Michael Griffin, head of the National Aeronautics and Space Administration, told Congress on Thursday that such a move would not be possible without in turn sacrificing some valuable science programmes.

The warning came as the House of Representatives Committee on Science pondered options in the face of the continued grounding of the space shuttle fleet.

Nasa put an indefinite halt on shuttle launches after a chunk of insulating foam fell off Discovery's large external fuel tank during lift-off last July.

The spacecraft completed its mission without a problem. But the space agency has grounded the whole fleet until engineers can figure out how to avoid such problems in the future.

A large piece of foam separating from the external tank and puncturing the protective thermal tiles on the shuttle Columbia was blamed for the fiery demise of the craft and its seven-member crew during re-entry in early 2003. 

Lean year

The US space agency is facing a lean year in space research

But contracts for the vehicle are not even expected to be awarded until the second half of this year, and Griffin said that if the new spacecraft was not operational in 2014, loss of critical expertise could occur.

"A longer gap in US human spaceflight capabilities will increase risk and overall costs and lead to even more delays," he told the committee.

"In addition," he added, "the US may risk a perceived, if not real, loss of leadership in space exploration if we are unable to launch our astronauts into space for an extended period when other nations are establishing or building on their own abilities to do so."

The US space agency is facing what some lawmakers characterized as "a lean year" as President George Bush's fiscal 2007 budget proposal calls for allocating for its needs under $16.8 billion.

Griffin said now "NASA simply cannot afford to do everything" and would have to sacrifice a scientific mission to Europa, Jupiter's moon, as well as several endeavors in the area of space astrophysics.