PLUTO, the solar system’s most mysterious planet, could finally surrender its secrets with the launch of the first probe designed to visit it.

The New Horizons mission — among the most ambitious ever launched by Nasa — is due to lift off on Tuesday on the first leg of a journey that could take up to 15 years to cross more than 3 billion miles of space.

Over such a distance, travelling at up to 30,000mph, it will have to navigate with pinpoint accuracy to pass Pluto at its intended altitude of 6,000 miles.

It will then have just 24 hours to bring its cameras and other instruments to bear before hurtling past and out of range.

A Nasa spokesman said that unlike recent missions to Mars and Saturn, Pluto’s weak gravity made it impossible to put the craft into orbit. However, the information gathered in that 24 hours could completely alter scientists’ understanding of the planet.

Even though Pluto was discovered 75 years ago, it remains the least-known planet. It was only last year that astronomers realised it had not one moon — Charon, discovered in 1978 — but three.

The problem is that Pluto lies so far from Earth and is only two-thirds the size of the moon. This means that even through a telescope it is about 50,000 times fainter than Mars.

Many astronomers argue it is not a planet at all but a larger member of the Kuiper Belt, the vast region of ancient, icy, rocky bodies that orbit the sun more than a billion miles beyond the planet Neptune.

One of the key tasks of the probe’s instruments will be to find out what Pluto is made of. There are indications that its surface includes different forms of ice composed of nitrogen, carbon monoxide, methane and water, but many other materials may also be present.

There is also speculation that, like Jupiter’s moon Europa, Pluto’s surface might cover an underground sea of water kept liquid by a molten core.

Alan Fitzsimmons, professor of astronomy at Queen’s University, Belfast, an expert on the Kuiper Belt, said the mission would shed new light on the birth of the solar system.

• At 6am British time today another Nasa enterprise was due to end dramatically with the return of the Stardust mission to collect dust and material from Comet Wild 2.

  The Stardust craft flew through the tail of the comet picking up debris to give scientists their first close look at such material. As it passes over Earth the probe will jettison a capsule that will parachute down to the Utah desert.

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