On Friday, June 30 at 7:18 a.m. EDT, (11:18 GMT) at T-minus 35 minutes, LO2 chill down was complete.
Engineers investigated a problem with the Nitrogen purge system which delayed LH2 tanking operations. LH2 tanking operations picked up at 7:36 a.m. and was completed at 8:10 a.m. EDT. At 7:48 a.m. the countdown clock was holdings at a planned 45 minute built-in hold. At 8:31 a.m. the launch team was polled for a final clear to launch. The planned T-0 of 8:38 a.m EDT was not possible due to a range safety violation. The hold was extended an additional 18 minutes to give the range time to move boats out of the launch area. Launch occured at 8:56pm.EDT (12:56 GMT). Boster engine cutoff occured at T+2:45 (12:59 GMT). Centaur main engine cutoff occured on schedule at T+26:08 (13:06 GMT).

On Tuesday, June 27, 2000 at 6:15 p.m. EDT, Following the Tracking and Data Relay Satellite (TDRS) and the Atlas IIA (AC-139) Launch Readiness Review, NASA managers decided to postpone the launch 24 hours to June 30 at 8:38 a.m. EDT. The delay is due to additional time necessary to review data from an additional test required on the centaur-stage engines.