The head of the U.S. space agency Wednesday faced tough questions from U.S. lawmakers who demanded to know what NASA is doing to improve the shuttle program after the Columbia disaster earlier this year.

The Senate Commerce, Science and Transportation committee hearing focused on a report issued last month by investigators looking into the Columbia shuttle disaster last February.

The report said there was a complacency and a willingness at NASA to downplay safety warnings in order to keep a schedule. It said there was a certain culture at the space agency that contributed to the Columbia mishap.

When grilled by senators as to when such a culture would end, NASA administrator Sean O'Keefe said it would be a long process. "We have got to be on the long haul proceeding in that direction. But I fully anticipate that we will see the beginnings of that change within six months to a year, to be sure," he said.

The head of the independent Columbia Accident Investigation Board, Harold Gehman, told the panel that Mr. O'Keefe was accountable for the mishap. But he said the systemic problems that led to the disaster had been in place for more than a decade.

Mr. Gehman said political leaders, including those in Congress and the White House over the past 10 years, deserve some of the blame for cutting the shuttle's budget. "It costs much more to operate the shuttle than everybody would ever admit," he said. "Over the years, what has happened, for one reason or another, people have tried to wring money out of the shuttle program in order to pay for other projects."

Senator Sam Brownback, a Kansas Republican, wondered whether restoring the shuttle program would be worth the cost. "Are we throwing good money after bad? There is a fair feeling that this is an older technology, it is a complex technology. We may just be at a point that it is time to say, 'Scuttle the shuttle', and move on to the next technology," he said.

Mr. Brownback said the United States should reassess its space program.

Senator Byron Dorgan, a North Dakota Democrat, said he does not want the United States to abandon space exploration. "I believe a society that stops exploring stops progressing," he said. "Space exploration has been very important for this country. I want it to succeed. I want it to continue."

Senator Bill Nelson, a Florida Democrat who was a crew member on a shuttle flight in 1986 and a staunch supporter of the space program, urged Mr. O'Keefe to fight any administration attempts to cut NASA's budget. "I think everybody including members of this committee that want to see our space program continue to be robust and continue to fill that desire of this nation to explore needs to know that you are going to be fighting, in the internal fights of the administration, with OMB and the White House, to make sure the monies are there for NASA," he said.

Shuttle flights were suspended after Columbia broke up over Texas, killing all seven astronauts aboard. Mr. O'Keefe did not say when the shuttles would fly again.