WASHINGTON, July 6 (Xinhua) -- The launch of NASA's Dawn spacecraft to explore two massive asteroids has been rescheduled to no earlier than Monday, July 9, NASA announced on Friday.

The launch window for Dawn on Monday will be 3:56 p.m. to 4:26 p.m. EDT (1956 GMT to 2026 GMT). It will be sent into space by a Delta 2 rocket.

The delay was prompted by difficulties with an aircraft that would gather data signals from the rocket during launch, and the availability of a tracking ship, NASA said in a statement.

Also, the weather forecast at Cape Canaveral Air Force Station in Florida raised concerns that a critical fuel loading operation would not be able to be completed Friday.

The spacecraft which had been originally set to launch Saturday was called off because thunderstorms and lightning at the launch pad prevented loading its fuel.

Seeking clues about the birth of the solar system, Dawn will first visit Vesta in 2011. And then in 2015, it will meet up with Ceres.