Tonights countdown milestones.

Rotating Service Structure begins move away from Atlantis to the park position (5:30 p.m.)
Perform ascent switch list
Fuel cell flow-through purge complete

Resume countdown at T-11 hours (9:55 p.m.)

Activate the orbiter's fuel cells (11:05 p.m.)
Clear the blast danger area of all nonessential personnel
Switch Atlantis’ purge air to gaseous nitrogen (11:55 p.m.)

Launch Day (Friday, May 14)

Complete final walkdowns

Enter 2-hour built-in hold at the T-6 hour mark (2:55 a.m.)

Complete tanking preparations
Launch team verifies no violations of launch commit criteria prior to cryogenic loading of the external tank
Clear pad of all personnel

Resume countdown (4:55 a.m.)

Chill down propellant transfer lines (4:55 a.m.)
Begin loading the external fuel tank with about 500,000 gallons of cryogenic propellants (about 5:05 a.m.)

Enter 2-hour, 30-minute built-in hold at T-3 hours (7:55 a.m.)

Perform inertial measurement unit preflight calibration
Align Merritt Island Launch Area tracking antennas
Complete filling the external tank with its flight load of liquid hydrogen and liquid oxygen propellants (about 7:55 a.m.)
Final Inspection Team and Closeout Crew proceeds to launch pad

Resume countdown at T-3 hours (10:25 a.m.)

Perform open loop test with Eastern Range
Crew departs Operations and Checkout Building for the pad (10:30 a.m.)
Complete closeout preparations in the White Room
Check cockpit switch configurations
Begin Eastern Range final network open loop command checks
Flight crew begins entry into the orbiter (11 a.m.)
Astronauts perform air-to-ground voice checks with Launch and Mission Control
Begin to close Atlantis’ crew hatch (12:05 p.m.)
Perform hatch seal and cabin leak checks
Complete White Room closeout
Closeout crew moves to fallback area
Primary ascent guidance data is transferred to the backup flight system

Enter 10-minute hold at T-20 minutes (1:05 p.m.)

NASA test director conducts final launch team briefings

Resume countdown at T-20 minutes (1:15 p.m.)

Transition the orbiter's onboard computers to launch configuration
Start fuel cell thermal conditioning
Close orbiter cabin vent valves
Transition backup flight system to launch configuration

Enter estimated 45-minute hold at T-9 minutes (1:26 p.m.)

Final launch window determination
Launch director, Mission Management Team and NASA test director conduct final polls for "go/no go" to launch

Resume countdown at T-9 minutes (about 2:11 p.m.)

Start automatic ground launch sequencer (T-9 minutes)
Retract orbiter crew access arm (T-7:30)
Start APU recorders (T-6:15)
Start auxiliary power units (T-5)
Terminate liquid oxygen replenish (T-4:55)
Start orbiter aerosurface profile test (T-3:55)
Start main engine gimbal profile test (T-3:30)
Pressurize liquid oxygen tank (T-2:55)
Begin retracting the gaseous oxygen vent arm (T-2:50)
Fuel cells to internal reactants (T-2:35)
Pressurize liquid hydrogen tank (T-1:57)
Deactivate bi-pod heaters (T-1:52)
Deactivate solid rocket booster joint heaters (T-0:50 seconds)
Orbiter transfers from ground to internal power (T-0:50 seconds)
Ground launch sequencer go for auto sequence start (T-0:31 seconds)
Booster gimbal profile (T-0:21 seconds)
Ignition of three space shuttle main engines (T-6.6 seconds)
Booster ignition and liftoff (T-0)

Preferred launch time: 2:20:07 p.m. EDT