ESA

: It's alive! Scientists successfully awakened Rosetta this afternoon, at which point the spacecraft tweeted "Hello, World" in about two dozen languages.

More than 300 million miles from Earth, a billion-dollar aluminum box is tumbling through space. Launched by the European Space Agency (ESA) in 2004, the Rosetta spacecraft has spent the last 31 months in deep space hibernation, drifting along a lazy arc toward the comet 67-P/Churyumov-Gerasimenko.

But on Monday, Rosetta's internal alarm clock is set to ring.

During its decade-long mission, Rosetta has circled Earth, swung around Mars, snapped photographs of asteroids, and even been misidentified as one. But the spacecraft's most important mission, for which it now awakens, is to study a comet up close. If all goes well, Rosetta's probe will be the first ever to land on the surface of a comet and study the frozen mass.

Rosetta is set to wake up at 10:00 GMT on Monday, Jan 20 (that's 4 a.m. Eastern). The spacecraft will fire thrusters to correct its attitude in space and then relay a confirmation message back to Earth. The ESA will be streaming live updates on its website throughout the day and live tweeting the event via @ESA_Rosetta.

The Rosetta spacecraft is not the first to study a comet's composition; that honor goes to the NASA Stardust mission, which won a Popular Mechanics Breakthrough Award in 2006. Stardust gathered debris from the 81P/Wild comet and returned samples to Earth for analysis. But the ESA expects to gain far more by studying a comet in its natural habitat, and over a period of months rather than seconds, says Gerhard Schwehm, a planetary scientist at the ESA. "Stardust produced a snapshot of a comet," Schwehm says. "We hope to produce a feature-length movie."

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Why put it in hibernation? "Rosetta gets its power from solar rays," Schwehm says, but the spacecraft's journey has taken it so far from the sun that its electrical systems became unreliable. "Since we could not get enough power to operate the spacecraft in all circumstances, it was considered much safer to put Rosetta into hibernation." On its approach to the comet, Rosetta is also getting closer to the sun. On Monday morning, Rosetta will finally drift close enough to our star that ESA will be able to activate its electrical systems.

By studying the composition of a comet, scientists hope to learn more about the conditions under which the solar system originally formed. "Comets are the most primitive objects in the solar system that are accessible to us," Schwehm says.

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