WASHINGTON (AP) - Superheated gases penetrated the left wing of the space shuttle Atlantis during its fiery return to Earth nearly three years before a larger breach in nearly the same location is believed to have doomed the shuttle Columbia and its seven astronauts, internal NASA documents show.

Atlantis suffered no irreparable damage after the May 2000 episode, and, after repairs, it returned to flight four months later. But after the incident, the National Aeronautics and Space Administration ordered fleetwide changes in how employees install protective wing panels and sealants.

The small leak through a seam in Atlantis' wing during its return from the International Space Station was disclosed in documents sought by The Associated Press under the Freedom of Information Act. NASA blamed the breach on an improperly installed seal that became dislodged, opening a quarter-inch gap between insulating materials.

Some experts expressed surprise that hot gases generated by the heat of re-entry, which can reach 3,000 degrees, ever had leaked inside a shuttle's wing. Although protective wing panels have been found damaged, even cracked, the Columbia disaster was widely believed outside NASA to have been the first such breach.

"Very little information about the flaws of the tile system ever make it into the open literature, so those of us who work on this ... seldom hear much about serious problems such as this one," said Steven P. Schneider, an associate professor at Purdue University's Aerospace Sciences Lab. "I've never heard this sort of leak occurred."

NASA said it later determined Atlantis' exterior wing panels were not damaged by the overheating despite being discolored from the high temperatures. Aluminum structures inside the wing "looked outstanding," NASA said.

Other parts immediately behind the wing panels were covered with a glassy material, apparently from melted insulating tile and other sealant material.

NASA spokesman James Hartsfield said all damaged parts were replaced.

The space agency formally reported the damage to its Program Requirements Control Board, an internal safety oversight body, which ordered fleetwide improvements in the installation of sealant materials before Atlantis was allowed to launch for its mission in September 2000. Atlantis is expected to be the next shuttle into space when NASA is cleared to resume flights.

The superheated gases never penetrated deep within Atlantis' wing, and a spokesman for the Columbia Accident Investigation Board, Woody Woodyard, said there was no serious threat to the crew or the spacecraft.

NASA blamed the Atlantis damage on improper installation of a seal between two protective panels on the shuttle's left wing, "called a butterfly gap filler," at the Boeing Co. plant in Palmdale, Calif., during an overhaul of Atlantis in late 1997. The mistake went unnoticed during subsequent inspections because the part could not be seen without removing protective panels, NASA said.

Engineers found the damage on Atlantis while investigating the mystery of a partially melted insulating tile. Removing two protective wing panels nearby and peering inside the wing structure, they determined the dislodged seal had created "a substantial flow path," according to NASA's internal reports. The gap measured just over one-quarter inch, about the width of a paperclip or a No. 2 pencil.

The protective panels, insulators and other hardware inside the left wing "shows various signs of overheating," NASA reported. Photographs showed charred and scorched components, including parts made from titanium and inconel, two of the most heat-resistant materials on the shuttle. Titanium melts about 3,000 degrees; inconel melts about 2,550 degrees.

The board investigating Columbia's Feb. 1 breakup determined that superheated gases penetrated protective wing panels that had been loosened by insulating foam that broke off its external fuel tank on liftoff and smashed against the shuttle. Investigators believe searing re-entry temperatures melted key structures inside until Columbia tumbled out of control and broke apart at close to 13,000 miles per hour, killing its seven astronauts.

Investigators remain uncertain over the size of the gap that permitted hot gases to penetrate Columbia's wing. But they believe it was as small as a one-inch slit running vertically up the wing for nearly 30 inches. In a test Monday, a chunk of foam blew open a dramatic 16-inch hole in parts of a mock-up of a shuttle wing.

Temperatures during a shuttle's return can climb to almost 3,000 degrees - nearly one-third as hot as the surface of the sun - along parts of the spacecraft, especially the leading edges of its wings. Damage there is considerably more likely to doom a shuttle than anywhere else. NASA requires immediate repairs when damage to the wing's protective panels exceeds four-hundredths of an inch, about the thickness of a dime.

Halsell said through a NASA spokesman, Allard Beutel, that the problem on Atlantis probably wasn't raised with the crew because it could be fixed so easily and because officials didn't assign particular significance to the mild damage inside the wing.

Col. Susan J. Helms, who has since left NASA to return to the Air Force, said she wasn't told about the breach but that she missed post-flight briefings because she was training in Russia for duties on the space station.

"Usually the crew will receive a rundown of all anomalies generated during the shuttle flight and should have heard about this," Helms said.

Both Helms and Mary Ellen Weber said they didn't know if other crew members were told.

"There are thousands and thousands of things that can go wrong, and the crew is very much aware this can happen," said Weber, who operated the robotic arm on Atlantis. "Certainly, when you learn about this, if it had progressed, it could have been much more dire."

Weber, now an associate vice president at the University of Texas Southwestern Medical Center in Dallas, described Atlantis' return to Earth as mostly routine and remembered seeing an orange glow from hot gases dancing outside the shuttle windows.

Although damage inside Atlantis' left wing was detected post-flight, NASA worried about the shuttle's return even before the discovery.

During liftoff, a 6-inch chunk of ice had smashed against the back edge of the right wing; so experts deemed it prudent to adjust Atlantis' flight to rapidly cool its wings prior to the fiery trip through the atmosphere, NASA documents showed.

It was impossible to know whether this cooling technique, called a thermal conditioning maneuver, also helped minimize heat damage inside Atlantis' defective left wing. NASA later determined damage on the right wing was relatively minor.

NASA did not consider ordering the thermal conditioning maneuver on Columbia because it believed such a move would have interfered with efforts to warm Columbia's landing gear tires for a safe landing.

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