Donald Pettit tapped to fly on Expedition Six. Photo: NASA

Donald Pettit, Ph.D., a member of the Expedition Six backup crew, will replace Donald Thomas, Ph.D., on the future mission to the International Space Station.  

Pettit has been training as backup flight engineer for this expedition and will join Expedition Commander Kenneth Bowersox (Capt., USN) and Russian cosmonaut Nikolai Budarin. The reassignment results from a medical issue that affects Thomas' long-duration space flight qualification.

"The demanding nature of long-term space flight requires a conservative approach to crew health issues, especially this early in the program," said Astronaut Office Chief, Charlie Precourt. "We have had to make such adjustments before; this again demonstrates the value of training backup space station crewmembers in parallel with the prime crews."

NASA selected Pettit in April 1996. He's a chemical engineer and served as a staff scientist at Los Alamos National Laboratory in New Mexico before joining the astronaut corps.

Selected by NASA in January 1990, Thomas became an astronaut in July 1991. He's a veteran of four space shuttle flights and has logged more than 1,040 hours in space.