NASA is planning to launch a new spacecraft on Thursday. The Dawn probe will explore the asteroid belt between Mars and Jupiter with the goal of improving our knowledge about the birth of the solar system.
If successful, Dawn will be the first robotic probe to put itself into orbit around two different bodies, thanks to an extremely efficient electric engine powered by xenon gas.
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. Liftoff of an unmanned rocket carrying Dawn is targeted for 7:20 a.m. EDT on Thursday from Cape Canaveral Air Force Station.
Dawn's last launch attempt in July was canceled after a string of technical problems and weather delays cut into the satellite's launch opportunity. NASA decided to launch its higher-priority Mars probe and then a space shuttle mission in August before returning Dawn to the launch pad.
The spacecraft is perched atop an unmanned Delta 2 rocket, which was 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.