The head of the U.S. Space Agency has conceded that there is no guarantee that the troubled space-shuttle program will launch another orbiter. NASA Administrator Michael Griffin spoke as crewmembers of the Space Shuttle Discovery expressed consternation that, despite exhaustive repair efforts, a piece of foam insulation once again broke away from an external fuel tank during last week's launch.

NASA is facing more questions for which it has few concrete answers. Days after the first shuttle launch in more than two years, Administrator Michael Griffin said NASA has no choice but to consider the possibility that further construction of the International Space Station will have to proceed without U.S. orbiters ferrying personnel, equipment, and supplies.

Mr. Griffin spoke on NBC's Meet the Press program.

"If we cannot fix the foam [problem], then we will have to figure out a different path to sustain the space station until we can get a new vehicle. As a matter of fact, we are currently doing design studies on just such things right now," he said.

The Bush Administration has called for accelerated development of a successor vehicle to the three-decade-old space shuttle, with a target date of 2010. But NASA officials say they had hoped for some 20 shuttle missions between now and then for a variety of missions, from space station construction to fixing the Hubble Telescope.

But that ambitious launch schedule has been thrown into doubt. It was foam debris that damaged the Space Shuttle Columbia and was blamed for the break-up of the vehicle upon its ill-fated return from orbit in 2003.

Administrator Griffin said last week's Discovery launch showed that, more than two years after the Columbia disaster, the foam issue has not been solved. He indicated that errors had been made or,

"We goofed [made a mistake] on that one," as he put it. "Certainly we were lucky. If it [the foam section] had broken off earlier and if it had followed a different trajectory, it could have hit the orbiter, as any piece of foam could, and could have done some damage."

Mr. Griffin said he has no reason to believe that Discovery is unsafe to return to earth. Crewmembers aboard the space shuttle have expressed similar confidence that Discovery is in good shape. But, in an interview on ABC's This Week program, pilot James Kelly said he was surprised and disappointed to learn that foam had peeled off the external fuel tank.

"The area where to foam came off is an area that was not examined or decisions were made not to look at it and not to test the foam there. I think we do need to address why that decision was made," said Mr. Kelly.

Despite problems and setbacks, former astronaut John Glenn said space exploration remains a worthy endeavor, and that a certain amount of risk is unavoidable, especially on space shuttles comprised of more than two million parts.

"It [the Space Shuttle] is the most complex machine ever built," said Mr. Glenn. "Any time you venture out of bed in the morning, you take some risk. I think there will always be risk, whether you drive an automobile or a spacecraft. I think it is worth the risk, because what we are trying to do with the shuttle is to complete the International Space Station. And that will enable us to do the research it was built to do in the first place - medicines and pharmaceuticals and things like that that are a benefit to people right here on earth."

Discovery is scheduled to leave orbit on Monday, August 8, one day later than originally planned. NASA says that, if any safety issues should arise concerning Discovery's return to Earth, the crew has the option of going aboard the International Space Station until a solution is found or another vehicle is launched.