Veteran Canadian astronaut Robert Thirsk will replace David Williams as the commander of NEEMO 7, the next "NASA Extreme Environment Mission Operations" mission to the undersea laboratory Aquarius.

The change in crew assignment is a result of a temporary medical issue related to Williams' qualification for this 11- day saturation dive mission. To maintain medical privacy, no further information about Williams' condition will be made public.

As a member of the NEEMO 7 backup crew, Thirsk has trained alongside Williams for months. Thirsk joins astronauts Catherine Coleman, Michael Barratt and Canadian surgeon Dr. Craig McKinley for the mission, which will focus on remote telemedicine experiments. The mission is set for Oct. 11-21 off the coast of Florida, near Key Largo.

"This is a temporary medical issue for Dave," 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."

"Of course, I'm disappointed I won't splash down for the NEEMO 7 mission, but I understand the necessity of the medical criteria that are in place for saturation diving," Williams said. "I know Bob will ensure all the NEEMO 7 objectives are met. I expect to return to full dive and flight status in the near future and look forward to working undersea on a future NEEMO mission," he said.