Dawn's Launch to the Asteroid belt mission to Vesta and Ceres has been moved to September. It seems that July's launch window is small and it might conflict with another mission being launched in August.
The launch of NASA's Dawn spacecraft, a mission that will explore the two largest objects in the asteroid belt in an effort to answer questions about the formation of our solar system, has been rescheduled to September.
The decision was made Saturday to move the launch to September after careful review by NASA's Science Mission Directorate officials, working with Dawn mission managers, the Dawn principal investigator, and with the concurrence of the NASA Administrator.
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. A September launch for Dawn maintains all of the science mission goals a July launch would have provided.
Launch Summer 2007
Mars gravity assist March 2009
Vesta arrival September 2011
Vesta departure April 2012
Ceres arrival February 2015
End of primary mission July 2015