NASA began the countdown for the launch of Space Shuttle Atlantis on mission STS-115 at noon EDT Thursday, Aug. 24, at the T-43 hour point. During this mission, Atlantis' crew will resume the construction of the International Space Station, which is the goal of the remaining space shuttle flights in the program.

The Kennedy Space Center launch team will conduct the countdown from the newly renovated Firing Room 4 of the Launch Control Center. The countdown includes 33 hours, 24 minutes of built-in hold time leading to a preferred launch time at about 4:30 p.m. on Sunday, Aug. 27, with a launch window extending about five minutes.

This mission is the 116th space shuttle flight, the 27th flight for orbiter Atlantis, and the 19th U.S. flight to the International Space Station. STS-115 is scheduled to last 11 days with a planned KSC landing at about 12:02 p.m. EDT on Sept. 7.

Atlantis rolled into KSC's Orbiter Processing Facility on Oct. 18, 2002, after returning from its last mission, STS-112. Its next mission was planned to be STS-114; however, during the program delays following the loss of orbiter Columbia, Atlantis was reassigned to mission STS-115. The orbiter rolled out of the facility's bay 1 and into the Vehicle Assembly Building on July 24. While in the building's high bay 3, Atlantis was mated to its modified external tank and solid rocket boosters. The entire space shuttle stack was transferred to Launch Pad 39B on Aug. 2.

The STS-115 crew includes Commander Brent Jett, Pilot Chris Ferguson, and Mission Specialists Joe Tanner, Dan Burbank, Heidemarie Stefanyshyn-Piper and Steve MacLean of the Canadian Space Agency.

During mission STS-115, Atlantis will dock with the station and the crew will perform three spacewalks. The astronauts will deliver and install the 17.5-ton, bus-sized P3/P4 integrated truss segment to the station's girder-like truss backbone. The new piece will include a second set of giant solar arrays, batteries and associated electronics. Together, the trusses and solar arrays will provide one-fourth of the total power-generation capability of the completed station.

To prepare for the extravehicular activities, the spacewalkers will perform a new "campout pre-breathing exercise." These crew members will reside in the station's airlock overnight, where the pressure will slowly be reduced. Harmful gases will thereby be removed from their blood, allowing them to gradually acclimate to the lower pressure they will encounter outside the station. Provisions are onboard to support as many as three additional spacewalks, if required to complete mission objectives.

For information about the STS-115 crew and the mission to the International Space Station, visit:

http://www.nasa.gov/shuttle

(end of general release)

COUNTDOWN MILESTONES *all times are Eastern

Launch-3 Days (Thursday, Aug. 24)

• Prepare for the start of the STS-115 launch countdown
• Perform the call to stations (11:30 a.m.)
• Countdown begins at the T-43 hour mark (noon)
• Begin final vehicle and facility closeouts for launch
• Check out backup flight systems
• Review flight software stored in mass memory units and display systems
• Load backup flight system software into Atlantis' general purpose computers
• Remove flight-deck platforms (8:30 p.m.)
• Complete preparation to load power reactant storage and distribution system (11 p.m.)

Launch-2 Days (Friday, Aug. 25)

• Activate and test navigational systems (1 a.m.)
• Flight deck preliminary inspections complete (4 a.m.)
• Enter first built-in hold at T-27 hours for duration of 4 hours (4 a.m.)
• Clear launch pad of all non-essential personnel
• Perform test of the vehicle's pyrotechnic initiator controllers
• Resume countdown (8 a.m.)
• Begin operations to load cryogenic reactants into Atlantis' fuel cell
• storage tanks (8 a.m.)
• Enter 10-hour built-in hold at T-19 hours (4 p.m.)
• Demate orbiter mid-body umbilical unit (4:30 p.m.)
• Resume orbiter and ground support equipment closeouts

Launch-1 Day (Saturday, Aug. 26)

• Resume countdown (2 a.m.)
• Final preparations of the shuttle's three main engines for main
• propellant tanking (2 a.m.)
• Begin filling pad sound suppression system water tank (4 a.m.)
• Pad sound suppression system water tank filling complete (7 a.m.)
• Close out the tail service masts on the mobile launcher platform
• Begin star tracker functional checks (9:50 a.m.)
• Enter planned hold at T-11 hours for 13 hours, 34 minutes (10 a.m.)
• Activate orbiter's inertial measurement units
• Activate the orbiter's communications systems
• Install film in numerous cameras on the launch pad (10:55 a.m.)
• Flight crew equipment late stow (3:10 p.m.)
• Move Rotating Service Structure to the park position (7 p.m.)
• Perform ascent switch list
• Fuel cell flow-through purge complete
• Resume countdown at T-11 hours (11:34 p.m.)

Launch Day (Sunday, Aug. 27)

• Activate the orbiter's fuel cells (12:45 a.m.)
• Clear the blast danger area of all non-essential personnel
• Switch Atlantis' purge air to gaseous nitrogen (1:20 a.m.)
• Enter planned 2-hour built-in hold at the T-6 hour mark (4:34 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 (6:34 a.m.)
• Chilldown of propellant transfer lines (6:34 a.m.)
• Begin loading the external tank with about 500,000 gallons of cryogenic propellants (about 6:44 a.m.)
• Complete filling the external tank with its flight load of liquid hydrogen and liquid oxygen propellants (about 9:34 a.m.)
• Final Inspection Team proceeds to launch pad
• Enter planned 3-hour built-in hold at T-3 hours (9:34 a.m.)
• Perform inertial measurement unit preflight calibration
• Align Merritt Island Launch Area tracking antennas
• Perform open loop test with Eastern Range
• Resume countdown at T-3 hours (12:34 p.m.)
• Crew departs Operations and Checkout Building for the pad (12:40 p.m.)
• Complete closeout preparations in the White Room
• Check cockpit switch configurations
• Flight crew begins entry into the orbiter (about 1:10 p.m.)
• Astronauts perform air-to-ground voice checks with Launch and Mission Control
• Begin to close Atlantis' crew hatch (about 2 p.m.)
• Begin Eastern Range final network open loop command checks
• 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 planned 10-minute hold at T-20 minutes (3:14 p.m.)
• NASA test director conducts final launch team briefings
• Complete inertial measurement unit preflight alignments
• Resume countdown at T-20 minutes (3:24 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 40-minute hold at T-9 minutes (3:35 p.m.)
• 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 4:21 p.m.)
• Start automatic ground launch sequencer (T-9 minutes)
• Retract orbiter crew access arm (T-7:30)
• Start mission recorders (T-6:15)
• Start Auxiliary Power Units (T-5)
• Arm solid rocket booster and external tank range safety safe and arm devices (T-5)
• 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 bi-pod heaters (T-1:52)
• Deactivate solid rocket booster joint heaters (T-1)
• 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)

CREW FOR MISSION STS-115

Commander: Brent Jett

• Pilot: Chris Ferguson
• Payload Commander (MS1): Joe Tanner
• Mission Specialist (MS2): Dan Burbank
• Mission Specialist (MS3): Heidemarie Stefanyshyn-Piper
• Mission Specialist (MS4): Steve MacLean

SUMMARY OF STS-115 LAUNCH DAY CREW ACTIVITIES

Sunday, Aug. 27

• 6:30 a.m. -- Crew wakes up
• 7 a.m. -- Breakfast
• *10:55 a.m. -- Television coverage from crew quarters
• Noon -- Weather briefing
• *12:10 p.m. -- Don flight suits
• *12:40 p.m. -- Depart for launch pad
• *1:10 p.m. -- Arrive at White Room and begin ingress
• *2:25 p.m.-- Close crew hatch
• *4:30 p.m. -- Launch

* Televised events (times may vary slightly) All times Eastern