Released by the Rosetta spacecraft on 12 November 2014, after a decade-long voyage across the solar system, Philae descended to the surface of comet 67P/Churyumov-Gerasimenko. If everything had gone to plan, Philae would still be close to the Agilkia touchdown point, selected by the orbital mechanics teams at the SONC (CNES, Toulouse). However, the cold-gas thruster, designed to keep the lander on the surface while it fired its harpoon anchors, did not function. The harpoons did not deploy either. As a result, Philae bounced twice over a period of two hours before finally coming to rest in the shadow of a cliff more than 1 km from Agilkia. Some of its science instruments functioned well, taking advantage of the unexpected bounces to gather additional data, while others were compromised by Philae’s orientation with respect to the surface, making it impossible to drill and extract samples for analysis.
The real problem, however, was that the lander was now in a region with much less sunlight. Philae’s solar panels were thus unable to collect enough energy to charge its battery and extend its science mission, which ended when power ran out, about 60 hours after touchdown.
Reconstruction of Philae’s touchdown and bounces on 12 November 2014 (Philae’s size and velocity and comet surface relief not to scale). Credits: CNES/Active Design, 2015.
While the images and measurements taken by various instruments on the Rosetta orbiter and Philae quickly narrowed down the lander’s likely location to a strip about 200 metres long, not far from the rim of a depression called Hatmehit on the comet’s small lobe, finding misplaced Philae in this rough, dark terrain seems less certain. After months of efforts and the announcement that the teams may have finally located Philae, doubts nonetheless remain. Pinpointing Philae’s exact position is vital if scientists are to make full use of the data gathered by its instruments, particularly CONSERT, designed to provide a much closer understanding of the structure of the comet nucleus. As the comet approaches the Sun, it will also help determine when illumination conditions become favourable for Philae to wake up and re-establish contact with the orbiter.
The quest to find Philae
On 12 November 2014 at 15:34 UT, Philae touched down on the comet’s surface and bounced. This first contact and rebound were caught by Rosetta’s NavCam (navigation camera) and OSIRIS narrow-angle camera, which also tracked Philae as it continued toward the Hatmehit depression. On Philae, the ROLIS down-looking camera captured views of the surface from as close as 10 metres altitude before the first rebound, while the ROMAP sensor made precise magnetic field measurements over the next few hours, including precise timing of the various contact points: 16:20 UT, 17:25 UT and 17:32 UT.
Philae finally came to rest at an unknown site now known as Abydos. Images taken by its CIVA camera have built up a panoramic picture of its immediate environment: it is thought to be in the shadow of a cliff of dusty ice. In the next few hours, measurements by the CONSERT instrument on Philae and Rosetta were used to narrow down the lander’s location to an ellipse measuring approximately 16 x 160 metres. Next, visual searches for Philae began.
The NavCam and OSIRIS images have been scrutinized in an attempt to find the lander, but nothing looks more like a glint of sunlight reflected by a solar panel than a glint of sunlight reflected by a piece of ice! Image resolution and illumination angle are also a factor. The highest-resolution images of the area of interest were taken in mid-December 2014, when Rosetta made a series of flybys of the small lobe at an altitude of 18 km. At this distance, the OSIRIS narrow-angle camera has a resolution of 34 cm per pixel. Philae’s body is 1 metre across, so would be visible on these images, but the Sun was at an angle of 90° to the site, casting large shadows.
The lander’s orientation and the fact it is probably lying at an angle and hidden in the rugged terrain further complicated the task for those involved in the search. For weeks, all their efforts were in vain. Each time a candidate was found, images taken from other angles or under different illumination conditions ruled it out.
Finally, a more promising candidate emerged. It was detected by Guillaume Faury of AKKA Technologies, a contractor working for the LAM astrophysics laboratory and the IRAP astrophysics and planetology research institute, by comparing images taken on 22 October at a distance of 8 km from the comet’s surface and other images taken on 12 and 13 December at 18 km from the surface. “Although the pre- and post-landing images were taken at different resolutions, local topographic details match well, except for one bright spot present on post-landing images, which we suggest is a good candidate for the lander,” says Philippe Lamy, astrophysicist at LAM (a CNRS/Aix-Marseille Université joint laboratory), who helped design and build the OSIRIS narrow-angle camera and has been tirelessly searching for Philae. “This bright spot is visible on two different images taken on 12 and 13 December, clearly indicating that it’s a real feature on the surface of the comet, not a detector artefact or moving foreground dust speck.”
Éric Jurado, head of space mechanics at the SONC (CNES, Toulouse), and his colleagues subsequently confirmed that it was indeed a serious candidate, since its position is compatible with the trajectory reconstructions they had performed and with the illumination and radio visibility constraints established since November. The candidate is located just outside the ellipse calculated by CONSERT, but improved terrain models and continued CONSERT data analysis may alter its position.
Plus, it is possible that physical changes had taken place at this location on the nucleus between the October and December images, perhaps as fresh material was newly exposed. This is unlikely, however, because illumination conditions did not change much over this period.
Ultimately, a definite identification will depend on new high-resolution images of the site in better lighting conditions, but closer flybys of the comet core are on hold, due to the increase in activity as it approaches the Sun. The teams will therefore have to wait until autumn, after the comet’s activity has subsided, when Rosetta should be able to safely operate in close proximity to the nucleus again. Between now and then, scientists hope that local changes at the surface will not have engulfed Philae or ejected it into space. Unless, of course, Philae wakes up and simply says: “Here I am!”
The candidate discovered by Philippe Lamy’s team (LAM) was identified by Guillaume Faury (AKKA Technologies). E. Jurado and R. Garmier at the SONC (CNES, Toulouse), A. Herique and Y. Rogez of the CONSERT team (Institut de Planétologie et d’Astrophysique de Grenoble) and P. Heinisch of the ROMAP team (IGEP, Braunschweig) were also involved.