A much-postponed launch of a spacecraft meant to explore two of the solar system's largest asteroids has been delayed again, this time until September.

NASA didn't want the launch of the Dawn spacecraft, which had already been delayed until July 15, to conflict with preparations for the August launches of the space shuttle Endeavour and a Mars lander mission, said NASA spokesman George Diller.

When the spacecraft does launch, it will embark on a yearslong journey to the asteroids Vesta and Ceres, between Mars and Jupiter.

The spacecraft will first visit Vesta, the smaller of the two bodies, four years from now. In 2015, it will meet up with Ceres, which carries the status of both asteroid and, like Pluto, dwarf planet.

NASA has rescheduled the launch several times. The original launch, planned for Saturday, was canceled because storms at the launch pad prevented loading its fuel. Also, a plane used to track the spacecraft after liftoff was having mechanical problems, and the tracking ship wasn't in the correct location. The launch date was changed to Monday and then July 15 before being postponed Saturday until September.

NASA has until the end of October to launch the spacecraft before the planetary bodies begin to drift apart. It will take another 15 years for them to get back together again, said Chris Russell, the mission's principal investigator.