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Rosetta's lander, Philae is scheduled to land on Comet 67P in November 2014. This will involve use of "harpoon" to anchor itself to make sure Philae remains attached to the surface despite weak gravity.

Due to the time lag, I understand we cannot have control over this in real time. Considering this, I have following questions about this scheduled landing:

  • Did we have sufficient information about the composition of this Comet to determine the thrust required for anchoring the harpoon? Or is it yet to be determined?
  • Will there be any auto-checks to verify successful anchoring?
  • What degree of control will we have about the exact location of the landing site?
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Rosetta's approach and odd orbit (described well in the question and answer here: Is this really Rosetta's orbit around 67P?) are designed to gather the necessary information needed to achieve a safe orbit and eventually land Philae.

The landing site is being selected now: "As many as five possible landing sites will be identified by late August, before the primary site is identified in mid-September. The final timeline for the sequence of events for deploying Philae – currently expected for 11 November – will be confirmed by the middle of October." (source).

This press release goes into a little bit more detail about the approach and landing: planned released from about one km, velocity at touchdown of about 1 m/s. Since the comet's mass is so small, the worry is that the lander may just bounce off the surface (according to Wikipedia the escape velocity is around a measly .46 m/s), thus the tether. So the landing site will need to be suitable for the harpoon.

In general, satellites control systems are able to make adjustments to maintain and transition to specified orbits / attitudes. Ground operators can send a desired configuration, and the control system will move to that configuration while controlling for any number of perturbations. Search "control theory" for more information about how this can be done.

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    $\begingroup$ Landing ellipse is nearly square kilometer in size. Landing will be on a ballistic trajectory at roughly 1 m/s targeting a site that's not in permanent shadow and not too dusty. Preferably not a clear rock though, one of the biggest worries is that Philae bounces off it, despite its harpoon, dampened legs w/ screws and slow approach. So they might also go for a site w/ low outgassing rate. Exact model is still worked on, pending more data on the comet. $\endgroup$
    – TildalWave
    Commented Aug 8, 2014 at 11:27
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    $\begingroup$ Press conference during the Rosetta comet approach. Landing site selection was one of the topics there. Will try to find some link but currently only have mobile on me, so it might take a while... There were some maps shared with comet's insolation, rotation axes (stable one axis rotation) and possible landing ellipses as an overlay. Most fell on the "day side" with some close to its complex day/night terminator. But they need more data first, more detailed topography and near-IR scans for thermal inertia. $\endgroup$
    – TildalWave
    Commented Aug 8, 2014 at 13:10
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    $\begingroup$ No rush, I found a similar press release... editing now. Please edit as you see fit with new info / sources. $\endgroup$
    – superdesk
    Commented Aug 8, 2014 at 13:36
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    $\begingroup$ Found a few links that discuss this what I mentioned in a bit more detail: NASA JPL Rosetta page: How will we choose a landing site? and BBC: Rosetta captures high resolution images of comet 67P that includes a bit better composite image that I was talking about. I'd edit it into your answer but doing such tasks on a mobile device is plain PITA, so please excuse my laziness. ;) $\endgroup$
    – TildalWave
    Commented Aug 8, 2014 at 15:59
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    $\begingroup$ And the first... The thrust is adjusted by the flight control system, which I mention in the last paragraph. As for verifying if the tether worked... I'm sure they do, but I didn't find anything specific to how they verify it. $\endgroup$
    – superdesk
    Commented Aug 8, 2014 at 18:34

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