The Philae lander fell asleep on the surface of the comet 67P/Churyumov–Gerasimenko

Panoramic image of the surface of the comet 67P/Churyumov–Gerasimenko around the Philae lander (Image ESA/Rosetta/Philae/CIVA)
Panoramic image of the surface of the comet 67P/Churyumov–Gerasimenko around the Philae lander (Image ESA/Rosetta/Philae/CIVA)

After the understandable enthusiasm felt last Wednesday for the success of the Philae lander with its landing on the comet 67P/Churyumov-Gerasimenko, the team had to face the harsh reality. The area in which it ended up wasn’t the planned one and there it received much less light than expected. The consequence is that its batteries ran out of power a few hours ago making it hibernate. But before it worked for a few hours and transmitted a lot of data.

All this happened because the harpoons system that was supposed to anchor Philae to the ground didn’t work and the information gathered since then allowed to establish that it touched down three times before stabilizing. The instruments work but Philae was receiving much less sunlight than expected with serious consequences for the energy it had available for its instruments.

Initially, the connection between Philae and the Rosetta space probe was unstable but then it was possible for the lander to transmit the first data. The analyzes revealed that Philae touched down at 15:34, 17:25 and 17:32 GMT. The first touch down took place within the designated area, call Agilkia, but then the lander bounced away, touched down for the second time, rebounded for a much shorter time and finally touched down firmly on the surface of the comet.

This unexpected turn of events caused two serious problems. First, Philae is quite stable but there’s no clear picture of where it stopped and any attempt to move it could worsen the situation. In the area that was chosen it would have received a good amount of light to power it, in the area where it ended up it has much less.

The data collected by Philae can be sent back to Earth only during short windows when Rosetta, which is orbiting around the comet 67P/Churyumov-Gerasimenko, passes over it. This made the scientific work even harder. The Philae team had to start making hard decisions from the beginning to try to collect as much data as possible in the short time expected before hibernating.

The Philae team activity at ESOC (European Space Operations Centre) was frantic in deciding how to use the instruments. Initially, they decided to limit the risks in terms of energy use but time was limited. Despite the risks for Philae’s stability, it was decided to move it using its legs to orient one of its solar panels more favorably. Its drill was also used because the analysis of the comet 67P/Churyumov-Gerasimenko’s soil is crucial to the mission.

Last night Rosetta passed again over the area in which it can communicate with Philae. There was fear that the lander might have already run out of battery power but it managed to complete the planned analyzes with its instruments and to transmit the results.

Philae might wake up if it receives more light. It could happen at any time, today or in the coming months when the comet 67P/Churyumov-Gerasimenko will be closer to the Sun but that depends on the lighting that will hit the area in which it ended up. The Rosetta spacecraft will continue its work from orbit and will remain ready to pick up any signal should come from Philae.

Philae’s work was much shorter than they hoped at ESA but its landing is still a success as it allowed it to do at least some of the science for which it was built. Therefore Stephan Ulamec, Philae’s team manager, is right to express the team’s pride for what they accomplished.

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