. The probe is done at this point. The goal is to work out as much scientific value as possible before it dies.
Without the ability to control its trajectory, its orbit quickly becomes wildly unpredictable due to gravitational interactions with Saturn's moons. As an example, the whole Grand finale set of orbits, going up to the inner edge of the rings and doing a set of atmosphere-scraping passes before diving in is not done with thrusters at all. Cassini's orbit is timed to cross Titan's orbit at each apogee. Every few orbits, they line up and Titan tweaks Cassini's orbit. This
diagram
shows how low Cassini goes with each orbit and also its interactions with Titan. (it's a confusing diagram, I know) The last pass by Titan sends Cassini right into Saturn. The last maneuvering burn was done quite a while ago, this is all just gravity interactions at this point. If we couldn't control where Cassini goes, it would eventually start wandering around Saturn in progressively crazier orbits until it smashed into
something.
There were a number of possible trajectories that they figured out. Many sent Cassini out into deep space where it wouldn't risk hitting any moons. But they managed to figure out this crazy set of orbits , using Titan's gravity to send Cassini inside the rings and then move up and down in that space and into Saturn. There was one maneuver that set all this up. Newton and Einstein are in the driver seat the whole way after that. That is two-fold in purpose - you don't need to worry about running out of fuel once you enter this set of orbits. (though there will be some tiny course correction burns that they might optionally do to tweak the height of the last 5 passes) The other is that even if Cassini smashes into ring particles and is destroyed, its debris will still follow the same path and all still end up in Saturn.
edit: to help explain, here's a video showing exactly how Cassini got into the Grand Finale orbits:
https://saturn.jpl.nasa.gov/resources/7508/
.
Here
is a diagram version of that video. The ring grazing orbits were the ones Cassini was doing right before going into the grand finale. It was orbiting outside the rings at the low point of the orbit (the perigee). But then they used the thrusters to nudge Cassini closer to Titan at the high edge of the orbit on one of those passes. Titan bends Cassini's orbit so that the perigee is now inside the rings. Further, the maneuver was really brilliantly designed so that every few orbits, Cassini passes close (but not as close) to Titan to keep tweaking the perigee. Looking back at the
first diagram from above
, you'll see that Cassini comes in pretty much dead center between the rings and Saturn. Then titan tweaks the next two orbits where the perigee goes much higher and actually runs through the lower edge of the rings. Then Titan tweaks it again and Cassini drops a bit lower for three orbits and back up into the rings for two more. Then Titan sends Cassini back to a mid-level orbit for 5 passes and then sends it to 5 orbits where Cassini is just grazing the top of Saturn's atmosphere. (This is where the last tiny thruster tweak might be used to move Cassini up or down, depending on how thick the atmosphere is) Then after that, Cassini makes one last pass by Titan which pushes the perigee far down into the Saturn atmosphere. It's an absolute marvel of orbital trajectory planning. I'm not sure if I ever seen any orbit planning that elegant. Basically no fuel used after the initial burn to get closer to titan at the beginning and even if Cassini gets destroyed at any point in the final 23 orbits, the remains will still end up safely in Saturn. Here's a
diagram
showing all the orbits in Cassini's mission for the last 14 years around Saturn. The ring grazing orbits are in yellow and the grand finale orbits are in orange.
This also has
huge scientific payoff. We get to go inside the rings for one. That allows us to get their mass, which we didn't know. That allows us to finally figure out how old they are and lots of other critical data to how they formed and how long they might last. We also get tons of data about the interior structure and composition of Saturn that we wouldn't be able to get any other way. Even crazier, this is occurring the same time as the Juno probe is also doing a highly elliptical, low-diving orbit around Jupiter. This allows us to do extremely detailed comparisons between Jupiter and Saturn when they're subject to similar conditions from the sun, etc. Really, it's an amazing thing that this set of orbits happened to be possible. It would have been worth doing even if Cassini weren't at the end of its life.
We are also trying to protect Titan and Enceladus from possible bacterial contamination. There's a very slim chance that those moons could harbor some sort of life and we don't want to get our grubby germs all over it. And yes, Earth bacteria can survive this length of time in space just fine. Many studies have shown that sporulating bacteria can survive for years in space as long as they're in a crevice or otherwise hidden from solar UV. We don't know the upper limit for how long those spores can survive in space but decades or even centuries is plausible. Titan, we've already possibly gotten dirty by dropping the Huygens probe on it. (Titan is likely very inhospitable to Earth life and Huygens was built and launched in a time when we were far less concerned about planetary protection) But still, it's best if we don't push our luck, now that we know that we shouldn't be doing this, even if hte odds of meaningful contamination are low. If we do end up discovering life on these worlds, we want damn sure to be able to study it without having to deal with homegrown microbes contaminating things.
The probability that there is life is pretty low and the probability that Cassini could actually infect those moons is even lower. But given that Cassini is already at the end of its useful life, this is the correct choice. The grand finale trajectory is an amazing bit of orbital mechanics that allows us to use the last gasp of a dying probe to get some amazing scientific data. The other options are basically dump it into deep space where it is of minimal value.
