The STS-118 mission countdown will start at 9 p.m. EDT Saturday at T-minus-43 hours, NASA said Monday. The countdown includes 27 hours and 3 minutes of built-in hold time leading to a targeted lift off at 7:02 p.m. EDT, Aug. 7. The launch window lasts 5 minutes.

During the 11-day mission to the International Space Station, Endeavour's crew will add another truss segment to the expanding station, install a gyroscope and add an external spare parts platform. The flight will also include at least three spacewalks and will debut a new system that enables docked shuttles to draw electrical power from the station to extend visits to the outpost.

If the system functions as expected, three additional days will be added to the STS-118 mission.

The mission is the 119th space shuttle flight, the 20th flight for Endeavour and the 22nd U.S. flight to the International Space Station. The mission would be Endeavour's first flight in more than four years.