FRANKFURT - A robotic space lander has surprised scientists by waking up and sending a signal to Earth, seven months after straying into the shadows of a comet where they feared it might be marooned for ever.
The European Space Agency said on Sunday that it had received signals from the lander, named Philae, late on Thursday, when it began "speaking" with its team on the ground for the first time since it went into emergency hibernation following a botched landing on the comet in November.
Scientists believe the space probe is receiving increasing amounts of sunlight as the comet speeds closer to the sun, enabling its solar panels to produce the power needed for it to send data.
"There's great excitement about it being back," ESA senior science adviser Mark McCaughrean told Reuters by telephone. "But we have to make sure it's not the last croak of a dying cowboy."
In the shadows, Philae's solar panels, which were meant to power the probe after its batteries ran out several days after landing, received far less than the expected six to seven hours sunshine per day. It went into hibernation on Nov. 15.
After reawakening, Philae "spoke" for 85 seconds with its team on the ground via its mothership Rosetta, which is orbiting the comet at a distance of about 6.5 km (4 miles). Analysis of the detailed data suggests the lander had been awake earlier but unable to make contact, ESA said.
"It's very fascinating and we're all very happy to have received this signal," project manager Stephan Ulamec told Reuters by phone. "The lander seems to be perfectly healthy."
Philae's official Twitter account also came back to life on Sunday, tweeting: "Hello Earth! Can you hear me?"
Scientists hope that samples drilled from the roughly 3-by-5 km comet by Philae will unlock details about how the planets - and possibly even life - evolved. The rock and ice that make up comets preserve ancient organic molecules like a time capsule.
The lander was released from Rosetta in November after a 6.4 billion km journey that took more than 10 years - a mission that cost close to 1.4 billion euros ($1.8 billion).
But harpoons to anchor Philae to the surface failed to deploy and it bounced twice before floating to rest two hours later. Scientists scoured the surface of Comet 67P/Churyumov-Gerasimenko for months in the hope that the lander would revive.
The mood at ESA shot from despondency to "unalloyed joy" when the first signal arrived, triggering a flurry of midnight emails and emergency meetings among scientists keen to see what Philae would reveal, said McCaughrean.
The challenge now is to assess the state of the lander itself, with scientists waiting for the next contact. There are still more than 8,000 data packets in Philae's mass memory that will provide information on what happened in the past few days, ESA said.
Comets date back to the formation of the solar system some 4.6 billion years ago. Some scientists suspect comets delivered water to Earth when they collided with the planet aeons ago.
"Comets are treasure chests of material from the birth of the solar system," McCaughrean said.
Scientists must now race to extract as much data from the comet as possible before its orbit takes it back away from the sun in several months' time and towards the outer reaches of the solar system.
At that point, the mother ship Rosetta will have burned most of its fuel. Unable to maneuver, scientists are likely to let it spiral slowly towards the comet's surface, making an increasingly detailed analysis from above until it comes to rest and loses contact with the Earth.