the launch was the first flight of the atlas v 501 configuration, and the first in four years to use a payload fairing. prior to the installation of the spacecraft, the atlas rocket was moved to the launch pad and performed a wet dress rehearsal on 2 april 2010. it was returned to the vertical integration facility the next day for final assembly. the x-37 arrived at the vif on 8 april. on 9 april, a 24-hour delay was announced. it subsequently slipped a further 24 hours after the landing of on mission sts-131 was delayed, as the eastern range could not have been reconfigured quickly enough to accommodate both events on the same day. after a series of delays, it was set for 19 april 2010. on 21 april, the atlas was rolled back out to the launch pad for launch. the launch window on 22 april opened at 23:52 utc, and closed at 00:01 on 23 april.