Dawn of a new era as Nasa launches 3 billion-mile asteroid mission

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Nasa, the US space agency, is planning to launch an unmanned spacecraft today to investigate two bodies in the asteroid belt between Mars and Jupiter that scientists believe could provide vital clues about the formation of planets, including the possible presence of water and even very basic forms of life.

The spacecraft, Dawn, will take off from Cape Canaveral in Florida – weather permitting – in the hope of reaching the bodies, called Ceres and Vesta, some time in 2011. It will stay with them for a further 16 months. The mission is expected to last eight years in all, with Dawn covering a distance of three billion miles.

Scientists believe the two bodies – in their different ways – are small proto-planets whose growth was stunted because of the gravitational pull of Jupiter, the largest of the planets in our solar system.

Dawn will take photographs and measure chemical, mineral and other data. "We're going to be visiting some of the last unexplored worlds in the inner solar system," Marc Rayman, the project's chief engineer told the Associated Press.

What we know so far is that neither body is an asteroid, despite being in the asteroid belt. Vesta, seen from earth, looks bone-dry, its surface apparently covered in volcanic basalts.

What makes it interesting, above all, is a big dent near its south pole, forming a crater measuring almost 300 miles across. One of Dawn's missions will be to establish whether Vesta could be a source of the meteorites found on earth.

Ceres, by contrast, is believed to be composed of as much as 25 per cent water. Spectroscopic studies have suggested that its surface is covered with clays, carbonates and other minerals that require water to form. The combination of materials that scientists believe Dawn will find there – water, organic compounds and salts – suggest it might be a good environment for the development of very basic life forms.

Neither Ceres nor Vesta is particularly big. Vesta is about 600 miles wide and Ceres just over half that size. They might look like neighbours from our vantage point, but they are in fact 38 million miles apart – which is one reason why the mission will take so long. Dawn will spend 10 months circling Vesta, then another six around Ceres.

One question scientists will be asking is why the two bodies are so different from one another. One answer could be simple distance from the sun – astronomers refer to something they call the "snow line" to distinguish warmer planets from colder ones. It could also have something to do with the influence of Jupiter, or any number of other factors.

Part of the excitement over the mission centres around the cutting-edge ion-propulsion thrusters that will power Dawn through outer space. This new technology is what makes the length of the mission possible, and is enabling Nasa to contemplate orbiting more than one celestial body for the first time.

The thrusters work by bombarding xenon gas with electrons and accelerating the ions that are created as a result into outer space. According to Mr Rayman: "It really does emit this cool blue glow, like in the science- fiction movies."