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Last Updated: Thursday, 27 September 2007, 12:12 GMT 13:12 UK
Successful launch for Nasa probe
Delta launch (AP)
Dawn's Delta launch vehicle climbs into the morning sky
Nasa's Dawn space probe has begun an eight-year journey to the Solar System's asteroid belt.

The US spacecraft, which will visit the small worlds of Ceres and Vesta, blasted-off at 1134 GMT (1234 BST) from the Cape Canaveral Airforce Station.

The probe's journey of nearly five billion kilometres will hopefully help scientists decipher how the Solar System formed.

The asteroids are thought to be the leftovers after the planets were made.

Dawn will reach Vesta in 2011 before going on to visit Ceres in 2015.

Ceres is almost spherical and is thought to harbour a layer of water ice some 60 to 120km (40 to 80 miles) thick beneath its hard surface.

At a meeting of the International Astronomical Union (IAU) last year, Ceres was elevated in status from merely the biggest body in the asteroid belt to a "dwarf planet" - the same designation now held by Pluto.

Ceres and Vesta (Nasa)
At 930km (580 miles) across, Ceres (L) is the biggest object in the asteroid belt
It is almost spherical and is classed as a dwarf planet
Vesta (R) is 525km (326 miles) across and has been resurfaced by volcanic flows
Ceres is thought to have an icy layer beneath its dusty surface; Vesta was resurfaced by ancient lava flows
Pieces of Vesta have fallen to Earth as meteorites
Vesta is very different. It is devoid of any water and appears to have been resurfaced by ancient lava flows.

Of particular interest is the giant crater - 460km (285 miles) across and 13km (eight miles) deep - at the object's southern pole. The massive collision that created this crater gouged out 1% of the asteroid's volume. Many of the meteorites that have fallen to Earth are thought to have originated in this space smash.

"Visiting both Vesta and Ceres enables a study in extraterrestrial contrasts," said Dawn's principal investigator, Christopher Russell, of the University of California, Los Angeles.

"One is rocky, and is representative of the building blocks that constructed the planets of the inner Solar System; the other may very well be icy, and represents the outer planets.

"Yet, these two very diverse bodies reside in essentially the same neighbourhood. It is one of the mysteries Dawn hopes to solve."

The mission's objectives include:

  • study internal structure and density
  • determine size, composition, shape and mass
  • examine surface features and craters
  • understand the role of water in controlling asteroid evolution

Dawn is expected to send back high-resolution images of these worlds, showing not just craters but also mountains, canyons, and clear evidence of volcanism.

Artist's impression of Dawn spacecraft   Image: Nasa/UCLA
The Dawn mission will be a "study in extraterrestrial contrasts"
Dawn will make a fly-past of Mars on its long journey. The spacecraft will orbit Vesta for about nine months, and Ceres for at least five months. The mission is scheduled to end in July 2015.

Visiting two objects and orbiting both is a capability that is only now becoming possible through the development of solar-electric engines.

Unlike the chemical rockets of yesteryear which gave probes a short, sharp impulse, the new generation of ion thrusters provide gentle but sustained propulsion. They work by ejecting charged atoms (ions) of xenon.

They take time to build up speed - hence Dawn's long mission profile - but they are very efficient and flexible, allowing spacecraft to hop from target to target.

Profile of mission (BBC)

Perfect lift-off for Dawn spacecraft

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09 Jul 07 |  Science/Nature
Nasa readies for asteroid mission
27 Jun 07 |  Science/Nature
Fresh look at dwarf planet Ceres
10 Oct 06 |  Science/Nature
Pluto vote 'hijacked' in revolt
25 Aug 06 |  Science/Nature

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