After several months of research, the teams of the Laboratoire d’Astrophysique de Marseille, those of the SONC and several scientists involved in the CONSERT and ROMAP instruments, believe that they have found the Philae lander jettisoned on comet 67P on 12 November 2014.
Released from its mechanical embrace with the Rosetta orbiter on November 12, 2014, after a decade of travel in the Solar System, Philae fell on the nucleus of comet 67P/Churyumov-Gerasimenko. If everything had gone as planned, Philae would still be right next to the landing point targeted by the SONC space mechanics teams (CNES, Toulouse) and christened Algikia at the time. But the cold gas thruster that was supposed to press the lander to the ground while its harpoons were sinking into the surface to dock it did not go off, the harpoons did not work and Philae bounced several times for 2 hours before crashing into a poorly lit cliff corner more than 1 km from Algikia. His scientific instruments were able to work more or less well, some taking advantage of the unexpected bounces to record additional data, others being handicapped by Philae’s orientation relative to the surface which did not allow them to drill and take samples for analysis, but, most seriously, it was really this drift to a region much less exposed to the sun.
Philae was unable to collect enough solar energy to charge its battery and extend its scientific mission, which ended with the depletion of its battery about 60 hours after arriving on the ground.
While images and measurements taken by various instruments from Rosetta and Philae made it possible to quickly pinpoint a landing zone with a wingspan of less than 200 m, located not far from the edge of the Hatmehit depression on the small lobe, finding Philae in this poorly lit chaos proved to be much more uncertain. After months of effort, and even though the probability that the searchers had finally located Philae seems very high, there is still some doubt. Locating Philae’s exact location is however essential to fully exploit the data collected by its instruments, especially those of CONSERT, thanks to which scientists will have a much more intimate understanding of the core structure. It will also make it possible to determine the period of time when the changing sunlight conditions on the approach to the Sun will favour the awakening of the lander and its reconnection with the orbiter.
On November 12, 2014 at 15:34 UT, Philae contacted the core surface and bounced back. This first contact and bounce were photographed by the navigation camera (NavCam) and by the orbiter’s high-resolution OSIRIS-NAC camera. The OSIRIS-NAC camera was even able to follow Philae as he headed towards the Hatmehit Depression. On Philae, the ROLIS camera provided images of the surface up to less than 10 m altitude before the first bounce and ROMAP gave precise indications of the magnetic field measured during the following hours, which made it possible to know the exact time of the various contacts: 16h20 UT, 17h25 UT and 17h32 UT.
Finally, once Philae was immobilized on an unknown site, named from Abydos, the CIVA cameras panned over his immediate surroundings: he was apparently stuck in the shade of a kind of dusty ice cliff. In the hours that followed, the CONSERT instrument, on Philae and aboard Rosetta, made measurements that were used to finally reduce the landing area to an ellipse of 160 x 16 m. Then the visual search for Philae began.
All the images taken by NavCam and OSIRIS were scanned to try to find the lander, but nothing looks more like the reflection of the Sun on a solar panel than the reflection of the Sun on a piece of ice! Naturally, the illumination of the area and the resolution of the images had to be taken into account. In mid-December 2014, while Rosetta was flying over the small lobe at about
At a distance of 18 km, the resolution of the OSIRIS-NAC images was close to 34 cm per pixel, which is more than enough to see Philae, whose central part is almost 1 m wide, but the Sun illuminated the area at an angle of 90° and the shadows were very large. The orientation of the lander and the fact that it was probably located at an angle in a recess in the terrain did not make the task any easier for the people who tackled the problem. For weeks, all their efforts were in vain. Each time a “candidate” was found, another image taken from a different angle or under a different lighting would allow it to be ruled out.
Eventually, one candidate stood out. He was detected by Guillaume Faury of AKKA Technologies, a company working under contract for the Marseille Astrophysics Laboratory (LAM) and the Institute for Research in Astrophysics and Planetology (IRAP), by comparing images taken on 22 October at nearly 8 km from the surface with others taken on 12 and 13 December at nearly 18 km. For astrophysicist Philippe Lamy (LAM, CNRS/Aix-Marseille University unit), who took part in the design and production of the OSIRIS-NAC camera and who tirelessly searched for Philae: “the images taken before and after the landing do not have the same resolution, but the topographical details correspond, except for a small light spot which is only seen on the December images and which therefore seems to be a good candidate! Moreover, the fact that this candidate is visible on the images taken on December 12 and 13 proves that it is not a reflection, an artifact on the electronic sensor, or even a dust passing in the field. »
Éric Jurado, head of space mechanics activities at the SONC (Science Operation & Navigation Center, CNES, Toulouse) and his colleagues have since confirmed that this candidate is to be taken really seriously, as his position is compatible with the trajectory reconstructions they have carried out and with the criteria of sunshine and radio visibility that have been determined since November. It is slightly outside the area calculated from the CONSERT data, but the analysis of this data is continuing and the area could be shifted to take into account the improvement of the terrain model.
In addition, it is not impossible that changes in the surface may have occurred between the October and December images causing new, brighter portions of material to appear, but this seems unlikely as the sunlight conditions varied little over the period.
Only new, high-resolution images of the area with good illumination would make a difference, but close overflights of the core are now outlawed because of its increased activity as it approaches the Sun. It will therefore be necessary to wait until autumn and the decrease in activity to return to the vicinity, in the hope that local changes in the surface will not de facto end the search by burying Philae or catapulting it into space. Unless, of course, Philae wakes up and simply says, “I’m here! »
The candidate discovered by Philippe Lamy’s team (LAM) was identified by Guillaume Faury (AKKA Technologies). Éric Jurado and Romain Garmier from SONC (CNES, Toulouse), Alain Herique and Y. Roger from the CONSERT team (Institute of Planetology and Astrophysics, Grenoble), P. Heinisch from the ROMAP team (IGEP, Braunschweig) were also involved.
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