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A comet-chasing space probe that has been in hibernation for almost three years has woken up again and sent its first signal back to Earth.

The European Space Agency has received the all-clear message “Hello, World!” broadcast from its Rosetta spacecraft some 800 million km away.

Rosetta was put into hibernation in 2011 to conserve energy for its long journey to meet comet 67P/Churyumov-Gerasimenko.

If everything goes as planned, the probe will rendezvous with the comet in the coming months and drop a lander on to its icy surface in November.

Rosetta is named after a block of stone that allowed archaeologists to decipher ancient Egyptian hieroglyphs. Scientists hope the probe’s findings will help them understand the composition of comets and thereby discover more about the origins and evolution of our solar system.

A scale model of the Rosetta spacecraft is pictured at the European Space Operations Centre in Darmstadt. Photo: Ralph Orlowski/Reuters

Comets are regarded as flying time capsules because they are essentially unchanged for 4.6 billion years.

Scientists have speculated that comets, which are essentially giant, dirty snowballs, may be responsible for the water found on some planets.

And like asteroids, comets also pose a theoretical threat to life on Earth.

Hopefully, Rosetta will reach 67P in the first half of this year and fly a series of complicated manoeuvres to observe the comet – a lump of rock and ice about four km in diameter – before dropping a lander on to its icy surface in November.

The Philae lander will dig up samples and analyse them with its on-board instruments.

The probe and its lander will keep sending back data until their batteries die or the debris streaming off the comet irreparably damages their sensitive instruments.

The mission is different from Nasa’s Deep Impact probe that fired a projectile into a comet in 2005 so scientists could study the resulting plume of matter.

Rosetta was put into hibernation in 2011 to conserve energy for its long journey to meet comet

The agency managed to land a probe on an asteroid in 2001, but comets are much more volatile places because they constantly release dust and gas that can harm a spacecraft.

Nasa is planning another space rock mission between 2019 and 2021. The agency is looking into sending a robotic spaceship to lasso a small asteroid and haul it close to the moon, where spacewalking astronauts would explore it.

Surrounded by four 100m lightning towers, the first Ariane 5G+, carrying the Rosetta probe, stands on the launch pad at the French Guiana Space Centre on February 24, 2004. Photo: European Space Agency

An artist’s impression of the probe’s Philae lander. Photo: AP/European Space Agency

Mission controllers cheer the first signal received from the Rosetta spacecraft. Photo: European Space Agency

Rosetta, the European Space Agency’s cometary probe with Nasa contributions, is seen in an undated artist’s rendering. Photo: Reuters/European Space Agency