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There is currently a question as to if the harpoons fired, that is undergoing review. But I have to ask, how important are they? From what I've heard, a single leg touching the right spot could be a sufficient hold, and two legs had to have connection to even fire the harpoons. If the lander can stick to the comet with just a leg or two, why does it need harpoons?

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The harpoons are intended to hold the lander to the surface long enough to get the ice screws on the lander's legs in securely. The ice screws are intended as the more permanent hold-down solution.

Bear in mind that the comet offers essentially no gravity; imagine trying to drive a wood screw into the ceiling without pushing the screwdriver upward at all. That's the ice screws, without the harpoons or hold-down thruster.

The harpoons might not be able to keep their hold very long; they're fired at high speed into an icy surface we don't know much about, that's likely to shatter, evaporate, etc. So the screws are needed as a longer-term solution.

So the plan was a three-part solution: the hold-down thruster to cancel the recoil of the harpoons, the harpoons and thruster to hold the lander in place until the ice screws were in.

Given that, and that (apprently) neither the thruster nor the harpoons worked, it's unclear right now (to me, not necessarily to ESA) how well the ice screws are driven in.


UPDATE: Aaaaand Philae apparently bounced twice and is now lying on its side. Poor fella.

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    $\begingroup$ Also, while Philae could just sit on the ground and make photos, in order to drill for samples it does need a firm anchor... or the moment the drill hits a harder layer Philae will turn into a helicopter. $\endgroup$
    – SF.
    Nov 13, 2014 at 19:56

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