The Rosetta spacecraft, launched in 2004 on a mission to rendezvous with a comet, study it and place a small probe on its surface, sent a signal that was received by NASA's Goldstone ground station in California at 1:18 p.m. EST, the European Space Agency reported Monday.
Operating on solar energy alone, Rosetta was placed into a deep space slumber in June 2011.
Rosetta's pre-programmed internal "alarm clock" woke up the spacecraft, warmed up its navigation instruments and aimed its main radio antenna, ESA scientists said.
"This was one alarm clock not to hit snooze on, and after a tense day we are absolutely delighted to have our spacecraft awake and back online," said Fred Jansen, ESA's Rosetta mission manager.
Rosetta is still around 5.5 million miles from its target, Comet 67P/Churyumov-Gerasimenko.
After essential health checks on the spacecraft the 11 instruments on the orbiter and 10 on the lander will be turned on and prepared for studying the comet, ESA officials said.
"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."