Here’s a look at today’s countdown events leading up to an on-time liftoff at 5:55:45 p.m. EST.
EST Event
4:45 p.m. Start flight open loop Flight Termination System test
5:09 p.m. Initiate fuel fill sequence
5:21 p.m. Begin 30 minute hold at T-4 Minutes
5:25 p.m. Weather Briefing
5:48 p.m. Status check to continue countdown
5:51:45 p.m. T-4 Minutes and counting
5:55:42 p.m. RD-180 engine ignition
5:55:45 p.m. Launch
5:56:03 p.m. Begin pitch/yaw/roll maneuver
5:57:07 p.m. Mach 1
5:57:18 p.m. Maximum Dynamic Pressure
6:00:00 p.m. Atlas booster engine cutoff (BECO)
6:00:06 p.m. Atlas booster/Centaur separation
6:00:16 p.m. Centaur first main engine start (MES1)
6:00:24 p.m. Payload Fairing jettison
6:14:01 p.m. Centaur first main engine cutoff (MECO1)
6:16:50 p.m. Cygnus spacecraft separation
~7 p.m. Cygnus solar array deploy
~8 p.m. Post-Launch News Conference on NASA Television