The U.S. space agency NASA is delaying the next space shuttle mission until at least March. It wants to give engineers more time to remove the threat of launch debris striking the orbiter. The decision follows a rebuke of the agency by some members of an oversight panel who accuse it of imposing launch pressures of the kind that led to the loss of the orbiter Columbia in 2003.

NASA says it will not send another shuttle this year to the International Space Station (ISS). It had hoped to launch Atlantis in September, but put that mission off last week until November. Now, the further flight delay will give technicians even more time to understand why a piece of hard insulation foam broke away from the external fuel tank during Discovery's July launch and to fix the problem.

It was such a chunk of foam that doomed Columbia and its seven-member crew by puncturing its wing during liftoff. NASA spent two-and-a half-years and millions of dollars to prevent another occurrence, and the July incident forced the agency to ground the shuttle fleet again until it could resolve the issue once and for all.

As a result, NASA will return three external fuel tanks to the manufacturer for analysis and redesign. Agency human space flight chief William Gerstenmaier says this means a shuttle cannot lift off before March, but emphasizes that March is only a planning target.

"We shouldn't be thinking of the launch date as March," said Mr. Gerstenmaier.  "We put a planning target out there and now we're going to do a detailed engineering assessment and analysis and start seeing if that makes sense. Then when a couple weeks have gone by and we've got enough intelligence together, then we can pick a launch date."

On Wednesday, NASA was accused of rushing Discovery to launch in July, skipping safety improvements that led to the repeat foam shedding. The accusation came from seven members of a much larger group of experts who had overseen NASA's shuttle safety modifications after the Columbia tragedy.

The full panel had concluded in June that Discovery was safe enough to fly, even though the space agency had not completed three of 15 alterations deemed important for a safe mission, including fixing the foam problem.

But the minority group's report this week argues that NASA put too much emphasis on meeting an unrealistic launch date for Discovery and that it did not learn the lessons of the Columbia tragedy.

NASA chief Michael Griffin says he welcomes those views, but defends the agency. He points out that it made huge improvements in shuttle safety and much less foam shed during Discovery's launch than during Columbia's.

"We've worked hard at NASA over the last two-and-a-half years to improve that situation that led to the loss of Columbia, but we don't suppose that we're done," said Mr. Griffin.  "One of the reasons that I was very receptive to the minority report was because we can't get done unless we're willing to all of the hard truths."

Mr. Griffin, who has been on the job four months, says he is trying to instill conservatism in the launch process and give engineers time to evaluate the foam issue.