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A region on the "head" of comet 67P/Churyumov-Gerasimenko, near the top of this image, is the target for Philae, the Rosetta mission's lander.

A region on the "head" of comet 67P/Churyumov-Gerasimenko, near the top of this image, is the target for Philae, the Rosetta mission's lander.

ESA/Rosetta/NavCam (CC BY-SA 3.0 IGO)

Rosetta mission primed for comet landing

DARMSTADT, GERMANY—The stage has been set for one of the most ambitious maneuvers in the history of space exploration: a comet landing. On Wednesday, the European Space Agency’s (ESA’s) Rosetta spacecraft will release a washing machine–sized lander, Philae, in the hope that it alights safely on the dusty, black surface of comet 67P/Churyumov-Gerasimenko.

Speaking Monday afternoon at a media briefing at ESA’s European Space Operations Centre in Darmstadt, Germany, mission managers stood and delivered relaxed, off-the-cuff remarks: They had done all they could. The lander’s commands had already been uploaded, and tonight, Philae would be turned on and warmed up. After that, the mission team was planning something rare: a good night’s sleep.

“I don’t deny that we are a bit tired, but we’re also very excited,” says Andrea Accomazzo, the mission’s flight director. “I don’t think we’ll land on a comet another time. We’re convinced we’ve done all we have to do and all that we could do.”

The Rosetta spacecraft has been orbiting 67P since August, when it arrived after a 10-year journey through the solar system. Rosetta’s initial orbits were tentative—the spacecraft had to map the feeble gravity field of the comet as it went—but it has now circled 67P at altitudes as close as 10 kilometers. In high-resolution images, the comet, 4 kilometers long and shaped like a rubber duck, has revealed surprising terrain: cliffs and boulders and smooth plains etched with mysterious sinuous patterns. Ice in the neck of the duck has already begun to evaporate, blowing off gas and dust—an activity that will only grow as the comet approaches the sun. In September, the mission team picked a landing site on the side of the duck’s head. In keeping with the mission’s ancient Egypt theme, the square-kilometer landing zone has been named Agilkia, after an island in the Nile River.

The orbiter has since backed away from the comet a bit. After a complicated set of maneuvers—best explained in this YouTube video from ESA—Rosetta will release Philae at an elevation of 22.5 kilometers. Accomazzo says that Rosetta can shove off Philae within only a small range of speeds, and that this speed correlates best to the 22.5-kilometer elevation. Moreover, at that elevation, the orbiter has better confidence in its precise position around the comet. The moment of release is planned for 4:03 a.m. EST on Wednesday.

Once in motion, Philae will drift down, unpowered, over the course of 7 hours, striking the surface at walking speeds. The mission team hopes to confirm touchdown at 11:02 a.m. EST. (The actual landing will have occurred nearly 30 minutes before that—the delay is caused by the travel time for radio signals to reach Earth.)

The lander, equipped with two harpoons and spinning screws in each of its three legs, will attempt to gain purchase. Rosetta project scientist Matt Taylor says that early results from some of the orbiter’s instruments show that the surface is slightly warmer than expected—an indication that it is more dusty and porous than icy. That could help the lander sink into the surface rather than bounce.

Yet a successful landing is by no means guaranteed. The mission team picked Agilkia because it had favorable characteristics: good illumination, relative flatness, and few boulders. “It was relatively easy to pick this site,” says Stephan Ulamec, the Philae project manager. Nevertheless, Ulamec says, some 20% of the landing zone is sloped at 30° or more—angles at which it becomes possible for the lander to topple. On top of that, there are many boulders that the lander could tumble over. And technically, Philae needs to perform its tasks perfectly.

But if all goes well, the historic moment in the making will also be a passing of the baton—from the engineers and project managers who got Rosetta and Philae to the comet in one piece to the scientists, whose data gathering will begin in earnest. “Up till now we’ve been doing science on the side,” Taylor says. “From this week onwards is where we start the main phase of the mission.”

To read more Rosetta coverage, visit our Rosetta collection page.