It was the second delay for the mission in as many days.

"Primary reasons for the move 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," the National Aeronautics and Space Administration said in a statement Saturday.

"A September launch for Dawn maintains all of the science mission goals a July launch would have provided," NASA further assured.

Dawn's launch was originally scheduled for Saturday, but because of weather was first delayed until Monday.

Even then, NASA said, a ship tasked with tracking the probe's launch into orbit would not have been in place by Monday as expected.

Dawn's eight-year mission will shed light on the earliest moments in the birth of the solar system 4.6 billion years ago by examining the two celestial bodies Ceres and Vesta in the asteroid belt between Mars and Jupiter.

Dawn is scheduled to enter orbit around Vesta in October 2011, proceed to Ceres in May 2012 and then begin orbiting Ceres in February 2015 -- traveling a total distance of 5.1 billion kilometers (3.2 billion miles).

Dawn's launch window, said NASA earlier, will remain open until the end of October, when the two asteroids begin pulling away from each other, making a rendezvous with both practically impossible. This will be the first NASA mission to orbit two separate objects.

NASA says the mission should provide a better understanding of the building blocks that formed the terrestrial planets and how the two "protoplanets" followed different evolutionary paths.

The space explorer, measuring 1.64 meters (5.3 feet) long and 1.27 meters (4.1 feet) wide when its solar array is retracted, is equipped with a high-definition camera and two spectrometers to study the two massive asteroids.

Ceres, discovered in 1801, is spherical and has a diameter of about 960 kilometers (596 miles). Scientists believe it may have a layer of thick ice under its crust, covering a rocky core.

Vesta, discovered in 1807, is smaller than Ceres but the third largest asteroid in the solar system. With a diameter of 520 kilometers (323 miles), Vesta has a rocky surface without a trace of water and a hot interior.

After having cancelled the Dawn project previously, NASA revived the mission in 2006 after an investment of 449 million dollars.

It is the ninth mission out of 10 planned in NASA's Discovery program that employs unmanned vehicles to explore space, often focusing on asteroids.

NASA has scheduled a news briefing for 11:30 am (1530 GMT) on Monday to explain the situation with Dawn and preview the launch of the Phoenix Mars Lander.

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