Noriega is being replaced due to a temporary medical condition that affects his qualification for flight. Because of medical privacy considerations, no other information on Noriega's condition will be made public. While he awaits a possible future flight assignment, Noriega will be assigned to a management role at NASA's Johnson Space Center, Houston.

Sellers will make his second trip on the Space Shuttle with the rest of the STS-121 crew: Commander Steven W. Lindsey (Col., USAF), Pilot Mark E. Kelly (Cmdr., U.S. Navy) and Mission Specialist Michael E. Fossum. STS-121 currently is assigned to Space Shuttle Atlantis and will carry supplies and equipment to the International Space Station. During the flight, the crew will test new Space Shuttle safety changes, procedures and equipment. The current target window for the STS-121 mission begins in May 2005.

Sellers' first space flight was on STS-112, an International Space Station assembly mission in 2002. Born and raised in England, Sellers earned a doctorate in biometeorology from Leeds University. He joined the astronaut corps in 1996.

Sellers previously had been assigned to STS-120. His replacement for that mission will be named at a later date.

Full biographies of NASA astronauts can be found on the Internet at:

http://www.jsc.nasa.gov/Bios/