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Comment by FuNe | Hacker News Reader

Why not worrying about the same thing about Saturn though? The best thing would be to slingshot Cassini in outer space like Voyagers but I guess they wouldn't have this option here.

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The moons of Saturn are small even compared to the earth. One could expect far more "wreckage" to survive the entry to a moon.

Getting into Saturn, however, is vastly different. Casssini has broken up into many smallish, white-hot pieces only to eventually merge into clouds of ammonia at -200C that are blowing at extreme velocities. Further down, there are clouds of water at 0C and then metallic liquid hydrogen. Perhaps pieces could end up there or on the rocky core? Saturn is a weird place.

How much worse than an autoclave is entry to Saturn?

Titan is actually larger than Mercury. Just something I learned yesterday while reading Cassini posts. It really puts the size of Saturn into perspective.

Mercury is also real damn small. Just a tiny bit bigger than Earth's moon.

Titan is larger, but also significantly less dense. Mercury is more massive than Titan.

Mercury is pretty much just a leftover iron core

Gas giants are a much less obvious place for native life to evolve, at least from our understanding of life. Enceladus appears to have liquid water, which makes it much more likely (still slim).

That said, Arthur C Clarke included giant city-sized lifeforms floating in the clouds of Jupiter in his 2001, 2010, 2061 series. And Isaac Asimov wrote "Victory Unintentional" in the robots series.

They actually did have this option, see

http://www.sciencedirect.com/science/article/pii/S0094576512...

However, it was evaluated as inferior to the option chosen (there was apparently no concern at all about contaminating Saturn itself); see the slide titled "EOM Options with Science Evaluation":

http://www.lpi.usra.edu/opag/march_08_meeting/presentations/...

I think the assumption is that any onboard life would be destroyed by the descent into Saturn.

This is the key point and deserves more attention. Mission control was not concerned with space junk impacting the moons. They were concerned with biological contamination altering potential evolutionary trajectories on any of the moons. Saturn's atmosphere was deemed to be sufficient to cause the requisite friction and head to cleanse Cassini from any such potential contaminants.

With Cassini in a close orbit around Saturn, there is not enough propellant to escape once again. And it would be going much slower than the Voyagers.

So practically speaking, leaving Saturn once it got there was never really an option.

Now, if you could find a way to scoop up some ring material or gas from the planet to refuel...

This is not true. You're right that it would quite slow, but end-of-mission options were investigated that would have sent Cassini all the way to Uranus or Neptune:

http://www.sciencedirect.com/science/article/pii/S0094576512...

Yep. I forgot that the Saturn system has lots of opportunities for energy boosts from the moons.

Titan specifically (none of the other moons are big enough to have any significant effect). Even if you're aware of that, it's a pretty surprising result, since it took so much more delta-v to capture into Saturnian orbit than it would to escape (mostly because we wanted to fly Cassini to Saturn and capture it into orbit in a reasonable amount of time; the trajectories that could've been used to escape from Saturn were quite slow).

Some of the options were pretty crazy. This paper: https://arc.aiaa.org/doi/abs/10.2514/1.42893?journalCode=jsr

noted that it would've been possible to escape (in the year 2014) to Jupiter (2021), then use gravity assists to also visit Uranus (2029) and Neptune (2061), or use Jupiter or Uranus to escape the solar system. Cassini also could've used any of Io/Europa/Ganymeda/Callisto's gravity to be captured in orbit around Jupiter, or entered into orbit around Neptune using Triton. I assume the main reason none of these options were chosen was that, with limited fuel remaining and the spacecraft not expected to remain operational forever, it was seen as more valuable just to stay at Saturn until end of mission (as well as pick up a bit more science diving the probe into the atmosphere).

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