ESA Selects Primary Landing Site For Rosetta Comet Study

Chuck Bednar for – Your Universe Online
Usually ‘X’ marks the spot, but for the ESA’s Rosetta orbiter, Site ‘J’ has been selected as the place where its Philae lander will touch-down on Comet 67P/Churyumov–Gerasimenko (67P/C-G), officials at the agency announced on Monday.
Rosetta, which is the first mission to ever rendezvous with a comet, will accompany 67P/C-G on its journey throughout the inner solar system and will measure the increase in activity as its icy surface is warmed up by the Sun, the agency said. The Philae lander will study the composition and the structure of the comet’s nucleus material, and will drill more than 20cm into the subsurface to collect samples for inspection in its onboard laboratory.
Now, the ESA confirmed that Site J, a region located near the head of the comet, had been unanimously selected as the primary landing site. Site J was selected because of the unique research potential it possesses, as well as the low risk to the lander in comparison to other locations. The 100kg lander is currently scheduled to reach 67P/C-G’s surface on November 11, and Site C (located on the comet’s body) will serve as the back-up destination.
Deciding on a landing point for Philae was not an easy task, according to Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center. He explained that the shape of the comet “makes it operationally challenging,” and that while “none of the candidate landing sites met all of the operational criteria at the 100 percent level,” site J on the irregularly shaped, 4 km wide head of the comet was “clearly the best solution.”
“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,” added 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 analyze pristine material, characterize the properties of the nucleus, and study the processes that drive its activity.”
The search for a suitable place for Rosetta’s lander to touch-down upon started once the probe arrived at the 67P/C-G on August 6. By August 24, Rosetta had collected enough data from a distance of 100 km to select five potential candidate regions, and those locations were further analyzed when the vehicle moved to within 30 km of the comet. Over the weekend, engineers and scientists met to review the data and select the primary and secondary 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,” the European space agency explained. “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.”
“The descent to the comet is passive and it is only possible to predict that the landing point will place within a ‘landing ellipse’ typically a few hundred meters in size. A one square kilometer area was assessed for each candidate site,” the ESA said. At Site J, most of the slopes are less than 30 degrees, which reduces the chances of the lander “toppling over during touchdown,” and the location also appears to have “relatively few boulders,” it added.
Furthermore, the location appears to receive enough light each day for Philae to recharge and continue its scientific operations beyond its initial battery-powered phase, the evaluation team concluded. Descent time to the surface of Site J is expected to take approximately seven hours, which should not compromise the lander’s on-comet observations by consuming too much of the unit’s battery during the actual landing itself.
While Site B was also considered as a potential backup, Site C was selected because of a higher illumination profile and fewer boulders, the agency said. Sites A and I appeared to be attractive candidates early on, but were ultimately eliminated because they failed to satisfy several essential criteria.
The ESA team will now have to prepare a detailed timeline to determine Rosetta’s precise approach trajectory in order to safely deliver Philae. The landing must take place before mid-November, as the comet’s activity levels are expected to spike as it grows closer to the sun.
As ESA Rosetta flight director Andrea Accomazzo explained, “There’s no time to lose, but now that we’re closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites. Of course, we cannot predict the activity of the comet between now and landing, and on landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that’s what makes this a risky operation.”
Once the lander is deployed by Rosetta, its descent will be autonomous, as it will follow commands prepared in advance and uploaded through mission control prior to separation, the ESA explained. As Philae lands, it will capture images and other observations of the comet’s environment, and once it touches down, it will become fixed to the surface using harpoons and ice screws. It will then create a 360 degree panoramic image of its location.
At that point, “the initial science phase will then begin, with other instruments analyzing the plasma and magnetic environment, and the surface and subsurface temperature,” the agency added. “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.”
The landing date should be confirmed on September 26 after the Rosetta team completes additional trajectory analysis, and the engineers and scientists will complete a comprehensive readiness review in the weeks that follow. The ESA will deliver a final verdict for a landing at the primary site on or around October 14.