on Rosetta, Philae, and C/67P

One of the more exciting things you can find at the American Astronomical Society meeting are new results in fields you don’t normally keep up with, and about space missions you’re not involved in . The team behind Rosetta and the Philae lander put out a very nice and detailed talk about what they learned about comet C67P/Churyumov-Gerasimenko from the first couple months of orbit and the 68 hours of battery life they got before the Philae lander’s batteries died.

As with most of the solar system presentations I’ve attended, this one gave me a great sense of the size and scale of the place: There’s a sheer mile-high cliff on this four-mile-wide comet, right where the “head” of the rubber duck meets the “body”.

ESA’s image of Comet C/67P. That ragged cliff overlooking the neck is a mile high.

That sheer cliff- in Rosetta’s pictures, it looks like a place now: an actual hill you could possibly climb, with rocks and pebbles and boulders just like on Earth. It would be a hell of a place to try base jumping. Actually, with that low gravity, you’d probably take many hours to land, and your main worry would be missing the ground and shooting off into your own orbit around the Sun.

Another Rosetta view of the cliff, looking like something out of fantasy artwork.

Rosetta actually had to dive bomb the comet when releasing Philae to ensure it was moving toward the comet with enough speed that they could predict where the lander would land (and then, of course, it bounced twice – apparently the subsurface ice was harder than expected – before landing on its side in some crevasse.

Philae’s own pictures of its landing site are pretty striking too. The landing zone looks like a cave… or a quarry (low budget scifi was right!)

My god, it’s full of rocks!

There are also a series of depressions in the comet’s surface – not craters, but former ice pockets that either blew outwards, or collapsed inwards. This is kind of cool; eventually, C/67P will run out of ice and will become an asteroid with a strange comet-like orbit. There’s a whole class of these asteroids, called “Damoclids”.

It’s cool that we can even consider craters on comets – that comets are sturdy enough to withstand cometary impacts is a relatively recent discovery. Not too long ago they were thought to be loose iceballs. C/67P looks solid and asteroid-y enough that rumors have been floating around that the scientists are wrong and it’s actually just a comet.

Yes, Comet C/67P is a comet. The cometary particles are just really dim (another photo from ESA)

One of the more interesting points covered in the American Astronomical Society meeting was the actual structure of the comet. I’ve already mentioned that it’s referred to as being shaped like a rubber duck. Some of the pictures (which I can’t find online, interestingly enough) show what the geologists on the team think is a fault line where the two lobes of the comet flex and rub against each other as the lopsided thing spins. They claimed this was a sign of mechanical instability. This has a couple of interesting implications:

  • The comet is formed from two pieces
  • The two pieces are still separate objects on some level
  • If the comet pieces are ‘mechanically unstable’, why is the comet still together? Might it break apart soon?

That last question is particularly interesting, because comets have, by definition, those releases of gas.

Of course, another planetary scientist, courtesy of Twitter, chimed in to the whole thing to say that C67/P is a single solid body, which would solve the mechanical instability problem. It doesn’t explain why so many small solar system bodies look like they’re two lobes, though.


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