Europe's Rosetta spacecraft ended a nearly three-year hibernation on Monday to prepare for an unprecedented mission to orbit a comet and dispatch a lander to the surface.
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Rosetta's on-board alarm clock went off a 5 a.m. EST, but it took the spacecraft seven hours to warm up its star trackers, fire up rocket thrusters to slow its spin, turn on its transmitter and beam a message back to Earth.
The probe, presently located about 500 million miles (about 800 million km) from Earth and just shy of Jupiter's orbit, is so far away that its radio transmissions, traveling at the speed of light, take 45 minutes to reach Earth.
Ground control teams got word that Rosetta was awake at 1:18 p.m. EST.
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"We have our comet-chaser back," said Alvaro Giménez, who oversees science and robotic exploration at the European Space Agency, said in a statement.
The mission has also recommenced the use of its Twitter account, @ESA_Rosetta, just after the spacecraft woke up. It tweeted "Hello, world!" in 23 European languages.
Rosetta, which carries a 220-pound (100 kilogram) lander called Philae, has been shut down for most of the past three years to save power. It is due to reach a 2.4-mile (4 kilometer) diameter comet called 67P/Churyumov-Gerasimenko in August.