DARMSTADT, GERMANY — Philae may be in a tough spot, literally. After touching down on comet 67P/Churyumov-Gerasimenko, the Rosetta mission’s robot has been beaming back striking but somewhat unsettling views of the comet.
Instead of being entirely in open space, the lander is flanked by what looks like a cliff. “We are not sure how far we are from the cliff, but we are in its shadow permanently, which may be a problem,” Jean-Pierre Bibring of the Université Paris Sud in Orsay, France, said at a media briefing on November 13. The lander is also not sitting perfectly on all three legs. It is almost vertical, with two feet on the ground and one in open space, Bibring said.
Being in the cliff’s shadow means that the lander’s solar panels get less sunlight than mission scientists had anticipated. The data suggest that the lander gets only 1.5 hours of sunlight a day where it has landed, rather than the six to seven hours it would have gotten if it had landed exactly on target, which it did — at first.
On the first touchdown, Philae hit the bull’s-eye. But the lander didn’t stop there. It bounced twice, touching down three times. In between the first and second touchdown, the lander shot a kilometer up into space and caught almost two hours of air time before it hit 67P again, then sprang another 20 meters in the air. The second bounce lasted 7 minutes. Together the bounces transported Philae from the original target site, Agilkia, to somewhere on the other side of the huge crater on comet 67P’s head. The team has not yet pinpointed Philae’s exact location.
The bouncing has left mission scientists scratching their heads about exactly what happened after the first landing. If Philae’s harpoons had fired and its foot screws had bitten into 67P’s surface, the lander wouldn’t be in this awkward position. But the bounce does have a positive spin. The acrobatics provide a major piece of information that scientists currently lack in the characterization of comets: a comet’s surface gravity, said planetary scientist Tony Farnham of the University of Maryland. The team cannot yet determine the exact surface gravity of 67P, Bibring said, but added that there was a good mix of fine powder and harder, rocklike material. Philae is collecting data that will soon provide more insight into this geology and the surface gravity of the comet.
The main concern right now, Bibring said, is trying to stabilize Philae so it can use all of its instruments to explore the comet. With Philae sitting on only two unanchored legs, using mechanical instruments like the drill SD2 could push or pull on the lander and send it back into space for another little hop or two — not something the team wants to risk. However, Bibring said the team is considering working very carefully with some of the mechanical instruments, like MUPUS, to try to jiggle Philae onto all three of its feet.
Philae has only 60 hours of battery power, and with the precarious position of the solar panels, the scientists are trying to cram in as much science as they can as quickly as possible. Decisions will be made soon about when to use the drill and other instruments that could affect Philae’s physical position. These instruments are the ones that will directly sample 67P’s surface, a critical element of the mission, Bibring said.
There’s a chance that after the batteries run out, Philae could go into a safe mode and come to life again in a few months when the seasons change and the solar panels can get more sunlight. But that’s a lot of speculation, said Philae project manager Stephan Ulamec of the German Aerospace Center in Cologne.
Bibring was adamant that despite the less than perfect conditions in which Philae now finds itself, the focus should remain on the here and now, with the fact that the team landed a robot on a comet, it’s alive and it has the ability to study 67P, however limited the time and capacity may be.
If Philae doesn’t execute its full mission, a piece of the puzzle is lost, but not the whole thing, Farnham said.