Europe’s comet-chasing Rosetta spacecraft emerged from 31 months of deep-space slumber Jan. 20 to reestablish communications with Earth.

The €1.3 billion ($1.76 billion) European Space Agency (ESA) mission made contact with NASA’s 70-meter-dia. ground station at Goldstone, Calif., 48 min. into a nail-chewing hour-long window that opened at 12:30 p.m. eastern.

The wakeup process began more than eight hours earlier, when Rosetta’s onboard “alarm clock” sounded at 5 a.m. eastern, initiating warmup of the spacecraft’s star-trackers, slowing its hibernation spin and switching on other systems before finally sending telemetry from its orbit 800 million km from Earth.

The signal was received some 44 min. minutes later at ESA’s ESOC space operations center in Darmstadt, Germany, at 1:18 p.m. eastern.

Built by Astrium Defense & Space and launched on an Ariane 5 in March 2004, Rosetta is on its way to rendezvous with Comet 67P/Churyumov-Gerasimenko, where it will study the nucleus of the comet and its environment for nearly two years and land a small probe on its surface.

During its decade-long journey Rosetta has made three flybys of Earth and one of Mars to build up enough speed on a trajectory towards 67P/Churyumov-Gerasimenko, which loops around the Sun between the orbits of Jupiter and Earth at a distance of more than 800 million km from Earth, and 185 million km from the sun. Along the way, it passed the asteroids Steins and Lutetia before being placed into a deep-space slumber in June 2011, during which it operated on solar energy alone as it cruised far from the Sun towards the orbit of Jupiter.

ESA says Rosetta’s orbit has brought it back to within 673 million km of the Sun, where there is enough solar energy to fully power the spacecraft. As the orbiter’s startrackers began to warm up, its thrusters fired to stop its slow rotation. A slight adjustment was made to Rosetta’s orientation to ensure that its solar arrays faced the Sun, after which the startrackers were switched on to determine Rosetta’s attitude.

Rosetta then turned directly towards Earth, switched on its transmitter and pointed its high gain antenna to send the signal announcing it is awake.

At more than 800 million km from Earth, it took almost 45 min. for the signal to reach mission controllers. Deep-space tracking dishes listening for the signal were NASA’s Goldstone site, Canberra station in eastern Australia and ESA’s New Norcia 35-meter antenna in western Australia.

As Rosetta gradually comes out of hibernation over the coming months, it is expected to be fully activated by May. In August, the orbiter will fly within 100 km of the comet, at which point it will slowly approach 67P/Churyumov-Gerasimenko at about a half a meter per second. In October, ESA will choose a landing site for Rosetta’s Philae lander, which is slated to be deployed in early November.

“We know very little about the comet’s rotation and dynamics,” says Thomas Reiter, head of ESOC operations. “We will only identify a safe landing scenario when we get there.”

Once Rosetta reaches Comet 67P/Churyumov-Gerasimenko, the orbiter will fly in different orbits in order to make the most detailed study of a comet ever attempted. It will be the first mission ever to orbit a comet’s nucleus and land a probe on its surface. It also will be the first spacecraft to fly alongside a comet as it heads towards the inner solar system, observing how a frozen comet is transformed by the sun’s warmth.

As it follows the comet, Rosetta will measure increased activity as the icy surface is warmed by the Sun. The Rosetta orbiter carries 11 instruments for remote sensing and radio science, and will study the composition, mass distribution and dust flux of the comet’s nucleus, as well as the comet plasma environment and its interaction with the solar wind.

The orbiter’s scientific instruments are accommodated on one side of the spacecraft, which will permanently face the comet during the operational phase of the mission.

The Philae lander is carrying 10 instruments, including a drill. It will focus on the composition and structure of the comet nucleus material, and will dig down more than 20 cm into the subsurface to collect samples for inspection by the Philae’s onboard laboratory. Until its release, the 100-kg (220-lb.) lander is carried on the side opposite from the large high-gain antenna dish. As Philae touches down on the comet, two harpoons will anchor it to the surface. The self-adjusting landing gear is designed to ensure that it stays upright, even on a slope, and then the lander’s feet will drill into the ground to secure it to the comet’s surface in the low gravity environment.