SPACE SHUTTLE MISSION STS-91 LAUNCH COUNTDOWN TO BEGIN MAY 30

NASA will begin the countdown for launch of Space Shuttle Discovery on mission STS-91 on May 30 at 9 p.m. EDT at the T-43 hour mark. The KSC launch team will conduct the countdown from Firing Room 1 of the Launch Control Center.

The countdown includes 26 hours and 10 minutes of built-in hold time leading to an opening of the launch window at about 6:05 p.m. on June 2. The launch window extends for 10 minutes. Managers are targeting a preferred launch time of about 6:10 p.m. However, an exact launch time will be determined during the T-9 minute hold after evaluating the orbital position of Russia's Mir space station.

This will be the third Space Shuttle mission of the year. STS-91 will mark the 24th flight of the orbiter Discovery and the 91st flight overall in NASA's Space Shuttle program. This is the ninth and final Shuttle flight planned to rendezvous and dock with Mir. STS-91 is currently slated to last 9 days, 19 hours, 53 minutes. Discovery is scheduled to land at KSC at 2:03 p.m. on June 12.

U.S. astronaut Andrew Thomas will join the other seven crew members aboard Discovery on Flight Day 3 following docking operations, ending more than four months of research onboard the space station. Discovery will undock from Mir on Flight Day 7. STS-91 features a series of firsts including Discovery's first time docking with Mir, the first on-orbit test of the Alpha Magnetic Spectrometer payload, and the first flight of NASA's super lightweight external tank. More than 3,000 pounds of supplies and water will be transferred to Mir during the four days of docking operations.

Discovery was rolled out of Kennedy Space Center's Orbiter Processing Facility bay 2 on April 27 and then mated with the external tank and solid rocket boosters in the Vehicle Assembly Building. The Shuttle stack was transported to Pad 39A on May 2.

The STS-91 crew consists of: Commander Charles Precourt, Pilot Dominic Gorie, Mission Specialists Wendy Lawrence, Franklin Chang-Diaz, Janet Kavandi, Valery Ryumin of Russia and Andrew Thomas.

The crew is scheduled to arrive at KSC at about 12:30 p.m., Saturday, May 30. Their activities at KSC prior to launch will include crew equipment fit checks, medical examinations and opportunities to fly in the Shuttle Training Aircraft.

• Prepare for the start of the STS-91 launch countdown
• Perform the call-to-stations (8:30 p.m.)
• All members of the launch team report to their respective consoles in Firing Room 1 in the Launch Control Center for the start of the countdown
• Countdown begins at the T-43 hour mark (9 p.m.)
• Begin final vehicle and facility close-outs for launch
• Check out back-up flight systems
• Review flight software stored in mass memory units and display systems
• Load backup flight system software into Discovery's general purpose computers
• Begin stowage of flight crew equipment

• Inspect the orbiter's mid-deck and flight-deck and remove crew module platforms
• Activate and test navigational systems
• Complete preparations for loading of power reactant storage and distribution system

Enter first planned built-in hold at T-27 hours for duration of four hours (1 p.m.)

• Clear launch pad of all non-essential personnel
• Perform test of the vehicle's pyrotechnic initiator controllers
• Open launch pad to personnel supporting PRSD load

Resume countdown (5 p.m.)

• Begin operations to load cryogenic reactants into Discovery's fuel cell storage tanks (5 p.m. - 1 a.m.)

Enter four-hour built-in hold at T-19 hours (1 a.m.)

• Demate orbiter mid-body umbilical unit and retract into fixed service structure
• Resume orbiter and ground support equipment close-outs

Resume countdown (5 a.m.)

• Start final preparations of the Shuttle's three main engines for main propellant tanking and flight
• Close-out the tail service masts on the mobile launcher platform

Enter planned hold at T-11 hours for 13 hours, 14 minutes (1 p.m.)

• Activate the orbiter's flight controls and navigation systems
• Begin startracker functional checks
• Activate orbiter's inertial measurement units
• Install film in numerous cameras on the launch pad
• Activate the orbiter's communications systems
• Flight crew equipment late stow
• Fill pad sound suppression system water tank
• Safety personnel conduct debris walk down

• Move Rotating Service Structure (RSS) to the park position (12:30 a.m.)
• Perform orbiter ascent switch list in crew cabin
• Start fuel cell flow-through purge

Resume countdown (2:14 a.m.)

• Activate the orbiter's fuel cells
• Configure communications at Mission Control, Houston, for launch
• Clear the blast danger area of all non-essential personnel
• Switch Discovery's purge air to gaseous nitrogen
• Complete inertial measurement unit activation

Enter planned two-hour built-in hold at the T-6 hour mark (7:14 a.m.)

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

Resume countdown (9:14 a.m.)

• Begin loading the external tank with about 500,000 gallons of cryogenic propellants (about 9:14 a.m.)
• Complete filling the external tank with its flight load of liquid hydrogen and liquid oxygen propellants (about 12:14 p.m.)

Enter planned two-hour built-in hold at T-3 hours (12:14 p.m.)

• Align Merritt Island Launch Area (MILA) tracking antennas
• Perform inertial measurement unit preflight calibration
• Close-out crew and Final Inspection Team proceed to Launch Pad 39A

Resume countdown at T-3 hours (2:14 p.m.)

• Perform open loop test with Eastern Range
• Crew departs Operations and Checkout Building for the pad (about 2:19 p.m.)
• Complete close-out preparations in the white room
• Check cockpit switch configurations
• Flight crew begins entry into the orbiter (about 2:49 p.m.)
• Astronauts perform air-to-ground voice checks with Launch Control and Mission Control
• Close Discovery's crew hatch (about 4:04 p.m.)
• Begin Eastern Range final network open loop command checks
• Perform hatch seal and cabin leak checks
• Complete white room close-out
• Close-out crew moves to fallback area
• Primary ascent guidance data is transferred to the backup flight system

Enter planned 10-minute hold at T-20 minutes (4:54 p.m.)

• NASA Test Director conducts final launch team briefings
• Complete inertial measurement unit pre-flight alignments

Resume countdown (5:04 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 planned 46-minute hold at T-9 minutes (5:15 p.m.)
(This is the final planned built-in hold. The countdown will resume to meet the preferred launch time of about 6:10 p.m. The exact launch time will be announced during this hold)

• Launch Director, Mission Management Team and NASA Test Director conduct final polls for go/no go to launch
• Launch team determines time of launch

Resume countdown at T-9 minutes (at about 6:01 p.m.)

• Start automatic ground launch sequencer (T-9:00 minutes)
• Retract orbiter crew access arm (T-7:30)
• Start mission recorders (T-6:15)
• Start Auxiliary Power Units (T-5:00)
• Arm SRB and ET range safety safe and arm devices (T-5:00)
• Start liquid oxygen drainback (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 retraction of the gaseous oxygen vent arm (T-2:55)
• Fuel cells to internal reactants (T-2:35)
• Pressurize liquid hydrogen tank (T-1:57)
• Deactivate SRB joint heaters (T-1:00)
• Orbiter transfers from ground to internal power (T-0:50 seconds)
• Ground Launch Sequencer go for auto sequence start (T-0:31 seconds)
• SRB gimbal profile (T-0:21 seconds)
• Ignition of three Space Shuttle main engines (T-6.6 seconds)
• SRB ignition and liftoff (T-0)

         Commander (CDR):  Charles Precourt 
             Pilot (PLT):  Dominic Gorie
Mission Specialist (MS1):  Franklin Chang-Diaz
Mission Specialist (MS2):  Wendy Lawrence 
Mission Specialist (MS3):  Janet Kavandi
Mission Specialist (MS4):  Valery Ryumin
Mission Specialist (MS5):  Andrew Thomas (down)