The race is on to reveal Pluto's secrets

Pluto, the outermost planet in our solar system and the only one yet to be visited by a spacecraft from Earth, might yield its secrets within the next decade.

This month Nasa has selected two proposals for a mission to that tiny frozen world 3.5 billion miles away. There, the Sun is just a small stab of light in the Stygian blackness. Beyond is interstellar space.

Although Nasa's 2002 budget request does not yet contain development funding for such a mission, the US Congress has asked the agency "not do anything precipitous which would preclude the ability to develop a Pluto-Kuiper mission". If funding is available, Nasa might launch acraft in 2004-06, to arrive at the edge of our solar system before 2020.

Pluto, discovered as recently as 1930, is unique among our family of planets. Named after the ancient Greek god of the underworld, it is not a rocky world like Earth, Mars, Mercury or Venus, or a gas giant like Jupiter, Saturn, Uranus or Neptune. Some astronomers question whether it is really a planet at all. At 1,430 miles in diameter, it is smaller even than our Moon. One conjecture is that it was once a moon that somehow escaped the gravity of its nearest neighbour, the blue ice-giant Neptune. Others suggest it is really a double planet with its moon Charon (fully half the size of Pluto and only discovered in 1978), a burnt-out comet or a large asteroid.

Astronomers class it as a Kuiper Belt object: part of a ring of celestial bodies made up of material left over from the formation of the rest of the planets, which have never been exposed to the higher temperatures and solar radiation of the inner solar system.

According to Dr Colleen Hartman, the Pluto programme director at Nasa's Office of Space Science in Washington DC, a Pluto-Kuiper Belt mission represents more than just a chance to visit the one planet not yet explored by spacecraft. "It's really an opportunity to look into a deep-freeze of history, which could tell us how our solar system evolved into what it is today, including the precursor ingredients of life."

The Plutonian surface and atmosphere may hold important clues to how we evolved on Earth. We know Pluto has large quantities of nitrogen ice and simple molecules containing combinations of carbon, hydrogen and oxygen ­ the necessary ingredients of life. These ices would be largely lost to space if Pluto had come closer to the Sun. Pluto's average temperature is thought to be about minus 240C. Sunlight is 0.00064 its intensity on Earth. These materials are locked in the atmosphere, a frozen sample of the primordial material from which the evolution of the solar system began.

The two Pluto proposals Nasa has selected were judged to have the best science value of the five submitted in April. Both include launch vehicle, spacecraft and instrument payload. Each would have remote imaging instruments and experiments to determine the geology and morphology of both Pluto and Charon (Charon was the ferryman who rowed the souls of the dead over the river Styx in Hades, a land of perpetual darkness).

The projects bring together groups from academia, industry and Nasa. "POSSE" (Pluto and Outer Solar System Explorer) will be led by Dr Larry Esposito of Colorado University with a team including Nasa's Jet Propulsion Laboratory, Lockheed Martin Astronautics and the University of California, Berkeley. "New Horizons: Shedding Light on Frontier Worlds" will be led by Alan Stern of Southwest Research Institute, Boulder with a team including Johns Hopkins University Applied Physics Laboratory, Ball Aerospace Corp, Stanford University and Nasa's Goddard Space Flight Center.

The two teams will each receive $450,000 to conduct a three-month study, after which Nasa will evaluate their content and technical and cost feasibility.

But time is pressing. The 2004 "launch window" is the last chance for a decade to take advantage of the "slingshot effect" offered en route by Jupiter's gravitational field to give the spacecraft an extra boost.

Pluto is also moving rapidly away from the Sun after reaching its nearest point ­ its perihelion ­ in the 1990s. A later launch might mean that the chance to study the Plutonian atmosphere could be lost for centuries. Scientists believe that the thin atmosphere will freeze to the surface as the planet's orbit moves further outward, with the next thaw not occurring until around 2230.