KENNEDY SPACE CENTER, Fla.--Less than six weeks before launch, astronaut Timothy Kopra, injured in a bicycle accident Saturday, was removed from the crew of the shuttle Discovery today and replaced by astronaut Stephen Bowen, a veteran spacewalker who flew aboard the shuttle Atlantis last May. Despite the last-minute crew change, NASA officials say Bowen should be able to complete a hurried round of refresher training in time for blastoff around February 24.

"As anybody would be this close to flight, [Kopra]'s disappointed," chief astronaut Peggy Whitson told reporters. "His crew made very tight bonds and so they are all very supportive of Tim...I think Steve will fit in well with this crew. He obviously doesn't have time to make a huge impact, he's going to just try to fill Tim's shoes."

Astronaut Timothy Kopra aboard the shuttle Endeavour in 2009.

Kopra was named to the all-veteran crew of shuttle mission STS-133 in September 2009, along with commander Steven Lindsey, pilot Eric Boe, Nicole Stott, Michael Barratt, and Alvin Drew. Kopra was assigned two key roles, serving as the flight engineer during launch and re-entry and the lead spacewalker for a pair of excursions with Drew.

Bowen, who completed two spacewalks last May during the most recent shuttle mission, is a Naval Academy graduate and veteran submariner with two previous shuttle flights to his credit. He will replace Kopra for the two spacewalks planned for Discovery's flight.

"Bowen is a very experienced EVA-er, five previous spacewalks and very capable in terms of qualifications in [NASA's spacesuit]. We felt with a very few additional [training] runs, he could pick up the timeline Tim and Al had worked out together previously and be able to pull them off with only an additional two runs in the [Neutral Buoyancy Laboratory training pool] for each of those two activities."

Kopra also served as Discovery's flight engineer, playing a critical role during ascent and entry. The flight engineer sits directly behind the shuttle commander and pilot, helping them monitor critical instruments, and double-checking procedures, both for normal ascents and entries as well as emergency aborts.

Whitson said there was not enough time to train Bowen for flight engineer duties in addition to the spacewalks and still make the next shuttle launch window. Instead, Drew will serve as flight engineer during ascent and Stott will take over for entry.

Astronaut Stephen Bowen aboard the shuttle Atlantis last May.

Citing medical privacy issues, NASA officials will not discuss the extent of Kopra's injuries or provide details on what happened other than to say he was injured Saturday in a bicycle mishap near his home in Houston. Multiple sources have said he broke his hip, but Kopra has not yet made any public comment.

With only two--and possibly three--shuttle missions remaining before the fleet is retired, the bike accident marked a difficult personal setback for Kopra. In years past, an astronaut who had to miss a flight for medical reasons could look forward to being worked back into the crew rotation for launch on a future flight.

But there are no available seats on the remaining shuttle flights. Whitson said NASA is holding open the possibility of returning Kopra to Discovery's crew if the mission encounters a major delay, giving the astronaut time to recover, but that appears to be a long shot.

She said Kopra, who is recovering at home, plans to help his crew as much as possible during training and to be ready if he gets another chance. If Discovery stays on schedule, Kopra presumably would be available for launch to the space station aboard a future Russian Soyuz rocket, although station crews for the next few years are already set.

Astronauts with flight assignments have been replaced in the past for a variety of reasons, but NASA has never had to replace a shuttle crew member this close to launch.

Astronaut David Griggs, scheduled to serve as pilot of shuttle mission STS-33, was replaced after he was killed June 17, 1989, flying a vintage aircraft. Two shuttle commanders--David Walker and Robert "Hoot" Gibson--were removed from flights in 1990 for disciplinary reasons after they violated NASA flight rules in unrelated incidents.

Jeff Ashby was replaced as the pilot of STS-85 in 1997 because of a family illness and Mark Lee was replaced on the STS-98 crew in 2000 for undisclosed reasons.

Gus Loria and Carlos Noriega also were replaced in 2002 and 2004 respectively because of undisclosed medical issues and Karen Nyberg was replaced on the crew of STS-132 in 2009 because of a temporary medical condition.

Just last week, NASA named a backup commander to train in place of astronaut Mark Kelly, the STS-134 commander. Kelly's wife, Rep. Gabrielle Giffords, was critically wounded in a Tucson shooting spree January 8 and it's not yet known if he will be able to carry out his assignment aboard Endeavour, scheduled for launch April 19.

NASA's most famous crew replacement occurred in 1970 when Ken Mattingly, command module pilot for the Apollo 13 moon mission, was exposed to measles. He was replaced by Jack Swigert just three days before launch.

NASA trained backup crews for Mercury, Gemini, and Apollo missions, as well as the first four shuttle missions, which were launched with two-man crews. But after that, with crews of six or seven astronauts, it was no longer practical to train backup shuttle crews and the practice came to an end.