Veteran NASA astronaut Leroy Chiao will replace William S. McArthur, Jr., as the commander of Expedition 9, the next mission aboard the International Space Station.

Leroy Chiao has been named to the Expedition 9 crew. Photo: NASA

The change in crew assignment is a result of a temporary medical issue related to McArthur's qualification for this long duration flight. Because of medical privacy considerations, no information about McArthur's condition will be made public.

As a member of the Expedition 9 backup crew, Chiao has been training alongside McArthur for months. Chiao joins Russian Flight Engineer and Soyuz Commander Valery I. Tokarev for the six-month mission. The Expedition 9 crew is scheduled for launch aboard a Soyuz spacecraft in April. European Space Agency astronaut Andre Kuipers is the Board Engineer rounding out the three-member Soyuz crew. Kuipers returns to Earth with Expedition 8.

The original Expedition 9 crew: Andre Kuipers, Bill McArthur and Valery Tokarev. Photo: NASA/Bill Ingalls

"This is a temporary medical issue for Bill," said Astronaut Office Chief Kent Rominger. "Because we are very cautious in our approach to crew health, we train backups for this kind of situation. We plan to assign Bill to another flight in the near future," he said.

"Obviously, I'm disappointed I won't fly on the Expedition 9 mission, but I understand the necessity of the medical criteria that are in place for long-duration space flight," McArthur said. "I know that Leroy will ensure all of the Expedition 9 objectives are met, and I look forward to flying soon on another Space Station mission."