A new study of comet 67P/Churyumov-Gerasimenko reveals that the southern hemisphere of the comet will present the safest landing site for ESA's Rosetta mission to deliver its lander, Philae.

The finding is based on three-dimensional computer models created by Jeremie Lasue and colleagues from the INAF-IASF and IFSI institutes in Rome, that predict the comet's activity during the first few months of Rosetta's encounter.

“Southern sites appear to be both the safest and the most scientifically interesting,” says Lasue. As a comet approaches the Sun it heats up, and its gases and ices begin to vaporize, which can lead to outbursts from the surface. The models predict how heat is transferred through the comet and how quickly its ices will vaporize as it approaches the Sun.

“When Philae lands, temperatures at the equator may rise above freezing and could fluctuate by around 150 degrees Celsius,” says Maria Cristina De Sanctis, co-author of the study. “However, the regions close to the south pole will keep more stable temperatures. From our present results, we’ve concluded that the southern hemisphere promises the best landing sites.”

As the comet approaches the Sun its diamond-shaped nucleus will be tilted such that its south pole will be bathed in full sunlight. According to the simulations, after several close orbits of the Sun, these far southern regions will become significantly eroded, meaning that by the time Philae lands, it will be able to access pristine cometary material just below the surface more easily, using its drill that has a range of 30 centimetres.

Furthermore, the southern hemisphere appears to offer the most stable landing conditions. At the time of landing, the comet's northern hemisphere will be receiving most sunlight, concentrating most of the outburst activity there. Scientists estimate that up to 30 kilograms of gas, and 50 kilograms of dust could be emitted per second, forming a coating of dust grains from the comet's interior on the surface. This could be up to 20 centimetres thick in the southern hemisphere, giving Philae lots of opportunity to sample the material from the comet's interior.

“Churyumov-Gerasimenko is a time capsule holding material from the birth of the Solar System,” says Lasue. “The nucleus’ southern hemisphere has been heavily eroded, so Philae will not have to drill down far to find those pristine samples. At the time of Rosetta’s rendezvous, gas will be escaping mainly from the northern hemisphere, so it will be safer for Philae to touch down in the south. In addition due to the orientation of the comet, the southern hemisphere will be protected from extreme temperature variations at the time of delivery.”

Rosetta will rendezvous with the comet in May 2014, with the lander dropping down to the surface six months later, where it will study the surface and sub-surface to determine the comet's evolution. As Rosetta approaches the comet more data will be available to hone in on the safest landing sites for Philae.

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