On 15 November 2014 at 01:15 CET, Philae's battery was exhausted and, after nearly 60 hours of operation on Comet 67P/Churyumov-Gerasimenko, the lander went into hibernation – in an unexpected place. Philae 'bounced' several times before landing in its current location, and its exact position has still not been determined. "We have been able to narrow down Philae’s location to an area of approximately 16 by 160 metres," explains Project Leader Stephan Ulamec from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The search for the three-legged lander is complex because even when fully illuminated by the Sun, Philae will be just a few pixels across in images acquired by the Rosetta orbiter's Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). It is possible that Philae is visible in several images, but this is not certain. Since 30 May 2015, the engineers and scientists at the DLR Lander Control Center (LCC) have been listening for signs of life from the lander. The possibility of contact is improving as the comet moves closer to the Sun, and the chances of Philae receiving sufficient heat and energy are increasing."
Landing site with shadow and cold
On 12 November 2014, Philae 'hopped' roughly one kilometre away from its planned landing site, Agilkia, to its current position, Abydos, over the course of two hours. The harpoons designed to anchor Philae to the comet failed to fire and the ice screws in its feet were unable to secure the lander to the surface. After the final landing, the work for the team in the DLR control centre began in earnest. The lander operated for almost 60 hours, commanded the 10 instruments on board and finally turned itself to optimise its orientation with respect to the Sun. It was realised then that the lander is in a position where it is both shaded and cold. Sunlight reaches the lander for only 1.5 hours of the comet's 12.4-hour rotation period. The thermal component of the Multi Purpose Sensors for Surface and Subsurface Science (MUPUS) attempted to hammer a probe into the comet; it discovered a hard layer of ice and was able to measure temperatures as low as minus 180 degrees Celsius. "The images acquired by the Rosetta Lander Imaging System (ROLIS) and the Comet Infrared and Visible Analyser (CIVA) also reveal a more rugged, shadowed environment; it is likely that the lander also tilted, but Philae’s exact position has not been determined," explains Ulamec.
Evaluation – pixel by pixel
After the first touchdown, the Navigation Camera and OSIRIS were able to acquire images of Philae on the comet's surface. Shortly before the final landing, OSIRIS imaged the Hatmehit region on the 'head' of the comet and discovered a slightly less dark region in the dark shadows. This shows the lander above a crater rim on the head of the comet. But Philae is only one metre across; in the images, it is only a few bright pixels. "It is extremely difficult to locate the lander in the uneven terrain and to know for sure if 'that is Philae'," said Ekkehard Kührt, a planetary scientist at DLR and a member of the OSIRIS team. "In addition, the Rosetta orbiter has had to increase its distance from the comet for safety reasons, due to the rising activity of the comet." Only by meticulous comparison of images acquired before and after landing might Philae be discovered in the images. Ideally, this would take place using time-separated images acquired under identical conditions of solar illumination. It is also possible that a bright spot discovered in the images is a reflection from a boulder on the comet’s surface, for example, which depended on the angle of the Sun when the image was acquired.
Many failures and a few promising candidates
In addition, the site must also meet other conditions; it must match the reconstructed trajectories of the lander and be close to the landing ellipse of 16 by 160 metres defined by the COmet Nucleus Sounding Experiment by Radio wave Transmission (CONSERT) instrument. For some possible Philae discoveries on OSIRIS images, these restrictions have negated them – a more detailed analysis showed that the bright pixels could not be Philae at its final landing site.
A promising candidate among the OSIRIS images, for example, showed Philae just outside the calculated landing ellipse. Here, scientists from the OSIRIS team compared images from 22 October 2014 – that is, before the landing – with images acquired on 12 and 13 December – acquired exactly one month after the landing. On 22 October 2014, the distance was about 10 kilometres from the comet nucleus; in December it was about 20 kilometres. The result of comparing the two images reveals a bright spot – possibly Philae. However, simulations conducted by DLR show that Philae was only very slightly illuminated by the Sun at the time of image acquisition on 13 December 2014: "The Sun was not illuminating the lower portions of the lander at this time," said Valentina Lommatsch from the Lander Control team at DLR. Thus, its reflection would only be seen on a few pixels in the image acquired from a distance of 20 kilometres.
Listening for Philae's signal
Philae's exact location could be determined when the lander wakes up from hibernation and provides further scientific data. For this, it needs to be able to generate at least five watts of power and have an operating temperature above minus 45 degrees Celsius. Then, the lander will switch itself into operating mode. Slightly more energy – a total of 19 watts – is needed to communicate with the DLR team on the ground. For the communications unit on board the Rosetta orbiter to be able to transmit the status of Philae, the orbiter's flight path must allow it to 'see' the lander. Currently, the Rosetta orbiter is flying about 200 kilometres above the comet. Since 67P/Churyumov-Gerasimenko is now always active and is ejecting gas and dust into space, the flight plan for Rosetta has become even more challenging. "In recent weeks, the team at the DLR Lander Control Center has been preparing for the operation of Philae and its instruments – now we hope that it will get in touch with us," says Ulamec.
Rosetta is an ESA mission with contributions from its Member States and NASA. Rosetta's Philae lander is funded by a consortium headed by DLR, the Max Planck Institute for Solar System Research (MPS), CNES and the Italian Space Agency (ASI).