Paris: The comet-chasing probe Rosetta has woken up and is operational after a 31-month hibernation, the European Space Agency says.
"Hello, world!" ESA said on Twitter, mimicking the signal sent back from deep space by the billion-dollar probe.
A webcast showed jubilant scientists at mission control in Darmstadt, Germany, as the all-is-well signal came in.
The probe, which is about 800 million kilometres from Earth and just shy of Jupiter's orbit, is so far away that its radio transmissions, travelling at the speed of light, will take 45 minutes to reach listening stations in California and Australia.
The spacecraft, which carries a 100-kilogram lander called Philae, has been hibernating for most of the past three years to save power.
It is due to reach a comet, called 67P/Churyumov-Gerasimenko, which has a diameter of four kilometres, in August.
Unlike previous comet probes, Rosetta won't just sail by. The spacecraft is designed to put itself into orbit around 67P for more than a year of close-up studies.
Comets are believed to be the pristine leftover remains from the formation of the solar system about 4.6 billion years ago.
Scientists hope the mission will provide more clues about how the solar system came into existence, much like the Rosetta Stone, for which the spacecraft is named, provided a blueprint for deciphering ancient Egyptian hieroglyphs.
"Rosetta should become a key element for our understanding of the history of the solar system," Stephan Ulamec, a Rosetta project manager, said in an interview with Reuters last month.
One of Rosetta's first tasks will be to scout for a suitable landing location for its Philae probe. Scientists are particularly keen to conduct organic chemistry experiments on samples drilled out from inside the comet's body.
"It would be really interesting to find out whether the organic chemistry that is relevant for life is there on comets," Ulamec said.
Engineers who designed the lander did not know what type of terrain they would find on the comet's surface. It is outfitted with twin harpoons laced with tethers that will be fired into the comet's surface to anchor Philae and keep it from bouncing back into space after touchdown.
Europe spent about 1 billion euros ($1.54 billion) on the mission, which is due to run at least until the end of 2015.