The conclusions do not necessarily mean another delay for the mission of the Discovery, scheduled to launch as early as July 13. The agency is to decide on a launching date after a final "flight readiness review" later this week.
But the findings are an embarrassment for the National Aeronautics and Space
Administration after two years of work to correct the problems that led to the loss of the shuttle Columbia and its crew of seven. The Discovery's return-to-flight mission has long been delayed as the agency has struggled to deal with the potential for lethal damage from ice or foam debris at liftoff.
"This has been a much more tortuous recovery than people anticipated with the loss of Columbia," said Dr. Howard E. McCurdy of American University, a historian who has written extensively on management and technology issues at NASA. "We are at a crossroads with the shuttle. Do we feel reasonably confident that we understand it enough to go on and fly with the known risks?"
The members of the advisory panel, led by two former astronauts, emphasized that NASA had significantly improved safety in the two years since the disaster. They also said they were making no recommendation as to whether it was time for the shuttle to fly again.
"That's not our charter," said Richard O. Covey, a co-chairman of the panel.
Instead, Mr. Covey said, the panel will present its conclusions on Tuesday to NASA's new administrator, Michael Griffin. "He can use that as advice for his determination," Mr. Covey said.
In a statement, Dr. Griffin did not address the review panel's individual conclusions but said it had performed "a valuable public service" that would help the space agency make its final decision about returning to flight.
"A vigorous discussion of these complex issues can make us smarter," he said, adding that he expected "a vigorous debate" in the flight readiness review.
The advisory panel, appointed to measure NASA's progress, concluded that the agency had fallen short of meeting the final 3 of 15 return-to-flight recommendations.
Those recommendations included eliminating all debris from the shuttle's external tanks, completing a more precise analysis of the risks of debris to the shuttle, and ensuring the ability to repair damage in orbit. Progress has been made all around, but none of the three, which Mr. Covey called "the most technically demanding," have been fully met.
In previous meetings, the panel signed off on 12 other preflight conditions, like improved documentation of shuttle systems and better photography during liftoff and in orbit.
The accident investigation board made other recommendations for the long term, most notably that NASA fix what it called the "broken safety culture" that led it to ignore or reject warnings that the Columbia had been damaged on liftoff.
Members of the advisory panel said on Tuesday that NASA's inability to fulfill the final three preflight conditions flowed in part from the high ambitions of the accident board. Foam falling off the external tank caused the damage that doomed the Columbia, and NASA has taken many steps to reduce substantially the amount of foam and ice that could fall off. It has not, however, been able to eliminate all debris or the risk of catastrophic damage from it.
"Although the program has performed an extensive effort to reduce debris for return to flight," the panel said, "there is still the potential for foam and ice to cause damage to the orbiter that exceeds safe entry limits. However, this potential has been significantly reduced."
Col. James C. Adamson, a former astronaut and a member of the panel that evaluated the five repair techniques that would be tested in the Discovery mission, said that repairing the shuttle in orbit would be "a tremendous technical challenge," and that the space agency was unable to design, develop and fully test any repair methods.
"We found that NASA fell short of meeting that recommendation," Colonel Adamson said, "although they had put forth a yeoman's effort in coming up with all of the options that they could conceive of for repair."
He also noted that the part of the external fuel tank that had shed a 1.67-pound chunk of foam on the Columbia's liftoff had been redesigned.
"The proximate cause of the damage to Columbia has been eliminated without a doubt," he said, adding, "Over all, they've significantly reduced the risk."
Even members of the advisory panel seemed surprised that they had voted to find all three efforts inadequate; subgroups had recommended a conclusion that NASA had met the recommendations for all but the repair systems.
As they discussed the findings, members like Susan Morrisey Livingstone, a former under secretary of the Navy, pressed for a more restrictive interpretation. "There's a couple of areas here I think need to be recognized are still works in progress," Ms. Livingstone said.
Mr. Covey said the determination to deny the agency passing grades grew as the panel wrestled with the new debris data from NASA. "There was a growing feeling in my mind that we might go 0 for 3 when we got into deliberations," he said. Then he quickly corrected himself. "It's not 0 for 3," he said. "It's partials in there."
Asked whether he, as a former astronaut, would climb aboard the Discovery next month, he said, "I would not have a concern about flying."