Michael Griffin, NASA's new head, told a Senate subcommittee that the U.S. must work to not lose access to space when the shuttle retires in 2010.
He said the proposed new spacecraft should be developed as soon as possible to be able to use it once the space shuttle retires.
Otherwise the U.S. would be unable to launch astronauts for four years.
But he warns that speeding up development of a new spacecraft will mean something else will have to be sacrificed. Griffin says that covering the cost of the shuttle replacement's accelerated debut may mean delays in some space station and exploration research.