History was made today as a spacecraft the size of a fridge executed the first successful landing on a comet. The European Space Agency (ESA) confirms that at about 16:00 GMT the unmanned Philae space probe touched down on the surface of comet 67P/Churyumov-Gerasimenko at the landing site known as Agilkia. The comet and spacecraft are 510 milion km (310 million miles) from Earth, so the news of the landing took 28 minutes and 20 seconds to reach mission control in Darmstadt, Germany.
The day before the landing, the 100 kg (220 lb) Phiale was turned on and its batteries charged for the first time since leaving Earth. There were some initial glitches as the batteries warmed more slowly than anticipated, but the spacecraft soon warmed up to its operational temperature. As the Rosetta mothership carrying Philae maneuvered into position, mission control carried out flight checks on the two spacecraft, sent command updates to Philae that allowed it to navigate autonomously to the landing site, and cleared the landing maneuver after a series of go/no decisions with the lander declared ready for separation at 02:35 GMT.
Despite a transient fault in the cold gas thruster aboard Philae, at 07:35 GMT Rosetta completed its final maneuver and the final permission was given to proceed with landing. At 09:03 Philae separated from Rosetta. During course correction maneuvers, communications with Earth were interrupted from either spacecraft, so they were programmed to operate autonomously.
According to ESA, Philae descended without power as 67P’s weak gravitational field, 1/100,000th that of Earth, drew the lander to it at a rate of about one meter per second (3.2 ft/sec). On contacting the surface, drills on each of the lander’s tripod base bored into the surface and a thruster on top of the spacecraft pressed it down to keep it from rebounding, until the harpoon on its undercarriage could deploy and anchor Philae in place.
Launched in 2004, Rosetta reached 67P by a circuitous route involving three flybys of Earth, one of Mars, and a long detour out beyond Jupiter as it built up enough speed to catch up to the comet. During this time, it passed close to the asteroids Šteins and Lutetia, and went into a 31-month hibernation to conserve resources until the comet rendezvous. Since its arrival on August 6, the orbiter has been mapping the comet in search of a suitable landing site for the Philae lander.
Today’s landing date was based on a window where there would be enough sunlight to power the lander, but not so much as to make the comet dangerously active. Meanwhile, the site was chosen based on a balance between the scientific value of the area against the safety of the lander. Agilkia has very little slope, few boulders, and abundant sunlight, yet contains many features of interest.
ESA says that Philae has begun taking panoramic images as part of a two-and-a-half day science mission using its suite of 10 instruments, which could be extended if its solar panels are able to charge its batteries.
The ESA animation below shows Philae’s landing on 67P.
Note: All mission times quoted are for when the signals were received on Earth from the spacecraft.