The US space agency (NASA) is getting ready to launch a piano-sized spacecraft to Pluto, the last unexplored planet in the Solar System.

New Horizons is set to lift off from Cape Canaveral, Florida, at 1824 GMT on Tuesday aboard an Atlas 5 rocket.

Despite being the fastest probe ever built, it will still take more than nine years to reach its distant target.

Anti-nuclear activists have staged small protests about the spacecraft's 33kg payload of plutonium fuel.

The $700m (£396m) New Horizons spacecraft will gather information on Pluto and its moons before - it is hoped - pressing on to explore the icy objects that reside in the region of space known as the Kuiper Belt.

This region, which lies beyond Neptune, consists of perhaps tens of thousands of icy objects spread out between 30 and 50 times the distance between the Earth and the Sun.

Some astronomers say Pluto is not a true planet at all, and should be classed instead alongside these icy relics from the formation of the Solar System.

"What we know about Pluto today could fit on the back of a postage stamp," said Colleen Hartman, a deputy associate administrator at Nasa. "The textbooks will be rewritten after this mission is completed."...