The Dawn spacecraft will now fly in September
The Dawn spacecraft was due to launch on Sunday from Cape Canaveral Air Force Station in Florida.
But US space agency managers decided to postpone the lift-off due to scheduling problems with another Nasa mission.
Officials felt the window for launching Dawn in July was too tight, and that it could affect preparations for the Mars-bound Phoenix spacecraft.
Phoenix will launch from the Cape in August on a mission to explore the Red Planet's northern polar region.
Asteroids are considered the building blocks of planets - primordial relics left over from the formation of the Solar System 4.6 billion years ago.
Dawn will carry out a detailed study of the structure and composition of these giants, shedding light on their evolution and the conditions in which they formed.
The mission will reach Vesta in 2011 before going on to visit Ceres in 2015.
Ceres - now formally classed as a dwarf planet - may harbour significant stores of water ice; Vesta is devoid of water and appears to have been resurfaced by ancient lava flows.
The launch window closes on 19 July; managers decided there was too little time to get the spacecraft on the right trajectory to fly past Mars on a gravity assist manoeuvre, which will help propel it to the asteroid belt.
The Dawn mission has experienced a number of hurdles on its way to the launch pad. It was cancelled by the US space agency in March 2006.
The decision caused a furore in the scientific community, prompting Nasa to reinstate the mission less than a month later.
In August 2006 the International Astronomical Union created a new category of celestial body known as a "dwarf planet" for large Solar System objects that were more significant than asteroids in general but not quite "true planets". Ceres was put in this category.