This is it, people — this is the spot on the comet where Rosetta's Philae lander will touch down on November 11th. It's called Site J, an area chosen for its unique scientific potential and minimum risk to the lander.
Choosing a landing site was not easy. Comet Churymov-Gerasimenko's strange, rubber ducky-like shape is presenting a host of operational challenges. And though the decision to land on Site J was unanimous, none of the candidate landing sites were able to meet all of the operational criteria at the 100% level. Site J was considered the best solution given the dearth of choices.
"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," noted Jean-Pierre Bibring, a lead lander scientist in a statement. "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."
A close-up of Site J.
Site J is on the "head" of the 4 km-wide comet. It was chosen from a pool of five candidate sites.
The ESA's Landing Site Selection Group had a number of things to consider. For one, they had to identify a safe trajectory for deploying Philae to the surface — one in which the density of visible hazards in the landing zone was at a minimal. At the surface, other factors included the balance of daylight and nighttime hours, and the frequency of communications passes with the orbiter.
Because the descent to comet will be passive, it will only be possible to predict a landing point for Philae within a "landing ellipse" about a few hundred meters in size. A one square kilometer area was assessed for each candidate site.
At Site J, the majority of the slopes are less than 30º relative to the local vertical, thus reducing the chances of the lander toppling over during touchdown. It also has relatively few boulders, and it receives sufficient daily illumination to recharge Philae; this will allow the lander to continue science explorations beyond the initial battery-powered phase. It should take Philae about seven hours to descend to the surface, a length of time that shouldn't suck up too much of the battery.
Site C has been chosen as the backup. It has a high illumination profile and few boulders.
From here, a detailed timeline will have to be prepared to determine the exact approach trajectory of Rosetta in order to deliver Philae to Site J. This must happen before mid-November, a time when the comet is predicted to grow more active as it moves closer to the Sun.
"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," noted ESA Rosetta flight director Andrea Accomazzo. "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 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.
"No one has ever attempted to land on a comet before, so it is a real challenge," says Fred Jansen, ESA Rosetta mission manager. "The complicated 'double' structure of the comet has had a considerable impact on the overall risks related to landing, but they are risks worth taking to have the chance of making the first ever soft landing on a comet."
There are two more dates we need to watch as the landing draws nearer. The landing date itself will be confirmed on September 26th, and the final Go/No Go for a landing at the primary site will be announced on October 14th.
Image credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.