It was an historic moment last autumn when the signal was received in ESA's German mission centre that the probe had landed on the comet's surface. But while the scientific world – and many in the wider world – were still celebrating, came the news that Philae had limited energy left for future communication.
It has been slumbering on the side of the comet ever since, but now it is closer to the Sun – around 320m kilometres away – meaning it is receiving twice as much solar energy as it did in November.
The ESA has turned its communication unit back on in the hope that the landing probe will soon be able to “talk” once again.
“Although it is probably still too cold for the lander to wake up, the prospects will improve with each passing day,” the space agency said.
“Philae is designed so that, since November 2014, it has been using all the available solar energy to heat up,” said Koen Geurts from the DLR Control Center.
“At this time, we do not yet know that the lander is awake. To send us an answer, Philae must also turn its transmitter – and that requires additional power.”
But there are still plenty of challenges for the little landing probe that could.
Internal temperatures must reach at least -45*C before Philae can break its hibernation, but its current site – called Abydos – receives less sunlight than the original planned location. It must be able to generate at least 5.5 watts using its solar panels to wake up and start charging its battery.
Once that happens, 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.
“It could be that the lander has already woken up from its winter sleep some 500 million kilometres away from Earth, but does not yet have sufficient power to communicate with Rosetta, which relays Philae’s signal back to Earth,” the government body said.
“Philae needs a total of 19 watts to begin operating and allow two-way communication.”
Rosetta is now orbiting the comet and will listen for a response until March 20. Communication will be attempted continuously because Philae’s environment could have changed since landing in November 2014.
“If we cannot establish contact with Philae before 20 March, we will make another attempt at the next opportunity,” said lander project manager Stephan Ulamec from the German Aerospace Center. “Once we can communicate with Philae again, the scientific work can begin.”