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Comment by 3chelon | Hacker News Reader

Can anyone explain why they would (a) crash, rather than attempt to land the probe on the comet's surface, and (b) turn the power off at the moment of touchdown?

Everything I've read (admittedly, not much) says they will get more data from crashing it, but that makes no sense to me. Surely landing it as intact as possible and leaving it running means it could send data whilst it has any remaining power, and maybe again in the future whenever it gets enough sunlight? Even if it was just a semi-intact archaeological curiosity for future generations, surely that would be better than wilfully destroying it?

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From the ESA FAQ

"As soon as Rosetta hits the surface, its main systems will be turned off, including the attitude and control systems, as well as the main transmitter, the latter in order to meet regulations aimed at avoiding interference on deep space network communications channels"

"as soon as Rosetta hits the surface, its high-gain antenna will very likely no longer be pointing towards Earth, making any potential communications impossible"

"One of the key features of the trajectory design is to minimise the spacecraft's relative velocity at impact. The current scenario predicts that the impact velocity will be around 90 cm/s, around walking pace."

I recommend reading the whole FAQ, it's very informative.

http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rose...

Useful info, thanks. So it's more of slo-mo crash landing then - that makes much more sense now. And the bit about the directional antenna also makes total sense.

And of course, they can't turn it off after they've touched down because they will likely no longer have a comm link!

Sure they can. It has a computer. Tell it "power down in 1 hour if you don't have a com link".

Yes, but there's a fairly high risk the computer or its power supply will malfunction on impact.

There is no need for that if statement. Rosetta's antenna needs to be very precisely oriented to maintain connection, and it is attached directly to the side of the spacecraft (as in, it cannot be aimed without moving the entire spacecraft). The moment Rosetta hits anything, the antenna will be knocked out of alignment, and if a corner of it is resting on the comet, it cannot orient itself properly to establish a link.

Basically, Rosetta has a highly directional antenna, and there was no chance it would still be pointing towards earth after landing. So they programmed it to switch off the transmitter on landing to avoid causing interference for other missions.

It "crashed" into the comet very slowly, so it was not smashed to pieces. It may be have bounced back and will eventually land somewhere else. We may never know.

As an addendum to that, people might find this interesting:

https://en.wikipedia.org/wiki/Galaxy_15

The tl;dr is that Galaxy 15 is a massive two-tonne geostationary communications satellite around Earth; in 2010 it went mad and started broadcasting hash. It basically became an uncontrolled jammer. It drifted out of its slot in geostationary orbit and started passing near its neighbour satellites, wreaking havoc. Several had to be moved to get them far enough away from Galaxy 15 to prevent interference.

Eventually, after about a year, its batteries went flat and it rebooted, and started responding to commands again. After checking out it was put back into its orbital slot and I believe it's still in service today.

They "crashed" it at a walking pace, at worst it's a bit twisted, definitely still an archeological curiosity. Why do that? Probably to try to get close-approach data that the detachable probe failed to. Why crash? I don't know if the vehicle even has the capability to retro-thrust itself to a stop but trying would be a fiddly maneuver and would likely mess up the science (puffing up an opaque cloud of dust, blasting the landing site). So approach-and-keep-going makes sense. But then the vehicle will lithobrake with predictable smashy consequences. So it's toast one way or the other; turning it off is probably just tidiness.

All the above is guesses but seems to make sense to me.

> …lithobrake…

Thanks for adding this to my vocabulary.

Seconded, "performed a lithobraking maneuver" is now on my list next to "rapid unscheduled disassembly".

> definitely still an archeological curiosity.

I was going to say that it'll probably get blown off when Churyumov-Gerasimenko nears the sun, but it's already done that, and Rosetta taught us that a comet isn't just a boring spherical ice-cow that outgasses equally in all directions anyway.

Maybe it'll survive to make another pass near the Earth! Maybe SpaceX will "rescue" it as a PR mission in the next few decades. It would make a pretty great addition to the Air and Space museum.

> ... lithobrake ...

OK, can't resist: https://www.youtube.com/watch?v=yeaeqIpopvI

I didn't read on, but my guesses:

- batteries will die when cold, they can't reanimate the probe when it comes back from the large ellipse of its orbit - probe has orbited the comet for 2 years, and has seen the most important period (closest to the sun). There's not much to learn from more pictures of that far away - landing would not work (no landing legs, not enough gravity, see Philae), and the camera is not made for it. - at the speed of impact (walking pace), it won't be crushed entirely, and will probably bounce a few times before settling on the comet. Philae is already there BTW, so that's an artifact.

Good call about the batteries - of course they will become totally useless after discharging and freezing.

I'm not sure about (a), but it would probably be very difficult to land it with the thrusters on the probe. Philae almost bounced off the comet and it was designed for that purpose...

For the second one, seems that https://twitter.com/AstroKatie/status/781814758404005888 has the response (with not a lot of details).

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