It is the only planet in our solar system as yet unvisited by spacecraft, but scientists believe it holds crucial information on the origin of our planet and of life itself. Nick Morrison reports on NASA's mission to Pluto.

IT is almost 30 years since Voyager embarked on its journey towards Neptune, the last mission to a previously unvisited planet. But, weather conditions permitting, the next few weeks will see the launch of a piano-sized capsule which will complete the initial reconnaissance of the solar system.

Sometime between next Tuesday and February 2, NASA hopes to see its New Horizons probe propelled into space, on a voyage which will take nine and a half years. If the launch takes place after February 2, another five years will be added to the journey time. Beyond February 15 and the delay is counted in centuries.

The destination is Pluto, the most remote planet in our solar system, and the only one yet to be visited by a spacecraft. And it is a mission that could shed light on the beginnings of our own planet, and even of life itself.

But it is a quest which almost never even reached the launch pad. Six years ago, NASA axed the project after its funds were cut. An expedition to the smallest planet orbiting the Sun - which many do not believe should be classed as a planet at all - fell down the list of priorities.

It took a groundswell of protest from mainly amateur astronomers, pointing out that it would be another 200 years before another similar opportunity arose, when Pluto would be as close to the Sun, for the space agency to change its mind on the $650m.

NEW Horizons will be blasted into space from Cape Canaveral in Florida on an Atlas 5 rocket. Travelling at about 47,000 miles an hour, its first destination is Jupiter, sometime in February or March next year.

The intention is to pass close to the giant planet and use its gravitational field to provide a sling-shot momentum, propelling the probe the rest of its four billion mile journey towards Pluto, with an estimated arrival of July 2015. Launching after February 2 means the opportunity for a rendezvous with Jupiter will be lost and it will arrive instead in 2020.

The spacecraft is just eight feet across and weighs about half a ton when fully fuelled. It carries seven scientific instruments, ranging from an optical telescope and a spectrometer to measure gas composition.

After mapping and measuring Pluto and its moon Charon, New Horizons will be reprogrammed for an encounter with an object in the Kuiper Belt, a mass of icy objects orbiting the Sun and the major source of comets which strike the Earth, including the comet believed to have wiped out the dinosaurs.

As well as being the furthest planet from the Sun - although its orbit means it is sometimes closer than Neptune - Pluto is the oddball of the solar system. It belongs in the third class of planets: where the Earth, Mercury, Venus and Mars are rocky worlds, and Jupiter, Saturn, Uranus and Neptune are gas giants, Pluto is an ice dwarf. But while there are more ice dwarfs than the other two categories combined, no spacecraft has yet been sent to one.

Pluto also has a less circular orbit than the other planets, it orbits around its poles instead of its equator - a characteristic it shares with Uranus - and is orbiting on a tilt of 17 degrees, more than twice as great as the next highest, Mercury, which tilts at seven degrees.

But this tiny planet, just two thirds the diameter of the Moon, may have much to teach us about the Earth's origins. The ice dwarfs are believed to be embryo planets, formed in the early days of the solar system more than four billion years ago. While other planets went on to bigger things, the ice dwarfs stopped growing. Thus, they are the bodies out of which larger planets formed, and they may hold important clues about the formation of planets.

"The further out you go, the more the material you find resembling what the solar system was before the planets formed," says Sir Arnold Wolfendale, professor of physics at Durham University and former Astronomer Royal. "It has been less knocked-about and condensed.

"If you go even further still, you come to very icy stages of development - these are the comets - and there is a lot of argument over the extent to which the water on the surface of the Earth came from comets, and of course water was crucial for us in the development of life."

Pluto is also one of only four objects in the solar system to possess a mainly nitrogen atmosphere, the others being Saturn's moon Titan, Neptune's moon Triton, and the Earth. Although its surface is about minus 223 degrees Centigrade, scientists believe its interior may be warm enough to house an ocean.

And it is the possibility of water that has led some scientists to speculate that primitive life may once have evolved on Pluto, although it is too cold and dark for it to still exist. Alan Stern, the mission's lead scientist, believes Pluto is central to our understanding of the origin of the solar system.

Exploring the planet and the Kuiper Belt, he says, is like carrying out an archaeological dig into the history of our system, allowing a glimpse into the early days of the formation of planets. But for now, scientists are crossing their fingers that conditions will be suitable for a launch over the next few weeks. If not, we could be in for a very long wait if we want to know more than the scantiest details about the solar system's most mysterious object, and perhaps even about how we came to be here.