Program manager John Shannon said Tuesday it costs $200 million a month to maintain the fleet flying.
Right now, the space shuttles are believed to retire this fall. Four extra missions are planned. Some in Congress, though, are pushing for more flights.
Last month, President Barack Obama killed NASA's Constellation program, which would have formed a shuttle successor.
Shannon says NASA by now has a fuel tank and set of boosters for one additional flight. He says getting other shuttle parts would not be a trouble.
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