Descent onto "head" of a comet
Philae, the lander of ESA’s Rosetta space probe, will descend onto landing site J on the surface of comet 67P/Churyumov-Gerasimenko mid-November
The intended touchdown site of the Philae lander to be deployed by ESA’s Rosetta space probe is almost in the centre of the “head” of the comet 67P/Churyumov-Gerasimenko. According to the Lander Team, which was in deliberations last weekend with representatives from the Rosetta Science Team and ESA in Toulouse, this region offers the best conditions for a safe landing followed by successful measurements when compared with other regions. In the initial selection of five possible landing sites which the Lander Team had short-listed at the end of August, the region now selected was designated as J. The scientists nominated region C, which is located on the side of the comet’s “body”, as the alternative landing site.
Images of landing site J taken in the last few days with the aid of OSIRIS, Rosetta’s scientific camera system, show a very rugged terrain. Nevertheless, computations showed that the chances for landing are good. These took into account the topography of the landing site and the mechanical properties of Philae’s landing frame. The precise spot on which Philae touches down can be determined only with an accuracy of around 500 metres. “This means we do not need one perfect landing spot, but a region where as many landing scenarios as possible have a happy end,” explains Hermann Böhnhardt from the Max Planck Institute for Solar System Research, Project Scientist for the landing mission.
“None of our five landing site candidates has fulfilled all criteria 100 percent, but landing site J is clearly the best solution,” says Philae Project Manager Stephan Ulamec from the DLR.
One important criterion is the number of large rocks strewn over the landing site. Researchers in the OSIRIS Team had taken a detailed look at the five landing site candidates during the last few weeks and recorded, measured and charted all discernible rocks in the process. “According to our present findings, landing site J presents Philae with comparatively few ‘stumbling blocks’,” says Holger Sierks from the Max Planck Institute for Solar System Research, Principal Investigator of the OSIRIS Team.
Landing site J affords the lander good communication with the orbiter
“We are very satisfied with the top of the comet’s head, from a scientific point of view as well,” says Böhnhardt. Initial measurements indicate the presence of organic material there and that it will possible to investigate the activity of the comet from this site. In addition, the CONSERT instrument should find good conditions for its measurements at the selected landing site. CONSERT is the only experiment of the Rosetta mission which is part of the orbiter and the lander. It aims to use radio waves to investigate the internal structure of the comet’s nucleus. A radio signal will be transmitted from the space probe through the nucleus to the lander and back. Due to the shape of the comet’s nucleus and the flight path of the orbiter not every spot on the surface of 67P is equally suited to penetrate the whole of the comet’s nucleus. Landing site J is one of the best sites for this task.
Moreover, the region selected allows frequent and good communication between lander and orbiter. This is important for the many scientific measurements which are to be undertaken during the initial two to three days after touchdown and in sequence with short intervals in the weeks thereafter. An important consideration is that Philae’s data storage capacities are limited. Measurement data must be transferred to the orbiter as quickly as possible in order to free up space for new data; furthermore, commands from the orbiter must reach the lander rapidly. The light conditions on the comet’s head are favourable for the subsequent measurements: Philae’s solar cells can produce enough electricity there to reliably recharge the batteries.
During the coming weeks, all of the orbiter’s instruments will investigate landing site J in more detail. OSIRIS, for example, will image this region with a resolution of 30 centimetres per pixel. Only in the event of an emergency – if the higher resolution were to discover hitherto unknown risks, for example – has the lander team also specified an alternative landing site. Landing site C is located on the side of the comet’s “body”. It, too, fulfils important criteria, but is slightly more difficult to fly to.
Rosetta is a mission of the European Space Agency ESA with contributions from the member states and the American Space Agency NASA. Rosetta’s lander Philae was made available by a consortium headed by the German Aerospace Center (DLR), the Max Planck Institute for Solar System Research (MPS,) and the French and Italian Space Agencies (CNES and ASI). Rosetta will be the first mission in history to fly to a comet, accompany it on its orbit around the Sun, and put down a lander on its surface.