Discovery's recent mission in space was prolonged by efforts to ensure the shuttle could return to Earth.
Two NASA engineering teams report on the effort to solve the space shuttle's safety problems. NASA has said the shuttle will not fly again until the agency can be sure that big chunks of foam won't fall off the orbiter's fuel tank during liftoff.
Falling foam doomed the shuttle Columbia in 2003, and despite a $200 million effort to fix the problem, at least one large chunk also came off Discovery several weeks ago.
NASA officials now say that so far the agency's engineers have found no easy answers to the problem. As a result, a scheduled September launch will likely face delays.