Rosetta, Philae and the unpronounceable comet

I have things to say, but I've been busy, so we've got a bunch of short videos.


Kelly Beatty has a good description of the landing on Sky & Telescope:

After free-falling toward Comet 67P/Churyumov-Gerasimenko for seven hours, ESA's Philae lander bounced on the surface twice before finally coming to rest.

It's been a historic day in planetary exploration. At 15:33 Universal Time, the European Space Agency's Philae spacecraft reached the surface of Comet 67P/Churyumov-Gerasimenko. When radio confirmation reached Earth some 30 minutes later, cheers erupted around ESA's control room in Darmstadt, Germany. (You can replay ESA's landing webcast here.) Never before had a spacecraft landed on a comet.

But Philae's descent wasn't over. The comet has such weak gravity, less than 1/100,000 that on Earth, that its escape velocity is only about 1 mile per hour (0.5 m per second). Philae landed about twice as fast, and bouncing around was a real concern. That's why the craft carries a pair of harpoons, designed to fire into the surface immediately after touchdown to anchor the lander. But the harpoons failed to deploy as planned. And a small thruster, whose only job was to provide a gentle downward force to keep the lander grounded, didn't fire either.

No one realized it at the time, but data from the probe and particularly magnetic-field measurements from its ROMAP instrument later showed that Philae did indeed bounce after touchdown — not once but twice!

The first rebound lasted some two hours, during which the probe spun slowly and drifted upward perhaps 1 km before coming down at 17:26 UT. The second bounce was briefer, ending at 17:33. So Philae actually landed on the comet three times before finally settling down. These unexpected encores explain why radio contact with the lander became intermittent, and why power from Philae's solar-cell arrays fluctuated for a while.

Why didn't the harpoons fire? Guns in Space (say it like Pigs in Space) may not be a good idea:

The harpoon propulsion system contained 0.3 grams of nitrocellulose, which was shown by Copenhagen Suborbitals in 2013 to be unreliable in a vacuum. - Wikipedia