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Comment by luckyt | Hacker News Reader

> Scientists [were] worried that when [Cassini] loses power, it could crash into a pristine moon, contaminating a place where we might someday search for life

Why is this so bad? Moons are pretty big compared to spacecraft, why are they so worried about the environmental damage of a single spacecraft weighing a few tons?

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Think fifty years in the future. The Herschel probe lands on Enceladus, armed with instruments that can sample the water and test it for presence of biological life. A hit! A world-changing discovery, we've detected DNA!

...but wait, hang on a second, check the records. Isn't this really close to where we predict that Cassini crash-landed after it ran out of fuel and contact was lost in 2023? Shit. Shit shit shit. We have no idea whether this DNA is actually from the oceans of Enceladus, or if it's from a hardy bacteria that hitched a ride on Cassini all the way from Earth, and thrived in the oceans of Enceladus.

If it's thriving in Enceladus, it might be out-competing the life that's already there. We may never know, we can't know what native Enceladus life was like, or if it even existed!

They're not worried about knocking down some space-trees; they're worried about it being impossible to assess scientific results with accuracy.

> We may never know, we can't know what native Enceladus life was like, or if it even existed!

To be fair we don't even know if life on Earth is native to it in the first place or not...

True, in fact I think a dominant theory is that life came from Somewhere Else, hitching a ride on an asteroid. Another one is that it formed around the volcanic vents in the ocean.

Solar wind makes live seeding from outside the solar system significantly less likely. It's possible, but considered far less likely than you might think. On the other hand life going from Earth to Mars/other moons etc is expected.

Finding life on Saturn's moons that looks eerily related to life on Earth would provide some weight to panspermia theories. But only if we could be sure that we didn't bring it there ourselves.

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.

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.

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).

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/...

It seems pretty unlikely that Earth life could accidentally seed any moon of Saturn, and extremely unlikely that it could out-compete life that's been adapting to that environment for eons. As in, even less likely than Earth's surface bacteria hitching a ride on probes down to volcanic vents on Earth and out-competing the life there. Maybe it would find a small niche to fill.

No matter how careful we are, if we do find life on another planetary body, genetically similar enough to be confused with present-day Earth life, the assumption should be that it is from Earth and there had been no other life. Unfortunately it leaves "are we alone?" unanswered, but it answers "can life exist beyond Earth?"

Cassini has an RTG, so a crash into something like Encaladus could potentially create a nice warm little oasis for Earth bacteria to live in. A few decades and some on the fringes of this oasis might adapt - there's believed to be liquid water there.

Don't have to out-compete if there's nothing there, and being similar to Earth life wouldn't let us rule it out - rocks from asteroid impacts are known to have travelled between Earth and Mars, so it's not outrageous for one to have travelled between Earth and Saturn.

If we find life on Enceladus that looks just like Earth life, the null hypothesis is that it came from Earth. That won't change, whether we crash a probe there or not. Finding extant life anywhere beyond Earth would still be a paradigm-shattering discovery, even if we must assume it arrived from Earth on a probe or ejecta.

The probe we send to test for life must, of course, be completely sterile to ensure it's not detecting its own hitchhikers.

> If we find life on Enceladus that looks just like Earth life, the null hypothesis is that it came from Earth.

Sure, and ruling that out is easier if we can say for certain we didn't crash an old space probe nearby.

The only way it would help is if we had some other way to rule out asteroid impact ejecta but not probe contamination. So we'd have: 1) Life on Enceladus that's 2) genetically indistinguishable from Earth life, which 3) we know didn't travel on a rock, but 4) could have travelled on a probe. We're getting into teapot-orbiting-Mars territory.

"no idea" and "impossible" is a stretch of the truth. Hyperbole.

There are lots of tools in the biologist's shed for this sort of thing, and we know how to use them (think of all the field work done anywhere on Earth, such as Antarctica... if the task of sorting life was impossible, we should defund all of that immediately).

We have an idea. It just makes it harder in the extremely remote chance that it crashes in such a way that it could contaminate an icy moon and have any passengers start growing wildly.

Personally, I think that, fundamentally and cosmically, the whole point of Gaia producing a species which can send probes to other planets is contamination. We're Gaia's reproductive system.

Shouldn't we expect life on Enceladus to be different enough to tell the difference?

I'd say when we can ensure no contamination there isn't any reason not to ensure no contamination.

What if it turned out that the DNA found was similar to ours? Now think of the difference in chances if we'd dropped Cassini on the surface or if we hadn't.

> Shouldn't we expect life on Enceladus to be different enough to tell the difference?

We should likely have as few expectations as possible. The discovery of an entirely different form of life on Enceladus would be as incredible as the discovery that life on Enceladus uses the same DNA building blocks as we do.

Cassini's mission profile was to do progressively more dangerous activities as the time allowed. Nearing propellant exhaustion, they investigated close to the rings and the planet's upper atmosphere before destruction. Contamination prevention was only a part of the decision.

"All solar systems missions are subject to constraints designed to minimize the unintentional impacts of Mars and, when applicable, the contamination of icy satellites such as Europa, Enceladus, and Ganymede." [1], also see [2]

"The Huygens probe which landed on Titan was not sterilized as the chances of finding life were considered insignificant. From further investigations it is evident that chances for life on Titan are higher than initially thought. Although forward contamination in this case is still considered unlikely, it reinforces the need to ensure we protect extra-terrestrial chances of life." [3], also see the second page of [4]

[1] https://planetaryprotection.nasa.gov/requirements [2] https://saturn-archive.jpl.nasa.gov/faq/FAQHuygens/ [3] http://www.spacesafetymagazine.com/space-exploration/extrate... [4] http://www.esa.int/Our_Activities/Space_Science/Cassini-Huyg...

They are worried Earth microbes on the satellite might eventually be confused for life on the moon.

>contaminating a place where we might someday search for life

Should it have crashed into a moon that we later find life on (especially should the life be remarkably similar to that here on earth), whose to say we didn't send it there via Cassini in the first place? Avoiding this problem entirely seems to be the best bet, also allowing them to collect atmospheric data from Saturn on the way out.

In some cases we could still estimate a probable answer.

Biologists can separate two cryptic species of living beings placed into an species complex, just taking a look to its DNA. The mutation rate among two isolated bacteria from earth and saturn would be noticeable. Unless we discover an active saturnian's nano-turism to pass the holidays on earth's oceans, the only reasonable answer to explain two identical or almost identical bacteria in both planets is a contamination event.

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