Pluto flyby just hours away

An artist's impression of New Horizons spacecraft Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/PA Wire

Nasa's New Horizons spacecraft is just hours away from flying past the last unexplored world in our Solar System.

Excitement is building at the American space agency as its probe closes in on Pluto after travelling almost three billion miles over the last nine years.

On board are the ashes of American astronomer Clyde Tombaugh, who discovered the remote icy object 85 Earth years ago, or a third of one Pluto year, as it takes much longer to complete one orbit of the sun.

When the probe was launched the aim was to reach the outermost of the sun's family of nine planets. But, seven months into the its epic journey, international astronomers downgraded Pluto's status to "dwarf planet".

Despite this scientists have discovered that Pluto is actually larger than many previous estimates. The Long Range Reconnaissance Imager (LORRI) on board New Horizons measures diameter to be 2,370 km (1,473 miles).

Residing in a region known as the Kuiper Belt, it is so far away that its light takes more than four hours to reach the Earth. And, because radio signals travel at the speed of light, engineers who communicate with New Horizons experience a significant delay between messages.

The spacecraft is due to make its historic fly-by at 12.49pm tomorrow UK time, passing within 12,500 kilometres (7,700 miles).

Its main imaging camera will send back the first detailed close-up pictures of the surface of Pluto, and is capable of showing features just 50 metres across.

The $700 million (£451 million) probe is about the size of a baby grand piano and, with a speed of more than 36,000 mph, is the fastest craft ever to leave Earth orbit.

The mission marks the conclusion of Nasa's quest to explore every planet in the solar system, starting with Venus in 1962.

Pluto taken by New Horizons Credit: NASA/JHUAPL/SWRI/PA Wire

Tomorrow's encounter with Pluto coincides with the 50th anniversary of the first ever fly-by of Mars by the Mariner 4 probe.

A tantalising early image received from New Horizons last week showed Pluto as a copper-coloured globe bearing a large bright spot in the shape of a heart.

Later photos taken from a million miles away have revealed cliffs, craters and chasms larger than the Earth's Grand Canyon.

Artist's rendering depicting Pluto's largest moon Charon rises over its frozen surface Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Pluto has a thin atmosphere of nitrogen, methane and carbon dioxide, which expands as the dwarf planet's elongated 248-year orbit takes it closer to the sun causing icy material on its surface to vaporise.

Scientists believe the dwarf planet may bear signs of past volcanic activity and could even have liquid water beneath its frozen surface.