CAPE CANAVERAL, Fla. - NASA is about to embark on an unprecedented asteroid-belt mission with a spacecraft aptly named Dawn. The 3 billion-mile, eight-year journey to probe the earliest stages of the solar system will begin with liftoff, planned for just after sunrise Thursday. Rain is forecast, however, 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. "For the people in the...