The Rosetta spacecraft was launched in 2004 with the mission of becoming the first probe to perform a soft land on the body of a comet. Rosetta finally caught up with Comet 67P Churyumov-Gerasimenko on August 6 and has been looking for a good place to land ever since. The list of potential landing sites was reduced to just five a couple of weeks ago. After reviewing all of the data Rosetta has delivered so far, Site J—located on the head of the comet—appears to be the best fit. Rosetta’s lander, Philae, will be making contact with the comet on November 11.

"As we have seen from recent close-up images, the comet is a beautiful but dramatic world—it is scientifically exciting, but its shape makes it operationally challenging," Philae Lander Manager Stephan Ulamec told NASA. "None of the candidate landing sites met all of the operational criteria at the 100-percent level, but Site J is clearly the best solution.”

When determining which landing site would work best for Philae, the team had to find a place that would be large enough to safely house the lander—about a square kilometer—with as few hazards as possible. They also needed to make sure that the site received enough sunlight and that Philae would be able to communicate often with Rosetta, who will be orbiting the comet and performing measurements.

If Site J doesn’t work out for whatever reason, Site C has been selected as a backup. Site C is located on the comet’s neck and is more exposed to sunlight than some of the other suggested sites. However, it is important that Philae touches down on the comet sooner rather than later. The comet is en route to the inner solar system. As it gets closer to the sun, the heat will make the comet more active by shedding off ice and dust. Too much activity could completely blind the landing, making it unsafe for Philae to attempt.

Backup site C. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM

"There's no time to lose, but now that we're closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites," Rosetta flight director Andrea Accomazzo added. "Of course, we cannot predict the activity of the comet between now and landing, and on landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that's what makes this a risky operation.”

Philae’s detachment from Rosetta and descent to the Site J will not be directed in real time. Rosetta is currently about 30 meters away from the comet, and Philae will have to make that journey alone. Based on the conditions nearing launch time, directions will be given to the lander. As soon as it hits the surface, Philae will secure itself to the surface and quickly begin to examine the comet in order to get its bearings and send information about how and where it landed.

Site J’s name is subject to change. ESA will be launching a contest soon, seeking names for this historic landing site.