A worker watches as the replacement Power Drive Unit for Space Shuttle Atlantis is lifted by crane toward the tail.

At Launch Pad 39A, a worker watches as the plastic-covered replacement Power Drive Unit (PDU) for Space Shuttle Atlantis is lifted by crane toward the tail. The PDU controls the rudder/speed brake on the orbiter. Atlantis is scheduled to lift off April 24 at 4:15 p.m. EDT on mission STS-101, the third flight to the International Space Station. The primary mission is to carry logistics and supplies to the Space Station, plus the crew will be preparing the Station for the arrival of the Zvezda Service Module, expected to be launched by Russia in July 2000