New Horizon measurements reveal Pluto is roughly two-thirds the size of the moon, and probably holds more ice beneath its surface than previously thought

A Nasa spacecraft that will hurtle past Pluto on Tuesday at more than 45,000 kilometres per hour has revealed the dwarf planet to be larger than scientists thought.

Fresh measurements from New Horizons, the first spacecraft to reach Pluto on the outer edge of the solar system, show that it is 2,370 kilometres across, roughly two-thirds the size of Earth’s moon.



Pluto image revealed by Nasa offers closest look yet at dwarf planet Read more

Alan Stern, the lead scientist on the $700m (£450m) mission, said the increased dimensions meant Pluto must hold more ice and less rock beneath its surface than researchers had expected. Pluto has been hard to measure with any accuracy from Earth because it is so far away, and its atmosphere creates mirages that can fool ground-based telescopes.

Other instruments onboard New Horizons confirmed that Pluto’s north pole bears an icy cap. The latest measurements beamed to Earth from the probe picked up chemical signatures of methane and nitrogen ice in the polar cap.



One early image received from New Horizons last week showed Pluto as an orangey globe bearing a large bright spot shaped like a heart. More recent images have revealed cliffs, craters and chasms larger than the Grand Canyon.

“The science we’ve already made is mouth-watering,” said Stern. “The Pluto system is enchanting in its strangeness and its alien beauty.”



New Horizons will perform its historic flyby at 12.49pm BST on Tuesday. But scientists must wait until 2am BST on Wednesday for the probe to make contact with Earth and confirm it has survived the encounter.

The most dangerous hazards for New Horizons are dust particles trapped in orbit around Pluto after being dislodged from its moons by meteorite impacts. A strike from a dust particle the size of a grain of rice could destroy the spacecraft, but the risk of such a disaster is low, at around one in 10,000.



The New Horizons spacecraft has spent more than nine years on its 4.8bn kilometre journey to Pluto, the last world in the solar system to be visited by a spacecraft. On board are seven sophisticated instruments and the ashes of Clyde Tombaugh, the astronomer who discovered Pluto in 1930.



When the probe blasted off in January 2006, Pluto was the ninth planet in the solar system. Seven months later, astronomers at the International Astronomical Union voted to downgrade the icy body to a dwarf planet, because it does not dominate its region of space in the way the other major planets do.



The spacecraft will take scores of photographs as it speeds past Pluto and its five known moons, Charon, Hydra, Nix, STyx and Kerberos. The images will give the first close-up view of the mountains and valleys of the unknown world, and its tenuous atmosphere seen as the sun rises and sets behind it. Instruments on New Horizons might even find evidence that it snows on the tiny world.



Nasa officials expect the first images from the flyby to be released on Wednesday night. The snapshots from onboard cameras will capture details up to 100 metres across, a vast improvement on those taken on approach, which pick out features about 15 kilometres across.

But scientists are in for a long wait to learn everything New Horizons sees. The probe will collect so much information as it passes Pluto that it will take 16 months to send it all back to Earth.



The mission marks the end of the US space agency’s bid to explore every planet in the solar system, starting with Venus in 1962. Tuesday’s flyby coincides with the 50th anniversary of the first ever fly-by of Mars by the Mariner 4 probe.