Scheduled For 1630 EDT

Three hours ago, NASA began the countdown for the launch of Space Shuttle Atlantis on mission STS-115, at the T-minus 43 hour point. During this mission, Atlantis' crew will resume the construction of the International Space Station -- 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 pm on Sunday, August 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 US flight to the International Space Station. STS-115 is scheduled to last 11 days, with a planned return to Kennedy Space Center at about 12:02 pm EDT on September 7.

Atlantis rolled into KSC's Orbiter Processing Facility on October 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 2003 loss of 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 August 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.

FMI: www.nasa.gov/shuttle