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Now, since Rosetta touched down on "Tschurri", as the comet is called in the german speaking world, I am asking myself how firmly does Rosetta sit on its comet?

Since on the comet very little gravity is acting due to the comets low mass, it would probably not take too much effort (force) to lift Rosetta from the comet's surface.

So could during one of the comet's next active periods Rosetta be blown from the comet's surface by e.g. outgassing?

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    $\begingroup$ Chances are most of the probe bounced off, after all it didn't land but it crashed. $\endgroup$ – GdD Sep 30 '16 at 14:28
  • $\begingroup$ @GdD According to the ESA the planned impact velocity was 90 cm/s, equivalent to a drop of just 4 cm in Earth gravity. I'd reckon it's mostly intact, but that might make a good topic for another question. $\endgroup$ – 2012rcampion Sep 30 '16 at 14:48
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    $\begingroup$ 67P's escape speed is on the order of 1 m/s (en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko) $\endgroup$ – Hobbes Sep 30 '16 at 15:13
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Sadly, we just don't know what happened to it, and we probably never will.

Rosetta may have come gently to rest in the snow-drift texture of the comet’s surface or bounced back into space, as the Philae lander did when it was dispatched in 2014. Rosetta’s precise fate will never be known, because it was pre-programmed to shut down all communication on contact and there are no telescopes on Earth powerful enough to see it. (Source)

Philae hit the surface around 1m/s and bounced quite high (twice) (at 38 cm/s for the first bounce). Rosetta hit the surface at 90 cm/s (same source as the quote above). Its possible Rosetta did the same and took damage when it landed. But this is just speculation. We won't know unless we send another probe to 67p to take pictures of the remains.

I have no clue if Rosetta could be blown off the surface by outgassing.

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    $\begingroup$ The problem with Philae was despin of the reaction wheel, which provided most of its propulsive force (and was to be neutralized by the harpoons which didn't fire). Rosetta? Likely similar... $\endgroup$ – SF. Oct 1 '16 at 7:00
  • $\begingroup$ Do you have a source for your Philae impact velocity, because a while ago I did a few calculations myself and predicted a impact velocity of 86 cm/s? $\endgroup$ – fibonatic Oct 5 '16 at 2:08
  • $\begingroup$ @fibonatic I can't find the source that said that, however I found two that said it bounced at 38cm/s, so maybe the first one I found was misquoting that. I'll update my answer. Here are my two sources. $\endgroup$ – Cody Oct 5 '16 at 15:43
  • $\begingroup$ @Cody So after Philae hit the surface for the first time, it bounced up at 38 cm/s, so you can't really compare this value with the speed with which Rosetta hit the surface with for the first time. $\endgroup$ – fibonatic Oct 5 '16 at 19:25
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It's very unlikely that the force of outgassing would be sufficient to shift Rosetta or Philae's position on the comet. When Rosetta was initially approaching 67P, the comet was estimated to be losing about a liter of water every second; averaged over a sun-side surface area of roughly 50 square kilometers, this amounts to something like .02 milligrams of water per square meter per second evaporating. The exact force exerted would depend on the velocity of the departing water molecules, but it's obviously not going to amount to much. Further, note that evaporation will be much slower directly beneath the spacecraft, because the surface will be shadowed.

Note that Philae has been at rest on the surface of 67P for about two years, a period which included 67P's closest approach to the sun, and therefore its most furious outgassing. It is not known for certain that it hasn't shifted position significantly, but its position in pictures taken from Rosetta in September of 2016 is consistent with its estimated position in mid-2015.

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