Philae’s wake up call

Europe’s tiny Philae lander on Comet 67P is now receiving twice as much solar energy as it did last November when it finally came to rest in a shaded spot.

A communication unit on the Rosetta orbiter has been switched on to call the lander and, although it may probably still too cold for the lander to wake up,  prospects improve with each passing day.

Several conditions must be met for Philae to start operating again. First, the interior of the lander must be at least at –45C before Philae can be induced from its winter sleep.

At its landing site named Abydos only a little sunlight reaches Philae – and the temperatures are significantly lower than at the originally planned landing location. The lander must also be able to generate at least 5.5 watts using its solar panels to wake up.

As soon as Philae ‘realises’ that it is receiving more than 5.5 watts of power and its internal temperature is above –45C, it will turn on, heat up further and attempt to charge its battery.

Once awakened, Philae switches on its receiver every 30 minutes and listens for a signal from the Rosetta orbiter. This, too, can be performed in a very low power state.

Philae needs a total of 19 watts to begin operating and allow two-way communication and it could be that the lander has already woken up from its winter sleep some 500 million km from Earth but does not yet have sufficient power to communicate with Rosetta, which relays Philae’s signal back to Earth.

The most likely time for contact is during the 11 flybys where the orbiter’s path puts it in a particularly favourable position with respect to the lander during comet ‘daytime’ – when Philae is in sunlight and being supplied with power by its solar panels.

Communication will be attempted continuously because Philae’s environment could have changed since landing in November 2014.

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