Astronaut Tim Kopra. Photo by NASA

The agency selected Steve Bowen to replace Kopra and said the crew change should not affect the mission's target launch date. STS-133 is slated for launch on Feb.24 from NASA's Kennedy Space Center in Florida.

However, Kopra could potentially rejoin the crew if for some reason STS-133 experiences a significant schedule slippage, said Peggy Whitson, chief of the Astronaut Office at NASA's Johnson Space Center in Houston. STS-133 has been grounded since its initial launch attempt last November due to weather and technical issues.

Whitson said Kopra is "doing fine and expects a full recovery." Further details about his condition were not disclosed due to medical confidentiality.

The crew swap-- six weeks before a scheduled launch—makes it the closest one to a launch date in the shuttle program's history. Astronauts have been replaced in shuttle missions eight previous times, Whitson said.

Bowen, who flew on Atlantis' STS-132 mission in May 2010, will be the first astronaut ever to fly on consecutive missions. He will begin training this week with the STS-133 crew, which includes commander Steve Lindsey, pilot Eric Boe, and mission specialists Alvin Drew, Michael Barratt and Nicole Stott. 

STS-133's 11-day mission to the International Space Station will deliver the Permanent Multipurpose Module--an external platform that holds large equipment and critical spare components for the station, as well as Robonaut 2-- the first human-like robot in space.

Last week, NASA also named a backup commander for Endeavour's STS-134 mission in April. The mission's regular commander, Mark Kelly, is caring for his wife, Arizona Rep. Gabrielle Giffords. 

STS-134 may be the final or second-to-last shuttle mission before NASA retires the orbiters this year.

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