Looking for a landing site
Researchers select five areas on the 67P/Churyumov-Gerasimenko comet where Philae could touch down.
Technically feasible to fly to, as level as possible, not too much shade, not too much sun − and scientifically interesting: these are the conditions which an area on the surface of the 67P/Churyumov-Gerasimenko comet must fulfil in order to be selected as the landing site for Philae, the lander of the European Rosetta space probe. In an initial selection, researchers and engineers have now nominated five possible candidates. Three of the potential landing sites are on the “head”, the other two on the larger “body” of the comet. The scientists believe that Philae could observe the activities of 67P close up from this vantage point.
“The deliberations were difficult, but we were able to identify a few sites which both the flight engineers and the scientists are very happy with,” says Hermann Böhnhardt from the Göttingen-based Max Planck Institute for Solar System Research, scientific manager of the landing mission who was involved in selecting the areas.
This was not an easy undertaking, because not only must it be possible to reach the perfect landing site safely and allow Philae to be operated without a hitch, it must also provide the necessary conditions for the planned scientific investigations. The team has now made a shortlist of five landing sites.
67P/Churyumov-Gerasimenko is an unusually shaped lump: a small part, the head, is connected to the larger body via a region shaped like a neck. “If one looks at the unusual shape and the global topography of the comet, it is certainly not surprising that many areas were immediately disregarded,” says Philae project leader Stephan Ulamec from the German Aerospace Center (DLR).
In the opinion of the researchers, promising candidates can nevertheless be found on both parts: three on the head, two on the body. However, the neck region, whose striking, bright colouring makes it of particular interest to many scientists, does not offer the necessary conditions for a safe landing and the subsequent, successful operation of Philae.
“The most important criterion was obviously that Philae can reach the areas in the first place,” says Böhnhardt. This does not apply to all regions of the comet, of course. Even if one takes into account all the velocities, trajectories and orientations of the space probe at the time Philae undocks and the different possible detaching speeds of the lander itself, the map of the comet still has no-go areas.
And equally important: from the landing site, there must be regular radio contact to the space probe in order to exchange operational commands and data. A surface which is as level as possible should also guarantee a safe landing. After the landing, at least six hours of sunlight are necessary every day for at least six months to charge Philae’s batteries with the aid of the solar cells. The Sun, however, must not shine for too long, either, otherwise Philae could overheat. As the deliberations showed, these conditions do not apply to the entire underside of the comet.
From a scientific point of view, further criteria are very important. Which sites afford a good view of the comet’s activity, for example? As the comet travels towards the Sun, the surface increasingly heats up. Even now highly volatile gases are evaporating from some parts and entraining dust particles as they do so.
Analysing this material is the task of the DIM instrument (Dust Impact Monitor), for example, which is led by scientists from the Max Planck Institute for Solar System Research. “We have preferentially selected landing sites which, according to our present knowledge, will afford us the opportunity to investigate the comet activity at close range, as it were,” says Hermann Böhnhardt.
The members of the selection committee base their decision on initial measurement data which the instruments aboard Rosetta have been recording since it arrived at the comet on 6 August. The images provided by OSIRIS, the scientific camera system, played a crucial role in this. From a distance of around 100 kilometres, they will provide initial clues about the topography of the body and make surface structures measuring two metres and more visible.
“Over the coming weeks we will take a very detailed and close look at the five landing site candidates,” says Holger Sierks from the Max Planck Institute for Solar System Research, scientific manager of the camera team. To this effect, a resolution of 50 centimetres per pixel will be achieved. “We will also undertake a more detailed investigation of the local activity that we can see even now in the jets which are streaming off.”
Other researchers hope also to obtain information on the locations of organic substances on the surface of the comet over the coming weeks. The COSAC (Cometary Sampling and Composition Experiment) instrument, for example, requires a landing site which is rich in organic compounds. This is where the gas chromatograph coupled to a mass spectrometer is to search for the building blocks of life.
This criterion could play an important role in the middle of September, when the selection committee will be meeting again. Two of the five nominated landing sites are then to be shortlisted and prioritised. “The process of selecting a landing site is extremely complex and dynamic. As we approach the comet, we will see more and more details which will influence our decision,” says Rosetta mission head Fred Jansen from ESA.
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 set down a lander on its surface.
BK / HOR