BEIJING, Sept. 26 (Xinhuanet) -- NASA is to embark on an unprecedented mission Thursday with a spacecraft named Dawn to explore an asteroid belt between Mars and Jupiter for clues to the birth of the solar system, according to the agency Tuesday.

    Dawn's targets are two of the largest bodies in the asteroid belt, Vesta and Ceres, which scientists believe hold clues to the solar system's formation.

    The liftoff is scheduled for Thursday morning, but rain is forecast and could force a delay.

    Scientists have been waiting for Dawn to rise since July, when the mission was put off because of the more pressing need to launch NASA's latest Mars lander, the Phoenix. Once Phoenix rocketed away in August, that cleared the way for Dawn.

    The spacecraft is perched atop an unmanned Delta 2 rocket, which is to boost Dawn into orbit around Earth so it can unfold its 65-foot (19.7 meter) solar wing panels and fire up one of its three ion engines for the four-year trip to Vesta.

    Dawn will slingshot itself around Mars in February 2009, but most of its speed will be built up slowly over time -- very slowly.

    The probe's engines work by pumping electrically charged ions of Xenon through an electric field, which accelerates the particles and prepares them for an 89,000-mph (142,400-kph) escape into space. The force of the expelled gas causes the spacecraft to move in the opposite direction.

    (Agencies)