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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?
pat_nafs
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1 Answers1

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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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superdesk
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    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. – TildalWave Aug 08 '14 at 11:27
  • Great comment! What's the source? – superdesk Aug 08 '14 at 12:51
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    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. – TildalWave Aug 08 '14 at 13:10
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    No rush, I found a similar press release... editing now. Please edit as you see fit with new info / sources. – superdesk Aug 08 '14 at 13:36
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    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. ;) – TildalWave Aug 08 '14 at 15:59
  • So far, based on your answer and comments above, my second and fourth sub-question about Philae's landing is answered. – pat_nafs Aug 08 '14 at 18:29
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    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. – superdesk Aug 08 '14 at 18:34
  • Ok, so based on the observations in next few weeks, scientists and engineers will determine the thrust with which the harpoon will be anchored. Verification - yeah, there should be some SOP for that. Thanks. – pat_nafs Aug 08 '14 at 18:45
  • Will wait to see if @TidalWave wants to make any edits in the answer before marking it answered. I would suggest addition/creation of the tag "Philae" to members with sufficient privileges – pat_nafs Aug 08 '14 at 18:51
  • @pat_nafs I'll let superdesk make any edits he/she deems fit, but a good call on the Philae tag, I already suggested someone does that in our [chat] since I was strapped to a mobile, but I guess nobody managed to do it. In excuse to most of us, it is summer in the northern hemisphere. :) I'm also not sure I understand your first subquestion. There's no such thing as a zero resistance reel, so the first level control would be how far from the target it is released. But I'd dare guess they also have some simple way of adjusting resistance to the tether reel, if needs be. – TildalWave Aug 08 '14 at 22:16
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    This article / gallery for the Training sessions for the Philae comet lander as performed by DLR (on a Philae replica, after Rosetta's launch) might also come handy. In general, it's mostly [the Germans](https://en.wikipedia.org/wiki/Philae(spacecraft)#International_Contribution)_ (DLR et al.) that worked on the Philae systems relevant to the question, so that's where I'd first look for more detailed info on its design characteristics. – TildalWave Aug 08 '14 at 22:25
  • What I really meant by my first sub-question is, have we already programmed for the thrust with which harpoon will be anchored. Considering the close resemblance with the second subquestion, I have merged the two. Sorry for the confusion and thanks for all the fish. – pat_nafs Aug 09 '14 at 06:00
  • The German link given by @TidalWave is brilliant. Here are key points "Nobody knows what the surface of the comet is like yet (they are simulating landing with different surfaces). How should Philae respond if individual subsystems malfunction? What are the first things to do following a successful landing? The engineers are rehearsing adverse events that the software will then need to resolve autonomously – without support from the ground. Shortly before arriving at its destination, the final software will be transmitted to Philae." A LOT needs to be still worked out. Hope we'll make it. – pat_nafs Aug 09 '14 at 06:06