Fresh out of an unprecedented power-saving sleep mode, Europe's comet-chasing Rosetta spacecraft awakened and phoned home Monday on the way to an enigmatic ball of rock and ice for a daring close-up inspection later this year.

Artist's concept of the Rosetta spacecraft. Credit: ESA
 
Out of contact with Earth since June 2011, Rosetta is about to conclude a 10-year sojourn through space and pull alongside comet named Churyumov-Gerasimenko in August, when the European Space Agency probe will become the first mission to ever orbit one of the "dirty snowballs" believed to harbor the building blocks of life.

European Space Agency officials say Monday's wakeup launches Rosetta into a year of firsts: rendezvousing with a little-known comet beyond the orbit of Mars, maneuvering into a series of jagged, imprecise orbits, surviving blasts from dust and ice crystals, then ejecting a hitchhiking robot named Philae to latch onto the comet with harpoons and ice screws.

Such a tricky encounter, set to begin this summer, has never been tried before.

"We have our comet-chaser back," said Alvaro Gimenez, 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."

Rosetta's on-board timer was programmed to go off at 1000 GMT (5 a.m. EST) Monday, but it took more than eight hours to receive a report on the spacecraft's condition. The probe roused itself from sleep, activated heaters and regained control of its orientation before aiming its high-power antenna toward Earth.

Admittedly nervous after waiting 31 months with no signals from the $1.7 billion mission, ground teams at ESA's control center in Darmstadt, Germany, were elated with the news.

Although Rosetta's signal made it to Earth within the expected window, the team had to wait a little longer than most officials expected. NASA-owned 70-meter (230-foot) antennas in California and Australia were trained on Rosetta's predicted location in the sky waiting on a peep from the probe 500 million miles away.

A video feed streamed from the Darmstadt control center finally showed a spike in the signal at 1818 GMT (1:18 p.m. EST).

"I think that's been the longest hour of my life," said Andrea Accomazzo, Rosetta's spacecraft operations manager.

"It's been a spectacular few moments of torture," said Martin Kessler, Rosetta's science operations manager.

The Rosetta control team, led by operations manager Andrea Accomazzo at center, celebrate at the European Space Operations Center in Darmstadt, Germany, after receiving the first signals from Rosetta after Monday's wakeup. Credit: ESA/J. Mai
 
The slumber was necessary to keep Rosetta going because it flew so far from the sun -- a maximum distance of 490 million miles -- that its solar panels could no longer generate enough power to supply the probe's control and communications systems. Engineers only left Rosetta's heaters on standby to turn on intermittently to keep the spacecraft's internal components warm.

Rosetta's control team will learn more about the spacecraft's condition in the coming hours and days. The signal initially received Monday was just a carrier tone, Rosetta's way of telling the ground team, "I'm alive!"

One of the first commands sent up to Rosetta after wakeup was to trigger a torrent of telemetry data detailing the status of every system aboard the spacecraft except its science instruments, which will be activated and tested in the next few weeks.

Rosetta's journey began March 2, 2004, with a middle-of-the-night blastoff aboard an Ariane 5 rocket from its French Guiana launch base.

The mission was a year late getting off the ground due to worries over the Ariane 5 rocket's reliability after a launch mishap in December 2002. The delay prompted a change in destination to Churyumov-Gerasimenko, colloquially known as 67P or C-G, an ice world four times larger than Rosetta's original target.

Since departing Earth a decade ago, Rosetta has returned for flybys three times and zoomed past Mars in February 2007, returning a spectacular self-portrait of the probe's solar panel backdropped by the stark landscape of the red planet.

Rosetta also logged flybys of asteroids Steins and Lutetia in September 2008 and July 2010, collecting data and imagery in a chance for bonus science on the way to the mission's ultimate objective.

Since lifting off in 2004, Rosetta's odometer stands at 3.8 billion miles.

The craft's extensive suite of cameras, spectrometers, dust analyzers and other science instruments will be switched on and tested in the next few months. In late March, the German-led Philae lander riding piggyback on Rosetta will be activated for the first time in three-and-a-half years to check its status.

Artist's concept of Rosetta and the Philae lander at the comet. Credit: ESA/C. Carreau/ATG Medialab
 
A major course correction maneuver is planned for May to change Rosetta's velocity by approximately 800 meters per second, or 1,800 mph, and adjust the craft's trajectory to arrive in the vicinity of Churyumov-Gerasimenko in August.

Rosetta's long-range camera should acquire the first images of the comet this spring, with the 3-mile-wide comet growing larger in the probe's apertures over the summer.

In August, Rosetta is scheduled for a delicate, untried maneuver to enter orbit around the comet.

Scientists are not sure what they will find there.

Some experts predict Rosetta will have to dodge chunks of rock, dust grains and ice blown off the comet as it is heated by the sun.

One finding by scientists who observed Churyumov-Gerasimenko with NASA's WISE infrared survey telescope estimated the comet flings off about 70 kilograms -- more than 150 pounds -- of dust every second at speeds of nearly 2,000 mph.

Rosetta carries a pair of solar panels extending 105 feet tip-to-tip. The unwieldy wings are not ideal for operating in close quarters with a comet.

"Hopefully, it will not affect the performance of the solar arrays," said Paolo Ferri, head of ESA's mission operations. "It may affect the performance of the optics. The risk that dust will deteroriate the spacecraft will grow" as Rosetta moves closer to the comet.

Rosetta will eventually move within 15 miles of Churyumov-Gerasimenko to gauge the comet's tenuous gravity field and map the surface of its nucleus. It will release Philae for a nail-biting descent to the comet in November.

Philae has its own ambitious research program.

The lander will use harpoons and ice screws to latch on to the comet before collecting panoramic imagery and drilling into the rock's subsurface to analyze samples from a depth of 30 centimeters, or about 1 foot.

Philae will overheat and succumb about three or four months after landing when Churyumov-Gerasimenko gets closer to the sun.

Rosetta will escort the comet more than a year, monitoring how the body reacts to greater solar heating as it moves toward perihelion, its closest approach to the sun, in August 2015.

"All other comet missions have been flybys, capturing fleeting moments in the life of these icy treasure chests," says Matt Taylor, ESA's Rosetta project scientist. "With Rosetta, we will track the evolution of a comet on a daily basis and for over a year, giving us a unique insight into a comet's behavior and ultimately helping us to decipher their role in the formation of the solar system."

Follow Stephen Clark on Twitter: @StephenClark1.