The European Space Agency's (ESA) Rosetta spacecraft should deposit its lander, Philae, on the southern hemisphere of comet 67P/Churyumov-Gerasimenko, EU-funded researchers are recommending. The team, which is supported in part by the EU-funded EUROPLANET scientists exchange programme, presented its findings at the recent European Planetary Science Congress (EPSC) in Rome, Italy. EUROPLANET is funded under the Research Infrastructures budget line of the EU's Seventh Framework Programme (FP7). Rosetta, which was launched into space in March 2004, is the first spacecraft to engage in the long-term exploration of a comet up close. Its destination is the comet 67P/Churyumov-Gerasimenko, a 'dirty snowball' 4 km (kilometres) across that orbits the Sun once every 6.6 years. Comets are among the most primitive objects in the Solar System, and scientists hope that Rosetta will generate new knowledge on what conditions were like during the early life of the Solar System. 'Churyumov-Gerasimenko is a time capsule holding material from the birth of the Solar System,' explains Jeremie Lasue of Los Alamos National Laboratory in the US, who was involved in the present study. On arrival at Churyumov-Gerasimenko in 2014, Rosetta will deposit Philae onto the comet's surface where it will be able to collect samples of material from beneath the surface. Over the next 13 months, Philae and Rosetta, which will remain in orbit, will gather detailed information on the comet as it approaches the Sun and travels away again. Selecting a landing site for Philae that is both safe and will deliver sound scientific data is far from easy. In this latest study, the team identifies the southern hemisphere as the most suitable target for Philae. According to the team, it will be easier for Philae to collect scientifically interesting samples in the southern hemisphere. Churyumov-Gerasimenko's nucleus is tilted at an angle of 45°, so the southern hemisphere will be subjected to the full glare of the Sun as the comet draws closer to the centre of the Solar System. Simulations show that the south pole will therefore be much more eroded than the north pole, making it easier for Philae to drill down into the comet and collect samples of cometary soil. Stability is also an important issue. As comets approach the Sun, they heat up and large quantities of gas and dust are emitted from their surface. While smaller specks of dust are swept away into the comet's coma, larger grains accumulate on the surface forming a layer of dust known as the dust mantle. The team has calculated that by the time of the landing, a coating 20 cm deep will have formed in the southern hemisphere compared to just a couple of centimetres in northern regions. In addition, the northern hemisphere will be illuminated by the Sun during the landing, and immense and rapid emissions of gas and dust would make landing there difficult. 'When Philae lands, temperatures at the equator may rise above freezing and could fluctuate by around 150°C,' adds Maria Cristina De Sanctis of Italy's Istituto Nazionale di Astrofisica, a co-author of the study. 'However, the regions close to the south pole will keep more stable temperatures,' she points out 'We would conclude [...] that the southern hemisphere area of the comet nucleus promises to present a safe landing site (low temperature variations, minimal gas and dust fluxes and good dust coverage) together with a highly eroded terrain and the potential for pristine material presence not too deep below the surface,' the researchers state. The astronomers arrived at their conclusions by developing and running three-dimensional computer models that predict the activity levels of the comet's nucleus from the moment Rosetta approaches the comet in early 2014 up until August 2015, when the comet comes closest to the Sun. Dr De Sanctis says: 'As more data on Churyumov-Gerasimenko becomes available to better quantify our results, we will be able to add to the picture and help prepare for a safe landing for Philae.' The EPSC is the major meeting in Europe for planetary scientists and is organised by EUROPLANET. EUROPLANET's goal is to bring together Europe's planetary science community through networking and joint research activities and by providing scientists with access to specialised laboratories and facilities as well as planetary science data, information and software tools.
Italy, United States