An American probe that will explore Pluto has woken from its slumber, after a nine-year journey to take a close look at the distant body for the first time.
"New Horizons is healthy and cruising quietly through deep space, nearly three billion miles from home, but its rest is nearly over," said Alice Bowman, the craft's operations manager at Johns Hopkins University Applied Physics Laboratory outside Washington.
The probe came out of hibernation on Saturday and transmitted a message to Earth.
New Horizons was launched in January 2006 and was in hibernation for about two thirds of its journey, to preserve its electric power and minimise resources needed to monitor it.
During its trip, NASA engineers woke the craft every few months to check if its systems were still functioning.
The craft's computer has also been sending a weekly message to Earth that takes four hours to arrive.
New Horizons begins its exploration of Pluto in January at a distance of about 260 million kilometres.
It will pass closest to Pluto, which travels on an elliptical orbit, in July, just before it ends its research.
During its mission, the probe will collect data on Pluto's topography and its largest moon Charon, giving astronomers an up-close look at the dim surfaces that are difficult to see from Earth.
The craft carries seven instruments, including infrared and ultraviolet spectrometers, a multicolour camera, a high-resolution telescopic camera and a space dust detector.
The instruments draw power from a radioisotope thermoelectric generator, which provides less power than a pair of 100-watt light bulbs, scientists said.
After its six-month investigation of Pluto, it will pass near other objects in the Kuiper Belt, a vast ring of debris.
In 2006, the International Astronomical Union withdrew Pluto's status as a planet given its small size, reclassifying it as a dwarf planet and leaving the solar system with eight planets.
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