WASHINGTON (AFP) -- The United States risks "loss of leadership" in space exploration, if it fails to replace quickly its ailing shuttle fleet with a new reliable space vehicle, the head of NASA has warned.

And that will not be possible without sacrificing some valuable science programs, Michael Griffin, head of the National Aeronautics and Space Administration, told Congress.

The warning came as the House of Representatives Committee on Science pondered options in the face of the continued grounding of the space shuttle after a chunk of insulating foam fell off Discoverys external fuel tank during lift-off last July.

The spacecraft completed its mission without 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 hitting the shuttle Columbia's protective thermal tiles is blamed for the 2003 fiery demise of the craft and its seven-member crew.

The shuttle fleet is now slated for retirement in 2010 and replacement by a new-generation spacecraft called the Crew Exploration Vehicle.

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 is 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 U.S. space agency is facing what some lawmakers characterized as a "lean year" following President George W. Bush's 2007 budget proposal that calls for allocating less than 16.8 billion dollars for its needs.

The White House intends to spend 72.4 billion dollars on military operations in Iraq and Afghanistan this year.

Griffin said now "NASA simply cannot afford to do everything" and will have to sacrifice a scientific mission to Jupiter's moon Europa, several endeavors in the area of space astrophysics as well as projects it had planned to undertake aboard the International Space Station.