It has been nearly four months since Philae landed on Comet 67P. Because the probe landed in the shadow of a cliff, it couldn't draw sufficient energy from the sun. But later this week, there's a slim chance the probe could awaken and send a signal to the Rosetta spacecraft.

Comet 67P is currently 320 million kilometers from the Sun. This means Philae and its extended solar panels could be receiving about twice as much solar energy as it was last November. It's probably still too cold for the lander to wake up, but ESA mission controllers are not taking any chances.

Here's what has to happen for Philae to wake up, according to the ESA:

Several conditions must be met for Philae to start operating again and allow the DLR Lander Control Center to put it to work. First, the interior of the lander must be at least at –45ºC before Philae can wake up from its winter sleep. At its new landing site – Abydos – only a little sunlight reaches Philae, and the temperatures are significantly lower than at the originally planned landing location. In addition, the lander must be able to generate at least 5.5 watts using its solar panels to wake up. It has not remained idle during hibernation.

"Philae is designed so that, since November 2014, it has been using all the available solar energy to heat up," says Koen Geurts from the DLR Control Center.

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

Which would be amazing. Should this happen, Philae would switch on its receiver every 30 minutes and listen for a signal from the Rosetta orbiter. It can do this in a very low power state. During this time, however, mission controllers won't know if Philae is awake. For that, they would need to receive a signal from the probe's transmitter, which requires a bit more power. It's actually conceivable that Philae is in fact awake but unable to communicate with Rosetta, which relays its signal back to Earth. For this to happen, Philae needs a total of 19 watts to begin operating and allow two-way communication.

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This is why the upcoming window between March 12 to 20 is so important. During this time, the Rosetta orbiter will be transmitting to the lander in hopes of receiving a response. The optimal times for contact will happen during one of 11 flybys where Rosetta's path will put it in a favorable position relative to the lander during the comet's "daytime" — that is, when Philae is in sunlight and being supplied with power by its solar panels.

Also, new commands have been sent to Philae to optimize the heating and provide energy savings to improve its chances of communicating. It may not have the energy to answer, but it could receive and execute those commands — a process known as "blind commanding" by the engineers.

Should Philae spring back to life, it will initially transmit a health report to Earth. From there, the controllers would evaluate the data, determine the state of the rechargeable battery, assess what's functional, determine the local temperature, and figure out how much energy the probe is receiving. The mission parameters would be adjusted accordingly. Ideally, it would be nice if Philae could still help the scientists perform some science.

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If contact cannot be established by March 20, the controllers will start planning for the next opportunity. And indeed, as the distance to the Sun decreases, the potential energy collected by the lander's solar panels is increasing.

[ ESA ]

Top image: The comet on March 6, 2015: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0