I’ve really wanted to discuss Comet Churyumov–Gerasimenko (67P) and the amazing Rosetta mission to it, but I’ve found the coverage of it to be really bad. So I’ve been reading everything I could and I think I’m in a position now to discuss it. And the first thing on my mind about the comet is its shape. Just look at the image on the left. When I first heard scientists describing it as a rubber ducky, I thought they must have become a bit stir crazy. But it does in fact look a whole lot like a rubber ducky — although one covered in iron flakes from a Wooly Willy.
But there is nothing particularly strange about this. Large asteroids can be relatively spherical. But the only one that is truly spherical is Ceres, which has a radius of about half Pluto. Comets are far smaller. The largest known is Comet Hale–Bopp, with a diameter of perhaps 50 km — roughly one-twentieth the size of Ceres. And most comets are much smaller than this — generally less than 10 km. So they have nowhere near enough gravity to affect their own shape. In the case of 67P, the small lobe is a bit more than 2.3 km across the and large lob is 2.6 km across.
What’s more, the basic shape of 67P is typical of comments. Comet Halley, for example, looks quite a lot like a peanut shell with two bumps on either side and and a thin shaft in between. Comet Hartley 2 looks much the same. And Comet Borrelly looks like a bowling pin. All of this is more or less what we see with 67P. I figure the reason for this is because of the sublimation of comets when they get close to the sun. The parts of the comet that vaporize most easily end up looking like bars that connect the less volatile areas — at least until the comet breaks in half.
The news from Rosetta is frustrating. As far as I can tell, we don’t know anything more about 67P than we did before. That’s mostly because Rosetta has been orbiting the comet since early August and it has been sending back data since September. The data are interesting. For example, both hydrogen and oxygen were found in its coma (tail). And no surface ice was found and the albedo (surface reflectivity) is very low. But these last two bits of information were known years ago (pdf).
The most exciting aspect of the Rosetta mission is the search for organic compounds — including nucleic and amino acids. I’ve always found the idea that earth was seeded with these building blocks of life to be far fetched. But a lot more amazing things are in fact true. If we discovered amino acids on 67P, it would blow my mind. And it really would change the way we look at the development of life in the universe. The coming decades would be very exciting indeed.
Sadly, we can only test for organic compounds by collecting samples. And it is not clear at this point whether Philae (the little lander that could) is going to be able to do this without launching itself off the comet. And the current situation can only be considered a crisis. The lander appears to be at the bottom of a cliff, so that it is only getting an hour and a half of sunlight for each 12 hour rotation of the comet. What’s more, only two of its three feet are on the surface of the comet. It is possible the issues can be worked out, but the scientists only have until Saturday to work it out, after which the battery on Philae will be dead.
I understand that for most people, the adventure story is enough: we landed on a comet! And there is no question that this is cool and the landing is a great engineering feat. But if it ends here with a couple of images and a little spectroscopy, this will be heartbreaking. But the scientists are collecting all the data they can while they can. As time runs out, they may go ahead and start digging, given they will have little to lose at that point. Here’s hoping they can work some kind of a miracle.