A European comet-chasing spacecraft that has been in hibernation for almost three years has woken up and sent its first signal back to Earth.
The European Space Agency (ESA) received the all-clear message "Hello World!" from its Rosetta spacecraft, 500 million miles away, shortly after 6pm.
Rosetta was put into hibernation 31 months ago to conserve energy for its long journey to meet comet 67P/Churyumov-Gerasimenko.
The spacecraft, launched by the ESA from Kourou in French Guiana nearly 10 years ago, is on a historic mission to drop a lander on the icy surface of a comet.
One of its first tasks will be to search for a suitable landing site for the box-like Philae lander, which will drill samples from the ground for analysis.
Philae will also capture panoramic images of the view from the comet's surface with an on-board camera.
Scientists hope Rosetta will answer important questions about the origins of the Solar System and the way comets evolve and develop.
Rosetta was put to sleep to conserve power as it headed to regions as distant as the planet Jupiter, where the Sun's weak rays provide only limited amounts of energy.
Unlike other long-distance space probes the craft has no nuclear batteries and instead relies on electricity generated by ultra-sensitive 15-metre long solar panels.
To make certain Rosetta woke up, the craft was fitted with not one but four quartz "alarm clocks".
The wake-up procedure involved switching on the probe's star-tracker navigation system, slowing its spin and warming up electrical components. This allowed Rosetta to orientate itself towards the Earth and start transmitting.
Scientists at the American space agency Nasa's 70 metre-wide deep space dish at Goldstone, California, were listening out for the extremely weak signal and relayed it to ESA's mission control centre in Darmstadt, Germany.
Rosetta, named after the block of stone that helped archaeologists decipher ancient Egyptian hieroglyphs, has already travelled round the Sun five times, picking up energy from the gravitational "slingshot" effect of Earth and Mars.
Comet 67P/Churyumov-Gerasimenko is a 2.5 mile-wide dirty snowball of ice and dust hurtling through space at 24,600mph.
Rosetta is due to reach the comet in August before deploying the Philae lander in November. Scientists have compared the mission to trying to land a fly on a speeding bullet.
Undated artistic sketch of Rosetta, the one billion-euro comet chasing space probe due to be launched in January 2003, whom the mission has been abandoned and a new mission will be found later, the European Space Agency (ESA) said 15 January 2003. Five or six other possible comets will be studied before a new target is selected, a decision likely not to be taken before the year's end. (Photo credit should read J.HUART/AFP/Getty Images)
This handout picture from the European Space Agency (ESA) retrieved on September 3, 2008 shows an artist's rendition of ESA's probe Rosetta.
Artist's impression of the European Space Agency (ESA) probe Rosetta with Mars in the background.
This handout image provided by the European Space Agency, transmitted by the space craft Rosetta, shows the asteroid Lutetia at closest approach July 10, 2010 between Mars and Jupiter in outer space.
This handout image provided by the European Space Agency, transmitted by the space craft Rosetta, shows a close-up view of a possible landslide and boulders at the highest resolution on the asteroid Lutetia July 10, 2010 between Mars and Jupiter in outer space.
This undated image provided by the European Space Agency ESA shows an artist's impression of the Philae lander
In this Dec. 10, 2013 file picture a European Space Agency, ESA, employee sits in the control room for the Rosetta mission at the ESA in Darmstadt, Germany.
This image provided by the European Space Agency ESA shows an artists impression of the Rosetta orbiter deploying the Philae lander to comet 67P/ChuryumovGerasimenko
In this 2013 file photo provided by the European Space Agency, ESA, employees work in the control room of ESA in Darmstadt, Germany.