(CNN) -- Like a groggy traveler after a long, cold night, the European Space Agency's comet-chasing Rosetta spacecraft woke up, warmed up and called home Monday before setting off on the final leg of its journey.
Rosetta shook off 31 months of sleep, during which it traveled into the dark reaches of the solar system, and contacted its operators Monday evening. Cheers erupted in the ESA mission control center in Darmstadt, Germany, when a signal from Rosetta arrived shortly after 7 p.m. (1 p.m. ET).
"The spacecraft is there, it's awake, and I think we are all overwhelmed," mission manager Gerhard Schwehm told reporters. Now, he said, controllers will have "two busy years ahead of them" as it approaches its target, Comet 67P/Churyumov-Gerasimenko, and follows it around the sun.
There was only an hour window for the signal to reach Earth, putting scientists on pins and needles.
"The spacecraft decided to make us suffer after two and half year," Paolo Ferri, head of ESA Mission Operations told CNN. "We came almost to the end of the window, we were very tense," he said.
The unmanned probe was launched in 2004 and is now nearly 500 million miles (800 million kilometers) from the Sun -- a distance that puts it just past the orbit of Jupiter. At that distance, the message it beamed back took 45 minutes to arrive.
When the signal finally arrived there were cheers of relief.
If all goes well, Rosetta will rendezvous with the comet in August and fly with it for two years. Scientists now have to learn how to maneuver the craft around a comet.
"It's flying in a very unusual environment for space flight with gas and dust and not much gravity to keep your obit stable," Ferri said.
Rosetta is also supposed to send a small lander to the surface of the comet in November. The mission will give scientists unprecedented insight into the makeup and structure of these ancient residents of the solar system, ESA officials say.
"This is groundbreaking science," Rosetta project scientist Matt Taylor told ESA television. "We're looking at where we came from, the evolution of our own solar system. And comets are looked upon as kind of a time capsule back to the beginning of the solar system."
Scientists believe comets, composed of rock and ice, may have brought water and maybe even the primitive building blocks of life to Earth.
Rosetta carries 11 scientific instruments; its lander has 10 more.
ESA controllers ordered Rosetta to shut down most of its systems in June 2011 to save on power as it sailed through the darkest reaches of the solar system. The amount of sunlight reaching the solar-powered probe is only 4% that on Earth, according to ESA.