Shuttle Atlantis astronauts will hold a mock countdown in Florida Thursday, as they get ready for what they say will be one of the most challenging missions ever.
The countdown test is part of the drill for each mission, and the installation of new solar panels on the space station by this mission is seen as seen as very complex.
Shuttle commander Brent Jett tells reporters the first six days of the mission are the most aggressive timeline in shuttle history as NASA resumes construction of the outpost.
Astronaut Heide Piper says she and the crew have been pumping iron to be ready for grueling, six-hour spacewalks.
NASA plans to retire the shuttle fleet once the station is complete in 2010. Atlantis' launch is set for August 27th.