The probe, which will explore the largest bodies in the Asteroid Belt, was due to take off on Friday. However, bad weather meant that the rocket could not be fuelled for fear of a lightning strike.
The launch was delayed again because there was no ship or aircraft available to relay telemetry data from the Delta II rocket, and the decision has now been made to push for a September launch.
"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," said Nasa.
"A September launch for Dawn maintains all of the science mission goals that a July launch would have provided."
Nasa does not have a window available in August because of the launch of the Phoenix Mars Lander, which is heading to Mars to look for evidence of water.
The Dawn probe will travel over three billion miles to two of the largest objects in the asteroid belt, Ceres and Vesta, and will send back information about the formation of the planets in the Solar System.
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