European Space Agency (ESA) scientists and engineers started breathing again today as the comet-chasing Rosetta space probe confirmed at 18:28 GMT that it had awoken from its 31-month hibernation. The news was announced via the @ESA_Rosetta twitter account, which tweeted: "Hello, world!"

The signal, which took 45 minutes to traverse the 807 million km (501 million mi) that separates the unmanned spacecraft from Earth, was received by NASA’s Goldstone ground station in California during the first window of opportunity to establish communications and was then forwarded to mission control in Darmstadt, Germany.

Launched in 2004, Rosetta had been in hibernation since June 2011 when a complex series of orbital maneuvers intended to build up the necessary velocity to catch up with comet 67P/Churyumov-Gerasimenko sent it 800 million km (497 million mi) away from the Sun, where its solar array couldn't provide enough power to keep its systems from freezing. Now, 673 million km (418 million mi) from the Sun, the onboard computer reactivated the spacecraft in a preprogrammed sequence at 10:00 GMT this morning.

Rosetta's wake up signal to Earth (Image: ESA)

The delay in sending the confirmation signal back to Earth was due to the need for Rosetta to take the time to warm up its navigation sensors, reduce the spin placed on it to keep it stable during hibernation, turn its solar array towards the Sun, and aim its high-gain antenna at the Earth.

According to ESA, Rosetta is still 9 million km (5.5 million mi) from the comet and faces months of recomissioning as mission control brings the spacecraft’s suite of scientific instruments back online. In late May, the probe will carry out a course correction, which will make it the first mission to orbit a comet. At about this time, the first images will taken of 67P/Churyumov-Gerasimenko from a distance of 2 million km (1.2 million mi).

After Rosetta's rendezvous with the comet in August, it will begin its main mission of mapping, studying its nucleus, analyzing the physical properties of gases expelled, and studying how the comet changes as it approaches the Sun.

If things go to schedule, on November 11 Rosetta will deploy the 100 kg (220 lb) Philae probe for the first controlled landing on a comet. Philae will use ice screws and harpoons to secure to the surface due to the extremely low gravity. Once there, it will drill into the comet, analyze the composition, and send back panoramic images.

"We have our comet-chaser back," says Alvaro Giménez, ESA’s Director of Science and Robotic Exploration. "With Rosetta, we will take comet exploration to a new level. This incredible mission continues our history of ‘firsts’ at comets, building on the technological and scientific achievements of our first deep space mission Giotto, which returned the first close-up images of a comet nucleus as it flew past Halley in 1986."

Source: ESA

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