Europe prepares to INVADE comet: Rosetta landing site chosen The European Space Agency has selected a site for the Rosetta space probe's lander, which is attempting to touch down on comet 67P/Churyumov-Gerasimenko. The region – labelled Site J – lies at the top of the rock, which is unevenly shaped and measures about 4km across at its widest point. The Rosetta craft is now within 30km …
There must be some way to deorbit it. I haven't seen any mention of how they're going to do this. The only real options are either (a) use a small engine on Philae itself, possibly a single-shot solid rocket; or (b) use Rosetta's main engine to do it.
For option (a), I *have* seen a reference that there's a small engine on Philae to hold it down while the screws dig in; but that was on a page talking about Lego.
Option (b) would be pretty ballsy, as what you basically end up having to do is to deorbit Rosetta, detach Philae, and then boost Rosetta back into orbit again. If the second boost fails, for whatever reason, then Rosetta will land alongside Philae, and I doubt it's designed to do that.
Admittedly, orbital velocity is only about 1 m/s, so it's not much of a burn.
Hm. Maybe they're using option (c): eject Philae with a big spring. Be nice to have some actual information...
Actually Philae does have an engine: an "Active Descent System (cold gas thruster)". clickety-linky
Icon: what Philae can do using the ADS.
Awesome --- thanks! That gives me the magic Google keywords to find this paper:
http://issfd.org/ISSFD_2012/ISSFD23_GC_2.pdf
So the lander ejection mechanism imparts a dV of between .05 and .5 m/s; then there's a backup spring which provides a fixed dV of .17 m/s; the ADS has a maximum dV of 1.85 m/s, of which they're going to use up to 1 m/s for a course correction during descent and the rest to hold the vehicle down while the anchors bite. That all sounds quite reasonable.
The paper discusses descent strategies, BTW, and is rather interesting.
a bit off-topic here but isn't it about time to officially change the name "Comet 67P/Churyumov-Gerasimenko" to "Rosetta's Comet?"
somebody call Churyumov and Gerasimenko and ask nicely.*
i'm holding my breath, hoping for a perfect landing and keeping the beer ready. if i can't reach all the way across the pond to buy the Rosetta team a beer, i'll just have to hoist one for them over here.
if this works, it'll be up there with (above?) the skycrane landing of Curiosity.
* with a stick if necessary.
In the general sense he's right. The thing about 67P/Churyumov-Gerasimenko is that it has very little gravity, and one could just jump off a slowly passing spacecraft and more or less fall onto the comet without employing a landing craft. Though if you're pedantic, you might want to call this 'landing' as well. For getting off again there are various options, such as a Skyhook-like rig (you just have to throw or fire the line up as balloons tend not to work too well in space), a jetpack or even just a pair of tensioned springs under your shoes, depending on how high the craft will pass over the comet
Why not send two or three?
I have to assume the biggest cost for a mission like this with a small unmanned object is operational (planning upfront and then keeping the team in place to track, land and assess results).
I'd assume that for long periods tracking two or three objects is hardly more onerous than tracking one and it must improve the odds substantially if you have a gap large enough to assess a first time failure before trying to land the second.
IMO, the better option would be not to send multiple craft, but have two or three landers aboard Rosetta. Getting a craft to some point in space, even if that's next to a moving object, is old hat; the tricky (and so far untried) bit is landing on the comet. With multiple landers you have a bigger chance of at least one succeeding, and if more than one lands successfully you can do science in multiple places.
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