Rosetta, Europe's comet-chasing spacecraft, has woken from its slumber.
A signal confirming its alert status was received by controllers in Darmstadt, Germany, at 18:17 GMT.
Rosetta has spent the past 31 months in hibernation to conserve power as it arced beyond the orbit of Jupiter on a path that should take it to Comet 67P/Churyumov-Gerasimenko in August.
Engineers will now finesse the probe's trajectory and prepare its instruments for the daring encounter.
One of the highlights of the mission will be the attempt to put a small robotic lander, Philae, on the surface of the 4.5km-wide comet. This will occur in November.
There were nail-biting moments in the Darmstadt control room as its flight engineers waited for the signal to come through. Three quarters of the way through the hour-long window of opportunity, they got what they were waiting for.
Gerhard Schwehm, mission manager for Rosetta, said: "After 31 months in hibernation, what is 45 minutes to wait?"
Andrea Accomazzo, the spacecraft operations manager, said: "I think it was the longest hour of my life, but also one of the most rewarding."
Monday's message, when it arrived, was a simple one - just a spike on the screens here at the European Space Agency's operations centre.
It was picked up in California by a 70m dish belonging to the US space agency, and then routed to Germany.
The signal contained no spacecraft telemetry, but its mere receipt from 800 million km away confirmed to controllers that Rosetta's automated systems were operating as expected.
In the coming hours and days, the Darmstadt team will talk to Rosetta to establish the full status of its systems.
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