WASHINGTON, July 7 (Xinhua) -- The launch of NASA's Dawn spacecraft, a mission that will explore the two largest objects in the asteroid belt in an effort to answer questions about the formation of our solar system, has been rescheduled to September.

The decision was made on Saturday after careful review by NASA's Science Mission Directorate officials, working with Dawn mission managers and principal investigators, NASA's official website said.

Primary reasons for the change were a combination of highly limited launch opportunities for Dawn in July and the potential impact to launch preparations for the upcoming Phoenix Mars Lander mission, set for early August, said the federal space agency.

The blastoff of spacecraft, originally slated for Saturday, was called off because thunderstorms and lightning at the launch pad prevented loading its fuel.

The Sunday launch attempt was also scrubbed because of difficulties with an aircraft that would gather data signals from the rocket during the launch, and the availability of a tracking ship.

NASA stressed that a September launch for Dawn maintains all of the science mission goals a July launch would have provided.

As scheduled, Dawn will first visit Vesta in 2011. And then in 2015, it will meet up with Ceres.