Space exploration: Destination Pluto

An interplanetary probe, delayed by high winds in Florida yesterday, is today scheduled to lift off from Cape Canaveral. Its mission? A nine-year journey to the most mysterious planet in the solar system. Steve Connor reports

The Atlas V rocket that will carry the New Horizons probe to Pluto

On the day, high-winds at Cape Canaveral in Florida prevented Nasa from launching the fastest spacecraft ever to leave the planet. But, weather permitting, the New Horizons probe will set off today on a nine-year journey to the smallest, coldest and least-understood planet in the solar system ­ the "ice-dwarf" world of Pluto.

It is the first mission to the most distant planet orbiting the Sun and scientists hope that it will shed some much-needed light on the darkest outer reaches of our vast planetary neighbourhood. "What we know about Pluto today could fit on the back of a postage stamp," said Colleen Hartman of Nasa. "The textbooks will be rewritten after this mission is completed."

Light from the Sun takes more than four hours to reach Pluto, which is so faint that it can barely be seen. As a result, it was the last planet to be discovered and now it is the last to be explored.

The probe will be carried into space by a giant Atlas 5 rocket. The launch, which was scheduled for yesterday, is now due to take place today.

New Horizons is a triangular, piano-sized spacecraft weighing 1,025lb. It will travel so far from the Sun that solar panels would be useless in the dim sunlight. This is why the probe has a nuclear-powered battery.

Its destination is some 3.1 billion miles from Earth and if all goes to plan the probe should arrive at Pluto in 2015.

Nine hours after launch, the probe should have passed the orbit of the Moon, the fastest time taken by any spacecraft to reach this point.

In just 13 months it is expected to have arrived at Jupiter where it will use the planet's massive gravitational pull to accelerate its speed still further by making a "sling shot" manoeuvre to catapult it on its way. Without the extra zip of a Jupiter swing-by, the New Horizons probe would take an extra five years to reach Pluto.

Pluto was discovered in 1930, and only then as a result of a fortunate accident that led a young American astronomer called Clyde Tombaugh to make a very careful survey of the night sky.

Calculations based on the motions of Uranus and Neptune, which later turned out to be erroneous, had predicted a "Planet X" beyond Neptune. Clyde started to search the sky for the mystery object and soon identified the ninth planet.

Pluto, which was named after the Roman god of the underworld, perhaps because it is in perpetual darkness, travels round the Sun once every 248 Earth years in a highly eccentric orbit. The orbit is so eccentric that it sometimes brings Pluto closer to the Sun than Neptune, which occurred between 1979 and 1999. Pluto is also odd because it rotates in the opposite direction to most of the other planets. In addition, Pluto is known to be unlike any of the inner rocky planets, such as Earth, or outer gas giants such as Jupiter ­ it belongs to a different type of planetary body known as an "ice dwarf".

Smaller versions of these stray objects make up a band of frozen material that orbits the Sun as a swarm of comet-like objects known as the Kuiper Belt, which was only fully recognised about 15 years ago.

In fact, Pluto is now considered to be the first Kuiper Belt object to be identified. Last year another Kuiper Belt object bigger than Pluto was discovered, raising questions about whether it too should be considered a planet. Indeed, following Tombaugh's death in 1997, some astronomers had suggested that Pluto was so small and insignificant that its status as a full-sized planet should be downgraded to that of a minor planet ­ a suggestion quashed in 1999 by the International Astronomical Union.

If Pluto orbited another planet it would be considered a satellite. Indeed, seven of the moons in the solar system ­ the Moon, Io, Europa, Gannymede, Callisto, Titan and Triton ­ are known to be bigger than Pluto, which is just 1,413 miles in diameter.

Pluto would cover about half the size of North America and its own moon, Charon, discovered in 1978, is not much smaller. This is why some astronomers have sometimes referred to them as a double planet.

Last year, the Hubble Space Telescope observed two additional moons around Pluto, which are some 5,000 times fainter than the planet itself. Hubble also captured a patchwork of light and dark areas on the planet's surface, which probably represents mixtures of different types of ice possibly coated with tar-like organic substances.

The light reaching Pluto is so weak that temperatures even in a "summer" rarely rise above minus 230C. It is believed that its atmosphere goes through a seasonal freezing and thawing, from solid ice to gas and back again each orbit.

"Not even Tombaugh and his mentors could have forecast how fascinating their new planet would turn out to be," said Alan Stern, the principal investigator of the New Horizons mission. "For the ninth planet was revealed to be the first known world with a satellite so large it could be called a double planet, a world with complex seasons and a chaotic orbit, and the only planet with an atmosphere that freezes out and then is reborn every orbit.

"Pluto, replete with polar caps and fresh snows of not one, but three exotic surface ices, methane, nitrogen and carbon monoxide, is an exotic wonderland on the ragged edge of the solar system's vast outer wilderness," said Dr Stern, a space scientist at the Southwest Research Institute in Boulder, Colorado.

The discovery of the Kuiper Belt fuelled a revolution in understanding the origin, architecture and richness of the outer solar system, he explained.

"If all goes as planned, New Horizons will cross the entire span of the solar system in record time and conduct flyby reconnaissance studies of the Pluto-Charon system in 2015 and then one or more Kuiper Belt objects before 2020," Dr Stern said.

But the technical problems facing the probe are formidable, even if it survives the rigours of launch. The spacecraft is fitted with a nuclear power supply which uses the heat of radioactive decay from 11kg of plutonium-238 to generate 200 watts of electricity ­ enough to power a couple of light bulbs.

Problems with the supply of the plutonium pellets almost jeopardised the mission. A security breach at the Los Alamos National Laboratory in New Mexico, where the pellets were fabricated, temporarily halted production. Nasa had to use left-over pellets from another space mission to ensure that everything would be ready for the launch.

New Horizons has not always been Nasa's favourite mission. The space administration initially thought that any mission to Pluto would be far too expensive to justify a journey to such a small, insignificant world.

After a political struggle, and lobbying by a 17-year-old high school student who was also a persuasive space enthusiast, the US Congress gave Nasa money for a smaller, more streamlined mission, despite the space agency's reservations. The total cost of New Horizons is now running at just over $550m (£312m).

The probe will carry instruments, including ultraviolet and infrared spectrometers, to study the composition of the planet's surface and atmosphere.

Once the probe has completed its flyby of Pluto and Charon ­ it will not be able to orbit the tiny planet ­ it will continue on its way to study the mysterious objects of the Kuiper Belt, which are thought to be remnants from the time when the planets first formed some 4.5 billion years ago.

Dr Stern said: "Exploring Pluto and the Kuiper Belt is like conducting an archaeological dig into the history of the outer solar system, a place where we can peek into the ancient era of planetary formation."

A detector will also count the grains of interplanetary dust collected on the nine-year journey, which the probe will spend in electronic " hibernation" with all systems shut down.

Each week the probe is programmed to send beacon signals back to Earth, which will take some four hours to travel the immense distance.

Once a year, engineers will wake up everything on board to check out the critical computer components, calibrate instruments and perform any necessary course alterations.

Speeding through the darkness of interplanetary space will take New Horizons nine years, even with the help of a Jupiter sling-shot. It will be a long, silent journey to the god of the underworld. But when the probe arrives in 2015, the final view should be well worth the wait.

The coldest planet

* Pluto was discovered in 1930. It was to be called Minerva, after the goddess of wisdom, until someone realised the name was already in use.

* Pluto does not feature in Gustav Holst's work The Planets because it was not discovered until 10 years after the premiere. However, in 2000 the British composer Colin Matthews wrote a movement called Pluto, to be played with the others, and dedicated to Holst's daughter, Imogen. It was performed at the Proms in 2000.

* The temperatures on Pluto never get above minus 230C, the air is a noxious mix of poisonous gases, and any gases that do exist freeze to solid ice once every Plutonian winter.

* Mickey Mouse's faithful dog, Pluto, made his debut in the 1930 film The Chain Gang, the same year the planet was discovered.

* The gravity on Pluto is so weak it would barely hold any humans who managed to get there. A man weighing 300lb on Earth would weigh just 20lb on Pluto.

* A single day on Pluto lasts 6.4 days on Earth.

* Pluto and its moon Charon orbit each other like two ends of a spinning dumbbell, which gives the impression that they are a double planet.

Susannah Orchard