Circle marks landing spot: Lander hopes to avoid jets of gas and dust that could complicate descent.

Circle marks landing spot: Lander hopes to avoid jets of gas and dust that could complicate descent.

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Comet landing site picked: Rosetta to aim for the J spot

The Rosetta mission has picked a destination on top of comet 67P/Churyumov-Gerasimenko for its Philae lander, mission managers announced today at European Space Agency (ESA) headquarters in Paris.

The area—site “J”—is near the top of the smaller lobe, or head, of the comet, which some have likened to the shape of a duck ever since the Rosetta spacecraft arrived at the comet on 6 August. The spacecraft is the first to accompany a comet as the sun heats it up and turns on its jets of gas and dust. The landing, to take place on 11 November, would be the first to scoop up a sample of dust and ice and analyze its composition.

Scientists and engineers picked the site, an ellipse with an area of about a square kilometer, from a shortlist of five candidates following a 2-day meeting in Toulouse, France. Site “C” will be the backup landing, in case site “J” proves troublesome on further inspection. “It is not a perfectly flat area,” says Stephan Ulamec, the Philae landing manager at the German space agency (DLR) in Berlin. “Even here the risk is high.”

The sites were evaluated on the basis of several criteria. One was surface terrain: Managers want to avoid areas with boulders, cliffs, or slopes that could cause the lander to topple. With two harpoons under its belly to help anchor it, Philae can cope with a variety of different material strengths on the surface. Early indications are that the dust at the surface is soft, fine-grained, and fluffy, and well within the lander’s capabilities.

Also important for selection were the dynamical considerations of the descent. For about 7 hours, the lander will fall, unpowered, under the feeble gravity of the 4-kilometer-long comet, and managers want the orbiting spacecraft to maintain as much radio contact as possible. They also want to avoid regions of active jetting so that outflowing gas and dust will not alter the lander’s trajectory. As such, they selected a region on the smaller lobe, which so far does not appear to be as active.

At the same time, the team doesn’t want to be too far away from all the action. Holger Sierks, the principal investigator for the orbiter’s main science camera, was excited that there are two cylindrical pits sitting just 500 to 600 meters away from the landing site—within view of the lander’s panoramic cameras. “We see early indications that these pits might become active,” says Sierks, a researcher at the Max Planck Institute for Solar System Research in Gottingen, Germany.

Finally, scientists also had a say. Most of all, they want a scoop of the black, organic-rich dust that they believe represents material that has been unaltered since the earliest days of the solar system, more than 4.5 billion years ago. As such, they want to avoid regions that have been active during the comet’s recent encounters with the sun. “We want to get material as pristine as possible,” says Philae lead scientist Jean-Pierre Bibring of the Institute for Space Astrophysics in Orsay, France.