Asteroid probe misses launch window
A science probe making the US space agency's first foray into the asteroid belt missed its launch window and won't now blast off until September, officials say.
September will be NASA's last chance for 15 years to explore its two targets, Vesta and Ceres.
These are two of the largest bodies in the asteroid belt, a vast area between Mars and Jupiter littered with primordial remains from the solar system's formation.
After September, the two bodies will begin moving apart from each other and the spacecraft will no longer be able to reach both.
Dawn is scheduled to enter orbit around Vesta in October 2011, proceed to Ceres in May 2012 and then begin orbiting Ceres in February 2015, travelling a total distance of 5.1 billion kilometres.
The space explorer, 1.64 metres long and 1.27 metres wide when its solar array is retracted, is equipped with a high-definition camera and two spectrometers to study the two massive bodies.
Ceres was discovered in 1801, is spherical and has a diameter of about 960 kilometres. Scientists believe it may have a layer of thick ice under its crust, covering a rocky core.
Vesta was discovered in 1807, is smaller than Ceres but the third largest asteroid in the solar system. With a diameter of 520 kilometres, Vesta has a rocky surface without a trace of water and a hot interior.
The spacecraft will be the first able to go into orbit around more than one target, thanks to its ion-powered engines that can stop and restart during flight using a fraction of the fuel of conventional chemical thrusters.
NASA last year cancelled Dawn, citing budget pressures and technical issues, but scientists appealed and won an additional US$100 million (A$116 million) to continue the program. Total mission costs will now be about US$450 million (A$524 million).
Since then, Dawn's launch has been delayed because of a series of technical problems, clashes with launches of other missions from Cape Canaveral and bad weather.