Deep depressions and dangerous cliffs have made it difficult for scientists to find a landing site for the Rosetta spacecraft that has been orbiting comet 67P/Churyumov-Gerasimenko.

However, after studying potential target sites for weeks, the agency announced on Monday that Rosetta's Philae robot is planned to touch down on the comet's "Site J" on November 11.

Comet 67P/Churyumov-Gerasimenko is located 440 million km from Earth and has a terrain that makes it difficult to safely land Philae. Images of potential landing sites showed cracks and slopes that could turn the probe over. Even the comet's flat surfaces contain boulders and fractures.

"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," says Stephan Ulamec, of the DLR German Aerospace Center.

A pre-mission analysis reported a 70-75 percent chance of a successful landing.

"At the end of the day, you'll only know when you land. Then it will have been either 100% or zero. That's the way it is," says ESA project manager Fred Jansen.

Scientists selected five potential landing zones in late August, narrowing the sites down to two last Monday. "None of the candidate landing sites met all of the operational criteria at the 100% level, but Site J is clearly the best solution," Ulamec says.

Site J is located on the "head" of 67P and has a relatively flat terrain, despite some cliffs. "There are flat areas, but there is also rough terrain," Ulamec says. "It's not a perfectly flat area as we probably would have hoped for a safe landing site." Images reveal few boulders and enough sunlight, which Philae will need to recharge its batteries.

Landing Philae from a distance of about 10km is what the ESA calls a one-shot opportunity. The agency has also selected "Site C," which is located on the larger lobe of 67P, as a back-up landing site in case the unexpected happens.

Site C contains depression, cliffs and hills, but smooth plains as well.

Both the primary and back-up sites will be further studied before final calls are made in mid-October.

Philae has been flying with Rosetta since its launch in 2004. The spacecraft arrived at the comet in early August after taking ten years to accurately position. The goal of the decade-long mission is to gain better understanding of our solar system and if the comets carried water to Earth.

Since the "hands-off" mission is the first of its kind, a successful landing on the comet will make history. The probe is expected to last until March because of overheating, but Rosetta will continue to orbit the comet for at least a year.

"I feel this is really a historic moment in science. It's unprecedented; it's a quantum step in cometary science, says Jean-Pierre Bibring, the co-principal investigator on Philae.

The ESA will hold a public competition to rename Site J.