NASA Administrator Michael D. Griffin predicted Friday that a newly appointed "tiger team" of experts will quickly discover why the space shuttle's external fuel tank loses potentially lethal chunks of insulation during launch, and did not rule out a shuttle flight before the end of the year.

Nevertheless, planners for the international space station formally requested that the shuttle Discovery extend its current mission by a day in order to transfer extra gear and supplies to the station, anticipating that a scheduled September flight would be postponed.

Griffin said that close inspection of Discovery did not turn up any significant damage, despite the loss of several chunks of foam insulation from the orbiter's external fuel tank during launch.

Discovery, carrying mission commander Eileen Collins and six other astronauts, arrived at the space station Thursday for an eight-day sojourn. The crew members spent Friday moving a nine-ton luggage capsule into the station, preparing for a spacewalk Saturday and inspecting the underside of the orbiter for damage.

Mission operations representative Phil Engelauf said at a Johnson Space Center news conference that Discovery pilot Jim Kelly and mission specialist Charlie Camarda used the shuttle's new 50-foot sensor boom to look at five nicks in the orbiter's underside heat shielding and two protruding felt "gap fillers" in the seam between two shielding panels.

None of the nicks was considered serious. John Shannon, the flight operations and integration manager, said flight controllers expect to clear Discovery late Saturday or early Sunday for landing at Kennedy Space Center.

Shannon said engineers counted only 25 "small dings" in Discovery's heat shielding -- compared with the 150 dings, including large scars, that shuttles have averaged over more than two decades in flight.

"Discovery is the cleanest bird we have ever seen," Griffin told reporters in a telephone news conference from his offices at NASA headquarters. "Almost everything we did to the external fuel tank worked."

The unalloyed success of Discovery's first few days in space contrasted markedly with the anxiety on Earth provoked by NASA's Wednesday announcement that it was grounding the shuttle fleet because the external tank had shed at least four unexpectedly large pieces of foam insulation during Discovery's launch Tuesday.

It was a 1.67-pound piece of foam that breached the shuttle Columbia's heat shielding in 2003, causing the orbiter to disintegrate on reentry and halting shuttle flights for 21/2 years. Tuesday's most worrisome piece of debris was a 0.9-pound chunk, which broke away from a thick foam ridge that runs along the side of the tank to protect electric cables and pressure lines from turbulence during launch.

Griffin said engineers gave this "protuberance air load ramp" a "huge amount of consideration" during the post-Columbia hiatus, discussing ways the ramp foam might be applied as well as possibly replacing it with something else or eliminating it.

"The decision was to leave it the way it was," Griffin said. "Obviously, that will not be the decision this time."

The ramp debate was also vetted and accepted by the NASA-appointed Return to Flight Task Force, assigned to review the agency's redesign work.

"We felt they had an appropriate rationale," Richard O. Covey, co-chairman of the task force and a former astronaut, said of the space agency in a telephone news conference. "Clearly, the rationale that we accepted was wrong, and the flight test [on Discovery] proved it."

Collins said in an in-flight interview early Friday that she was "very surprised" to learn that the tank had once again shed large pieces of debris: "In the end I'm disappointed this has happened, but it's something I believe we can fix."

Griffin agreed, suggesting "folks have overreacted a bit" to the decision to halt flights until the new tiger team solves the debris problem. He offered no timetable but insisted "we're not conceding" the possibility of launching a another shuttle mission before the end of the year. A two-week launch window opens in September, and there is also a four-day window in November.

Griffin also said he has no plans to alter a rigorous timetable imposed by President Bush's initiative to complete assembly of the space station by 2010, retire the shuttle and put humans back on the moon by 2020.

Still in play, he added, is a possible shuttle mission to service the Hubble Space Telescope. He said he will not revisit an earlier plan to send a robotic spacecraft to the orbiting observatory.

Despite Griffin's optimism, Engelauf told reporters that mission planners had been asked to put together a package of tasks and activities that Discovery's crew might undertake if the September flight is postponed.

Engelauf said Discovery's crew is doing its best to conserve electricity and onboard fuel so it could add another day to the mission to accomplish extra duties. He said the mission began with eight hours of "margin" and now has 21 hours.

Engelauf said the astronauts -- nine strong, counting space station mission commander Sergei Krikalev and flight engineer John Phillips -- would spend the extra day doing tasks that required more than two people, as well as deferred maintenance and anything else on the September shuttle's agenda that could be accomplished now.

The astronauts will also pillage Discovery for anything the space station can use, including specialty tools, spare laptop computers and consumables of all kinds. Engelauf also said it is important for Discovery to carry as much trash as possible back to Earth.

Soichi Noguchi of Japan floats through the space station's Destiny laboratory.