NASA said Friday that fixing the space shuttle's external fuel tank will delay the liftoff of the shuttle Atlantis until the final days of the September launch window and, perhaps, beyond.

William H. Gerstenmaier, space station program manager, said engineers have decided to target Sept. 22 as the earliest Atlantis launch date, with four attempts possible before the window closes five days later.

Atlantis was at the Kennedy Space Center preparing for launch in case the shuttle Discovery, docked at the space station, suffered damage grave enough to require that it be abandoned.

But NASA "stood down" the emergency launch Thursday after clearing Discovery for reentry. Discovery was set to undock Saturday in preparation for a landing at Kennedy early Monday morning.

The launch window for Atlantis's regular mission will open Sept. 9, but the chances of making that date dimmed abruptly when Discovery's external tank shed four large pieces of foam insulation during its July 26 launch.

Discovery emerged relatively unscathed, but NASA grounded the fleet until the cause of the foam loss is found and fixed. Foam debris damaged the shuttle Columbia, which disintegrated over Texas 21/2 years ago, killing all seven astronauts aboard.

NASA Administrator Michael D. Griffin has predicted a "quick turnaround" in the foam investigation, and he said the agency is not ruling out an Atlantis launch before the end of the year.

"Until we know where we are, I would rather be optimistic and try for success," Griffin said at a Johnson Space Center news conference Friday.

Gerstenmaier, named by Griffin to oversee the foam investigation, said he has dispatched an engineering team to NASA's Michoud Assembly Facility in Louisiana, where the external fuel tanks are made. There they were conferring with a shuttle program team and a third team from Lockheed Martin Corp., the tanks' manufacturer.

Although the external tank lost four unacceptably large pieces of foam, the early focus is on a one-pound chunk that broke away from a ridge designed to protect cables and pressure lines from turbulence during launch.

"The first step is to take a look at the data from this flight," Gerstenmaier said. "Then we'll look at future tanks." Only then, he said, will investigators decide whether to remove Atlantis's tank and modify it, or fly it as is.

On Friday, commander Eileen Collins and the rest of her seven-member Discovery crew spent their last day at the space station before undocking for reentry preparations.

Friday's most difficult job was to re-stow the Italian-built storage bin Rafaello aboard the shuttle. During nine days at the station, astronauts unloaded the bin and other cargo space, delivering 6,900 pounds of parts, equipment and supplies. The shuttle crew will bring home 6,300 pounds of worn-out gear and trash.

Later, House Majority Leader Tom DeLay (R-Tex.), who represents the area where the Johnson Space Center is located, and Sen. Kay Bailey Hutchison (R-Tex.), chairman of the Senate Commerce subcommittee on science and space, toured Mission Control and wished the astronauts a happy landing.

"I had a wonderful visit with your husband, and he was complaining mightily that he had to get the school supplies," Hutchison told Collins. Collins and husband Pat Youngs have two young children.

"Looks like I might have some cleaning up to do when I get home," Collins replied.

The Rafaello module carried supplies for the space station. It will hold trash during Discovery's return to Earth.