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Comment by stronglikedan | Hacker News Reader

They're going to fly between the planet and it's rings - but how? I was watching a show on Voyager 1 where they had originally thought about doing the same, but were glad they didn't. When Voyager got close enough to image the rings, there was quite a bit of debris between the planet and the rings. Apparently, it was enough to ensure the destruction of the spacecraft. Do they now know enough about that debris field, and now have accurate enough "control" (calculated by astrodynamics), to avoid a collision?

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From this NASA link [1] it looks like when Cassini crosses through the plane of the rings, it does that almost at a right angle. The width of the rings is at most 1km, and at a speed of about 20km/s, each crossing will take at most 0.05s. The average distance between rocks in the rings is a few meters (and Cassini is about 4m by 6m), so even if Cassini crossed the rings themselves the probability of a crash would be about 50%. The average distance between space debris in the space between rings and between rings and the planet is probably hundreds of times lower, so I would think that the probability of a crash is very small, easily less than 1% per crossing. For Voyager, a crash probability in the neighborhood of 1% was most likely unacceptable, but for a spacecraft scheduled to be destroyed on 15-Sept-2017, this is probably ok.

[1] https://saturn.jpl.nasa.gov/mission/spacecraft/navigation/

Now that the primary objectives of the mission are completed, they are engaging in more and more dangerous orbits to gather even more data. If they do have a mission ending strike.. it goes out doing science.

From what I remember, the plan was Voyager crossing the Cassini division and they were glad when they realized it was not as particle free as we used to think at the time. The region between the planet and the inner ring is much cleaner.

Cassini has been around Saturn for a long time now and I don't think there will be too many surprises on its final cruise.

And, if there are, Cassini will just have discovered a new way to destroy space probes.

Even if they're taking huge risks going through a debris field, they're destroying the craft soon after anyway. If it actually is risky, it's probably worth the risk.

They're going to do it 22 times over 24 days before the descent into Saturn. The only thing I can think of is that the 2,000 km gap is close enough to avoid the debris, and perhaps they couldn't have calculated that close a fly-by with Voyager 1 with the knowledge and technology of the day.

It might have been a trajectory limitation. A major goal for Voyager 1 was to fly by Titan, and it may not have been possible to both fly through the gap and fly past Titan given the limited fuel available.

They might not have known the gap was there, either. Much of what is now known about the structure of Saturn's rings was first discovered by the Voyagers.

Is it possible that there is a rock big and fast enough to divert Cassini someplace undesirable, such as leaving a streak through a ring, crashing into a moon, or perturbing something else from its natural state?

Cassini is very small, compared to pretty much everything around Saturn. So, no. You could drop a billion Cassini spacecrafts into the rings without perturbing anything.

I don't mean that Cassini will shift a moon from its orbit, but the introduction of foreign materials, for example, could change things.

Perhaps I'm putting too much emphasis on the naturally pristine state of things around Saturn, but I've read that also is a priority of researchers, at least in some circumstances.

"its rings"

And there are gaps that have been identified, but they don't care if the spacecraft gets destroyed now, so that's why they're doing these bold maneuvers now

re "its rings". I'm never quite sure whether to correct grammar/spelling or not in HN comments. On the one hand I know there are lots of people here for whom English is a second language and who might find it useful. On the other hand, I don't want to appear dickish. At work I generally ask my non-native-English-speaking colleagues if they want me to point out errors and then act accordingly. And every now and then one of them challenges my correction and I discover I'm wrong ;-)

As a non-native speaker, I think there are some classes of errors like confusing homophones that native speakers are much more likely to make, e.g. "could of", their/there/they're, or even the above-mentioned its/it's. The latter two are covered very extensively in lessons, and "could of" is just so absurd to anyone who's had to study to learn the language that it doesn't even compute.

I've generally decided not to comment on people's spelling and grammar on HN because it doesn't really add to the discussion. I do sometimes make a point of italicising the correct spelling in my response if I have something substantial to add to the discussion :)

Good idea. Thanks.

Yeah I concede correcting some mistakes might get annoying sometimes

But as a non-native speaker myself the homophone mistakes really bother me (though I concede native speakers are likely to make these kind of mistakes in their native language - and I include myself in this)

But it's a continuing effort, however I noticed this kind of mistake has increased in past years. I suspect autocorrect is to blame

It will be on a ballistic trajectory into Saturn, which will happen debris collision or not, so what does it matter?

Can they somehow ensure this 22 orbits in advance (e.g. using Titan's gravity to change the trajectory slightly on each orbit)?

It seems so.

https://www.youtube.com/watch?v=pMvc2sEMdJ0

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