Sep 27, 2007
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.
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.
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:
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.
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.