On August 6, the Rosetta probe rendezvoused with Churyumov-Gerasimenko. Now comes the hard part: picking a suitable location for the spacecraft's Philae lander. The site must be chosen soon because, as the comet gets closer to the sun, its icy surface will begin to change.
Churyumov-Gerasimenko is on a 6.5-year orbit around the Sun and is currently 324 million miles away from it. The Philae landing is expected to take place in mid-November when the comet is still about 279 million miles away. From that point on, both the lander and the orbiter will observe how the comet's surface material changes over time, as it nears its closest approach to the sun—114 million miles in August 2015—which will subject it to eight times the amount of sunlight it's receiving now.
But, where to land? As the European Space Agency reports:
For each possible zone, important questions must be asked: Will the lander be able to maintain regular communications with Rosetta? How common are surface hazards such as large boulders, deep crevasses or steep slopes? Is there sufficient illumination for scientific operations and enough sunlight to recharge the lander's batteries beyond its initial 64-hour lifetime, while not so much as to cause overheating?
To answer these questions, data acquired by Rosetta from about 100 km [62 miles] distance have been used, including high-resolution images of the surface, measurements of the comet's surface temperature, and the pressure and density of gas around the nucleus. In addition, measurements of the comet's orientation with respect to the sun, its rotation, mass and surface gravity have been determined. All of these factors influence the technical feasibility of landing at any specific location on the comet.
By September 14, five candidate sites will have been assessed and ranked, leading to the selection of a primary landing site, for which a fully detailed strategy for the landing operations will be developed.