NASA hopes to have the so-called Orbital Space Plane launched to the international space station by 2008, to serve at least initially as a lifeboat.
"NASA is proceeding with OSP development before we - the Congress, the White House and NASA - have reached any agreement ... on appropriate NASA goals for human space flight beyond the international space station," Reps. Sherwood Boehlert and Ralph Hall wrote to NASA Administrator Sean O'Keefe.
The letter was dated Oct. 21 and released publicly by the congressmen Monday.
Boehlert, R-N.Y., the committee chairman, and Hall, D-Texas, the ranking minority member, said it is too soon to know whether the Orbital Space Plane will significantly increase crew safety and support long-term space flight goals. They noted that NASA's budget request for fiscal 2004 is "clearly no longer credible."
The congressmen urged O'Keefe to defer the program until an inter-agency review is completed, approved by President Bush and evaluated by Congress.
"Without such consensus on a shared vision, progress on all of these steps will stall and public support for the nation's civilian space program will inevitably founder," they wrote.
NASA spokesman Robert Mirelson said O'Keefe will address the concerns and the project will be "within reasonable budget constraints and safety issues."
The space plane would be launched by unmanned rockets; NASA has yet to settle on a design.