Rough Landing Putting Comet Probe’s Science Team to the Test

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UPDATED at 12:24 p.m. EDT

PARIS — Europe’s Philae lander bounced twice on Comet 67P on Nov. 12 before settling in its current rocky spot where sunlight is rare, making battery power all the more precious and forcing managers to rethink how to make the best use of the 10 experiments, program managers said Nov. 13.

Less than 24 hours after Philae’s landing on Comet 67 some 500 million kilometers from Earth, mission managers were waking up to the fact that they have a successful landing and a functioning lander, but not at all in the spot they had targeted.

The lander was designed to touch down softly, then be pushed to the surface by cold-gas thrusters for several seconds so that two harpoons could pin Philae to the surface, aided by foot screws.

The thrusters did not function and mission managers now believe that, for reasons unknown, the harpoons did not deploy.

As a result, Philae bounced off its targeted landing spot and floated for about two hours, reaching an elevation of about 1 kilometer, before making another landing, bouncing again for a seven-minute float and eventually settling about 1 kilometer from the target zone.

Marc Pircher, director of Toulouse Space Center of the French space agency, CNES, told a press briefing that the lander appears relatively stable at its new location, although this cannot be certain and one of its three feet is not on solid ground. What is more, he said, the area is rocky and Philae appears stuck in a kind of grotto where its solar panels receive only 90 minutes of sunlight every 12 hours.

The original landing spot provided between six and seven hours of sunlight per 12-hour day.

Pircher said the solar panels show signs of having survived the bounce-and-touchdown sequence with only part of their capacity intact.

At a later press briefing at the European Space Agency’s European Space Operations Centre in Darmstadt, Germany, officials said the solar arrays do not appear damaged but that the poor lighting of the landing area, with a cliff next to it, will limit solar exposure and challenge the recharge of batteries.

Stephan Ulamec, Philae program manager at the German Aerospace Center, DLR, said Philae’s instruments automatically switched on at first touchdown and continued operating as the lander lifted off and descended over the next few hours.

Philippe Gaudon, CNES’s program manager for the Rosetta mission — Rosetta is the mothership that spent 10 years chasing Comet 67P and jettisoned Philae on Nov. 12 — said that given the lack of sunlight at the new site, Philae’s total battery life, which would have been 60 hours at the intended target zone, is now more like 50-55 hours.

Pircher said the good news is that Philae appears stable at the new spot and its antennas are oriented in such a way as to be able to communicate with Rosetta.

“We have communications and we have power, and Philae is working,” Pircher said. “That’s the essential thing. We will now need to spend some time figuring out a revamped experiment-deployment scenario to take account of the realities of where it ended up.”

Gaudon said eight of Philae’s 10 experiments already had begun providing data. The two others — the Alpha X-ray Spectrometer and the Multi-Purpose Sensors for Surface and Subsurface Science — have not been activated fully insofar as they require mechanical movement of Philae.

“We have action-reaction concerns,” Pircher said. “If we undertake to move a part of Philae, it could force a reaction that might destabilize the entire lander and move it. The place where the lander is now we never really examined because it has few of the characteristics we wanted.”

Pircher said mission managers for now are disinclined to attempt to deploy the two harpoons that were supposed to secure the lander to the surface.

Using the harpoons without the thrusters to counteract the action would risk forcing Philae off the surface again, with no telling where it would end up, he said.

Jean-Pierre Bibring, principal investigator for the CIVA instrument, including six cameras and a spectrometer, said the fear of moving Philae by deploying one of its mechanical systems should no be seen as an abandonment of the subsurface-drill instrument.

The drill is an integral part of Philae, Bibring said, and while the management of the lander’s power resources and current stable physical location will start with the less-risky sensors, it is likely that an attempt will be made to use the drill instead.

Bibring raised the possibility that ground teams could find a way to deploy Philae hardware so as to provoke a movement that would reorient its solar arrays to a more favorable position.