Editor's Note: The launch window is roughly 10 minutes long. To maximize ascent performance, NASA targets the middle of the window. A final hold in the countdown at the T-minus nine-minute mark will be extended as required to hit the desired launch time.

EDT...........EVENT

Tue  05/11/10

03:30 PM......Call to stations
04:00 PM......Countdown begins

Wed  05/12/10

02:00 AM......Fuel cell reactant load preps
07:30 AM......MEC/SRB power up
08:00 AM......Clear crew module

08:00 AM......Begin 4-hour built-in hold
08:00 AM......Clear blast danger area
08:45 AM......Orbiter pyro-initiator controller test
08:55 AM......SRB PIC test
09:55 AM......Master events controller pre-flight BITE test
12:00 PM......Resume countdown

01:30 PM......Fuel cell oxygen loading begins
04:00 PM......Fuel cell oxygen load complete
04:00 PM......Fuel cell hydrogen loading begins
06:30 PM......Fuel cell hydrogen loading complete
07:30 PM......Pad open; ingress white room

08:00 PM......Begin 4-hour built-in hold
03:00 PM......PRSD offload
05:00 PM......Crew module clean and vacuum
08:30 PM......OMBUU demate
09:00 PM......Remove APU vent covers
09:30 PM......MPS 2000 psi GSE

Thu  05/13/10

12:00 AM......Countdown resumes

12:00 AM......Main engine preps
12:00 AM......MECs 1 and 2 on; avionics system checkout
02:00 AM......FRCS Tyvek cover remova/inspect
06:30 AM......Deflate RSS dock seals; tile inspection
07:00 AM......Tile inspection
07:00 AM......TSM prepped for fueling

08:00 AM......Begin 13-hour 54-minute hold
08:00 AM......L-1 engineering briefing
08:15 AM......Crew weather briefing
08:30 AM......Crew orbiter/payload briefings
09:30 AM......ASP crew module inspection
09:30 AM......OIS communications check
11:20 AM......JSC flight control team on station
11:30 AM......Comm activation
12:00 PM......Crew module voice checks
01:00 PM......Flight crew equipment late stow
05:30 PM......RSS to park position
06:30 PM......Final TPS, debris inspection
07:00 PM......Ascent switch list
09:54 PM......Resume countdown

10:14 PM......Pad clear of non-essential personnel
10:14 PM......APU bite test
11:04 PM......Fuel cell activation
11:54 PM......Booster joint heater activation

Fri  05/14/10

12:24 AM......MEC pre-flight bite test
12:39 AM......Tanking weather update
01:24 AM......Final fueling preps; launch area clear
01:54 AM......Red crew assembled
02:39 AM......Fuel cell integrity checks complete

02:54 AM......Begin 2-hour built-in hold (T-minus 6 hours)
03:04 AM......Safe-and-arm PIC test
03:54 AM......External tank ready for loading
04:17 AM......Mission management team tanking meeting
04:45 AM......NASA TV fueling coverage begins
04:54 AM......Resume countdown (T-minus 6 hours)

04:54 AM......LO2, LH2 transfer line chilldown
04:58 AM......Final crew medical checks
05:04 AM......Main propulsion system chill down
05:04 AM......LH2 slow fill
05:34 AM......LO2 slow fill
05:39 AM......Hydrogen ECO sensors go wet
05:44 AM......LO2 fast fill
05:47 AM......Crew medical checks
05:54 AM......LH2 fast fill
07:49 AM......LH2 topping
07:54 AM......LH2 replenish
07:54 AM......LO2 replenish

07:54 AM......Begin 2-hour 30-minute built-in hold (T-minus 3 hours)
07:54 AM......Closeout crew to white room
07:54 AM......External tank in stable replenish mode
08:09 AM......Astronaut support personnel comm checks
08:39 AM......Pre-ingress switch reconfig
09:00 AM......NASA TV launch coverage begins
09:54 AM......Final crew weather briefing
09:59 AM......Crew suit up begins
10:24 AM......Resume countdown (T-minus 3 hours)

10:29 AM......Crew departs O&C building
10:59 AM......Crew ingress
11:49 AM......Astronaut comm checks
12:14 PM......Hatch closure
12:44 PM......White room closeout

01:04 PM......Begin 10-minute built-in hold (T-minus 20m)
01:14 PM......NASA test director countdown briefing
01:14 PM......Resume countdown (T-minus 20m)

01:15 PM......Backup flight computer to OPS 1
01:19 PM......KSC area clear to launch

01:25 PM......Begin final built-in hold (T-minus 9m)
01:55:59 PM...NTD launch status verification
02:10:59 PM...Resume countdown (T-minus 9m)

02:14:59 PM...Orbiter access arm retraction
02:14:59 PM...Launch window opens
02:14:59 PM...Hydraulic power system (APU) start
02:15:04 PM...Terminate LO2 replenish
02:15:59 PM...Purge sequence 4 hydraulic test
02:15:59 PM...IMUs to inertial
02:16:04 PM...Aerosurface profile
02:16:29 PM...Main engine steering test
02:17:04 PM...LO2 tank pressurization
02:17:24 PM...Fuel cells to internal reactants
02:17:29 PM...Clear caution-and-warning memory
02:17:59 PM...Crew closes visors
02:18:02 PM...LH2 tank pressurization
02:19:09 PM...SRB joint heater deactivation
02:19:28 PM...Shuttle GPCs take control of countdown
02:19:38 PM...SRB steering test
02:19:52 PM...Main engine start (T-6.6 seconds)
02:19:59 PM...SRB ignition (LAUNCH)