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Engineers on the Rosetta team are struggling to figure out why they cannot get a solid communications lock with their Philae lander, and have come up with several explanations.
One possible explanation being discussed at DLR’s Lander Control Center is that the position of Philae may have shifted slightly, perhaps by changing its orientation with respect to the surface in its current location. The lander is likely situated on uneven terrain, and even a slight change in its position – perhaps triggered by gas emission from the comet – could mean that its antenna position has also now changed with respect to its surroundings. This could have a knock-on effect as to the best position Rosetta needs to be in to establish a connection with the lander.
Another separate issue under analysis is that one of the two transmission units of the lander appears not to be working properly, in addition to the fact that one of the two receiving units is damaged. Philae is programmed to switch periodically back and forth between these two transmission units, and after tests on the ground reference model, the team has sent a command to the lander to make it work with just one transmitter. As Philae is able to receive and accept commands of this kind in the “blind”, it should execute it as soon as it is supplied with solar energy during the comet’s day.
They have heard from Philae since July 9. Because of comet activity they have had to pull Rosetta back away from the nucleus to protect it, and are now focusing its primary activity on observations, not communications with Philae. They will continue to try to bring Philae back to full functioning life, but the priority is now the comet.