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The European Space Agency's Rosetta spacecraft is in orbit around comet 67P/Churyumov-Gerasimenko, shown here. It will stay there through the end of 2015 and send a lander to the comet's surface this fall.


Landing Site Chosen for Spacecraft's Daring Rendezvous With Comet

Finding a safe place to set down the European Space Agency's Rosetta lander was harder than expected.

Scientists announced Monday morning the spot where a small robot will touch down on the surface of a comet, in what they hope will be the first soft landing on a comet, as opposed to a crash landing.

When the European Space Agency's Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko this summer after a ten-year journey, scientists realized that landing on the lumpy chunk of ice and rock would be trickier than originally thought.

After a survey for sites that was a complex game of pin-the-lander-on-the-comet, the decision announced Monday is considered the best among the challenging options for setting down the probe, called Philae, on the comet’s craggy surface.

"There is flat area, but there is also some rough terrain," Stephan Ulamec, Philae lander manager at the German Aerospace Center, says about the site. "It's not a perfectly flat area as we probably would have hoped for a safe landing site."

Called site J (for now), the selected landing area presents an added hazard: It's near two active, gassy pits.

If all goes well, the lander will touch down somewhere inside a one-kilometer-wide target ellipse later this fall. The team has also selected a backup site in case site J loses its luster over the next months, as more high-resolution images come in.

Ten-Year Journey

The Philae lander is riding aboard Rosetta, the European Space Agency's interplanetary explorer tasked with unlocking secrets hidden in comets. Launched ten years ago, Rosetta took a serpentine path through the solar system as it raced to catch its speeding target, comet 67P/Churyumov-Gerasimenko, moving at up to 135,000 kilometers per hour. After a 6.4-billion-kilometer journey, the probe pulled into orbit around oddly shaped 67P/C-G in early August, and will stick around through the end of 2015.

Later this fall, Rosetta will descend toward 67P/C-G and nudge little Philae closer to the four-kilometer-wide comet. For about seven hours the dishwasher-size probe will inch toward the cosmic snowball, snapping photos, collecting samples of its atmosphere, and measuring solar wind and dust.

As it approaches 67P/C-G, Philae will fire a set of harpoons and tether itself to the frozen surface. Once parked, it will begin collecting scientific data almost immediately, and will spend the next several months drilling into and studying the material both on and beneath the comet's surface.

Because 67P is so far away, scientists won't have any control over the landing maneuver as it's happening. Instead, they'll program and upload the commands ahead of time. Then they'll wait for the lander's transmission, relayed through Rosetta.

Before scientists saw the comet, they had calculated a 75 percent chance of success at best, says ESA's Fred Jansen, Rosetta mission manager. The team hasn't updated that figure, but odds are those chances haven't improved.

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Landing site J has been selected as Philae's target. It's relatively flat, has few boulders, and is close to active pits. Landing site C, on the comet's main body, is the backup site.

Landing on a Rubber Duck

Three weeks ago the team members identified five candidate landing spots; over this past weekend, they met in Toulouse, France, to choose the best of the five.

Figuring out where to park Philae ended up being unexpectedly tricky. The comet's unusual shape and dramatic terrain surprised mission scientists, who weren't expecting their target to be shaped like a jagged rubber duck (or a fish, depending on the angle of view).

"We had been considering the comet as a gray potato for a number of years—based on near-Earth-based measurements—so the actual shape of the comet really twisted my melon," says Rosetta project scientist Matt Taylor. While the comet's two lobes are accessible, the strip of material connecting the lobes is not.

And in addition to its odd shape, the comet's surface is not smooth. It's covered in hazards such as giant boulders, cliffs, and cracks. Meeting any one of those upon landing could result in disaster for Philae. "It might capsize," Ulamec says, noting that the terrain "is scientifically fascinating, but on the other hand, not really lander-friendly."

Scientists also needed to contend with the lander's energy requirements. Philae's batteries are solar powered, so the lander needs to find a spot with about six hours of sunlight per 12.4-hour comet rotation—enough to charge the batteries, but not so much that it overheats.

"Putting those factors together, there are not too many areas good for Philae's landing," Ulamec says.

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