At a briefing for reporters following an intensive - and apparently at times contentious - Flight Readiness Review, Griffin described the key safety issue that had been bedeviling NASA engineers: the persistent risk that more large pieces of insulating foam still might break off of the external tank during launch and damage Discovery sufficiently to endanger the orbiter and its crew during re-entry.

A piece of foam weighing approximately 1.5 pounds (0.7 kilograms) broke off during shuttle Columbia's launch in January 2003. Traveling at about 500 miles (800 kilometers) an hour, the foam punched a large hole in the leading edge of Columbia's left wing.

Two weeks later, the hole allowed superheated gases to enter the wing's internal structure during re-entry, eventually leading to the spacecraft's disintegration and the deaths of its seven crew members.

Despite that tragedy, and despite the lingering risks, Griffin said he was confident that NASA could protect the Discovery crew - even if a large piece of foam did break off and damage the orbiter.

"I do not see this situation as a crew-loss situation," he said. "We're not in the situation we were with Columbia where we did not know we had a problem."

Griffin explained that NASA engineers had made several critical improvements to the shuttle that should minimize the chances of any significant damage caused by the foam, but the agency has prepared for the worst. Additional and improved video cameras will inspect Discovery during and after launch to track for damage.

If necessary, the crew will take temporary refuge at their destination, the International Space Station, until shuttle Atlantis could be launched on a rescue mission. And, as a last resort, the Russians could send up a Soyuz spacecraft to bring the crew home.

Griffin described the potential foam problem as "a programmatic risk, not a crew-loss risk, and if we're going to fly, we're going to have to accept some programmatic risks."

Despite those risks, he said, NASA engineers insist that no further design changes to the foam can be made until more data are accumulated from the next shuttle flight.

Wayne Hale, the shuttle's program manager, supported Griffin's position. "If we find out significant information on this (next) flight, we'll plug it back into the (foam) design," he told reporters.

As adamant as Griffin was about getting Discovery back to flight, he also acknowledged that another catastrophic accident - such as Challenger in 1986 and Columbia in 2003 � would almost certainly mean the immediate end of the shuttle program.

"I think at that point, we're done," he said, in response to a question about another shuttle crew loss.

On the positive side, however, Hale said he thinks NASA has "a good shot at getting three flights off this year," assuming no major mechanical failures and no hurricanes strike central Florida in 2006.

Discovery's countdown currently is set to begin June 28. Following Discovery's launch in July, shuttle Atlantis is scheduled to lift off to the station in August. A third mission for the year involves an orbiter and launch window that are not yet determined.

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