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