European Spacecraft Touches Down on Comet

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UPDATED at 4:26 p.m. EDT

PARIS — Europe on Nov. 12 successfully placed a 100-kilogram lander on a comet 500 million kilometers from Earth following a seven-hour descent from a satellite that had been chasing the comet for 10 years.

Mission managers at the European Space Agency’s European Space Operations Centre in Darmstadt, Germany, confirmed that the Philae lander had touched down, apparently softly, on Comet 67P. The lander appeared to be in good health and was sending signals.

Less clear was whether it had secured a stable purchase on the comet’s surface. Early telemetry suggested that the two harpoons that were to drive into the comet, up to 2 meters in depth, to fix Philae into place may not have deployed correctly.

The possibility of making another attempt at harpooning the surface had not yet been confirmed.

Rosetta and Philae managers had known all day Nov. 12 that fixing Philae to the surface would be even more complicated than originally thought because its Active Descent System of cold-gas thrusters, designed to push Philae against the surface for several seconds before harpoon deployment, did not function.

Philae teams had hoped that the early indications of the thruster dysfunction might have been an error message, but the post-touchdown telemetry confirmed that the system did not work.

The fact of touching down on a comet is itself an exploit. What is yet unclear is whether Philae, equipped with 10 experiments and designed to operate for at least 60 hours to characterize the comet’s surface, interior and chemical composition, will be able to perform much of its work.

Comet 67P has very little gravity. The Philae lander’s equivalent weight on Earth would be about 1 gram — a small sheet of paper landing on a floor. It would not take much on a comet shedding dust and water every day to disturb the lander’s position if it is not fastened tightly to the surface.

At a press briefing, ESA officials said they were still sorting through occasionally confusing telemetry purporting to show what happened to Philae.

Stephan Ulamec, Philae program manager at the German Aerospace Center, DLR, said that as difficult as it is to land on a comet, it is also difficult to determine what happens once on the surface.

Philae landed in the targeted zone and is functioning, Ulamec said. But its harpoon anchoring system did not deploy despite earlier indications that it had. He said it was unclear whether Philae landed in a sandbox-type surface composition, and that the variations in some of the telemetry needed to be further studied overnight.

What is also certain, he said, is that Philae’s instruments have already begun sending science data to the Rosetta orbiter for relay back to Earth.

From this data, he said, it appeared that Philae landed, bounced and then relanded a second time. Fluctuations in the radio link to the Rosetta satellite lead to this conclusion, he said.

ESA’s Rosetta flight director, Paolo Ferri, said at the briefing that Philae landed right on target, even assuming a bounce and second landing.