The countdown began Monday for a two-week scientific research flight by NASA's oldest space shuttle, the Columbia.
Meteorologists gave 80 percent odds of good weather for the launch, set for 8:54 a.m. Thursday. The only concern was possible high wind.
The five veteran crew members _ four men and one woman _ will spend 14 days conducting medical and structural experiments and testing a new robot-arm grapple system. Columbia also will carry 12 rats as part of a bone and immune cell study.
Most of the primary experiments will be commanded by remote control from the ground.
This is will be Columbia's 16th flight.