September 15, 2014 – Rosetta’s lander Philae will target Site J, a region on Comet 67P/Churyumov–Gerasimenko that offers unique scientific potential, with hints of activity nearby, and minimum risk to the lander compared to the other candidate sites.
Site J is on the ‘head’ of the comet, an irregular shaped world that is just over 4 km across at its widest point. The decision to select Site J as the primary site was unanimous. The backup, Site C, is located on the ‘body’ of the comet.
The 100 kg lander is planned to reach the surface on November 11, where it will perform indepth measurements to characterise the nucleus in situ, in a totally unprecedented way.
Choosing a suitable landing site has not been an easy task.
“As we have seen from recent close-up images, the comet is a beautiful but dramatic world – it is scientifically exciting, but its shape makes it operationally challenging,” says Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center. “None of the candidate landing sites met all of the operational criteria at the 100% level, but Site J is clearly the best solution.”
The race to find the landing site could only begin once Rosetta arrived at the comet on August 6, when the comet was seen close-up for the first time. By August 24, using data collected when Rosetta was still about 100 km from the comet, five candidate regions had been identified for further analysis.
Since then, the spacecraft has moved to within 30 km of the comet, affording more detailed scientific measurements of the candidate sites. In parallel, the operations and flight dynamics teams have been exploring options for delivering the lander to all five candidate landing sites.
Over the weekend, the Landing Site Selection Group of engineers and scientists from Philae’s Science, Operations and Navigation Centre at France’s CNES space agency, the Lander Control Centre at DLR, scientists representing the Philae Lander instruments and ESA’s Rosetta team met at CNES, Toulouse, France, to consider the available data and to choose the primary and backup sites.
A number of critical aspects had to be considered, not least that it had to be possible to identify a safe trajectory for deploying Philae to the surface and that the density of visible hazards in the landing zone should be minimal. Once on the surface, other factors come into play, including the balance of daylight and nighttime hours, and the frequency of communications passes with the orbiter.
A one square kilometre area was assessed for each candidate site. At Site J, the majority of slopes are less than 30º relative to the local vertical, reducing the chances of Philae toppling over during touchdown. Site J also appears to have relatively few boulders, and receives sufficient daily illumination to recharge Philae and continue science operations on the surface beyond the initial battery-powered phase.
“We will make the first ever in situ analysis of a comet at this site, giving us an unparalleled insight into the composition, structure and evolution of a comet,” says Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France. “Site J in particular offers us the chance to analyse pristine material, characterise the properties of the nucleus, and study the processes that drive its activity.”
Both Sites B and C were considered as the backup, but C was preferred because of a higher illumination profile and fewer boulders. Sites A and I had seemed attractive during first rounds of discussion, but were dismissed at the second round because they did not satisfy a number of the key criteria.
Once deployed from Rosetta, Philae’s descent will be autonomous, with commands having been prepared by the Lander Control Centre at DLR, and uploaded via Rosetta mission control before separation.
During the descent, images will be taken and other observations of the comet’s environment will be made.
Once the lander touches down, at the equivalent of walking pace, it will use harpoons and ice screws to fix it onto the surface. It will then make a 360° panoramic image of the landing site to help determine where and in what orientation it has landed.
The initial science phase will then begin, with other instruments analysing the plasma and magnetic environment, and the surface and subsurface temperature. The lander will also drill and collect samples from beneath the surface, delivering them to the onboard laboratory for analysis. The interior structure of the comet will also be explored by sending radio waves through the surface towards Rosetta.
In conjunction with its lander, Rosetta will be in orbit around comet 67P/Churyumov-Gerasimenko. Boulder’s Southwest Research Institute (SwRI) Space Science and Engineering Division built one of three instruments aboard Rosetta that is funded by NASA.
The instrument, known as Alice, is probing the origin, composition and workings of the comet, gaining sensitive, high-resolution compositional insights that cannot be obtained by either ground-based or Earth-orbital observations. The ultraviolet wavelengths Alice observes contain unique information about the composition of the comet’s atmosphere and the properties of its surface.