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

Rosetta was awakened by its internal alarm clock at 10:00 GMT on Monday. The European Space Agency then received the all-clear message "Hello World!'' from the spacecraft about 800 million kilometres away shortly after 18:00 GMT.

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

If all goes as planned the probe will rendezvous with the comet in the coming months and drop a lander onto its icy surface in November.

For almost 10 years, Rosetta has been travelling through space.

Chris Riley, a science professor at the University of Lincoln, told Al Jazeera that "the main function of this probe is to orbit and map the comet".

After revolving around the comet, it will then harpoon a device to its surface to analyse its composition.

Rosetta has been circling the sun on a widening spiral course, swinging past Earth and Mars to pick up speed and adjust its trajectory.

The mission will perform several historical firsts, including the first time a spacecraft orbits a comet rather than just whizzing by it to snap some fly-by pictures, and the first time a probe has landed on a comet's nucleus.

Scientists believe the comet was formed about 4 and half billion years ago. The study will help determine whether comets brought water or even life to Earth.

Until the end of 2015, the probe will gather data on the comet's surface and examine how it changes as its nears the sun.