Inspectors were ordered to feel for similar cracks on wing panels of other shuttles. They found none, but NASA now acknowledges its testing might have missed deterioration on shuttles like Columbia because of difficulties detecting such flaws without removing wing panels and cutting them apart.
"There is no technology right now to do effective, nondestructive testing," NASA Administrator Sean O'Keefe said in an interview with The Associated Press. "It's a conundrum, one we really have to get better at and have to really figure out."
But CBS News Space Consultant Bill Harwood says that may not be true.
"One of the things the board investigating the disaster has said they're going to do is recommend that NASA begin using machines that work sort of like cat scans, that actually show you the interior composition or health of these leading edge panels between flights, which would be able to detect a problem before launch," he said.
With visual inspections, "you can look at the outer surface and the inner surface, but you can't really see in-between, where these carbon composite layers are that provide the protection from heat during re-entry," said Harwood.
The National Aeronautics and Space Administration knew that visual inspections of wing panels were inadequate. An engineering study a year before the fleet inspection concluded that underlying damage to wing panels "can extend significantly beyond" anything seen on the outside.
One of the study's authors, Ignacio Norman, later participated in intense debates within NASA during Columbia's mission about whether it might return safely.
Columbia was NASA's oldest shuttle when it disintegrated above the earth Feb. 1, killing the crew. Investigators have focused on its left wing and the type of protective panels cited in the earlier safety review.
Documents reviewed by the AP showed that when the shuttle Discovery returned from space after its March 2001 mission, inspectors were alarmed to discover a 2-inch tear caused by corrosion on a left wing heat-resistant panel. They called the flaw "serious." Inspectors had looked at the same panels three hours before liftoff and found no such damage.
The damage was among the most significant following space missions in recent years, because damage to a wing's leading edge is considerably more likely to doom a shuttle than anywhere else. NASA requires immediate repairs when damage to the protective panels exceeds four-hundredths of one inch, about the thickness of a dime.
At the time, inspectors said Discovery's safety was not jeopardized. They concluded the damage was caused by small amounts of oxygen slowly penetrating the U-shaped panel's surface and weakening its outer coating of silicon carbide, a brittle material. The coating protects the leading edge of a shuttle's wings from temperatures that can climb to nearly 3,000 degrees during re-entry.
The engineers feared similar corrosion damage to wing panels on other shuttles, especially older ones. They speculated this corrosion appeared on Discovery because it had just returned from its 29th flight, a record number at the time. Columbia was on its 28th mission when it was destroyed.
It was a dramatic illustration of recurring corrosion problem that has frustrated NASA for years. In recent weeks, it has increasingly captured the attention of the board investigating the Columbia accident.
Citing reports since 1992 of pin-size holes especially on older wing panels, investigators have openly speculated whether similar corrosion may have fatally weakened a panel along Columbia's left wing. That is where a 2-pound chunk of insulating foam off the fuel tank smashed against it 81 seconds after liftoff.
NASA has said it closely studied the corrosion problem from 1995 to 1997. It ordered more inspections, began recoating panels after every 16 flights and placed new restrictions on how many flights each panel could make. None of the panels aboard Columbia had exceeded those limits.
Damage to Discovery was so worrisome that NASA additionally ordered inspectors to begin feeling for cracks along wing panels before every shuttle mission. Months before Columbia's breakup, it also began testing a new technique, thermography, to scan for cracks inside panels — with mixed results so far.
Harold Gehman Jr., a retired Navy admiral heading the investigation, has indicated the board soon will recommend that NASA improve testing to find flaws on older shuttles.
Safety inspectors routinely swarm over returning shuttles, looking for cracked insulating tiles or other damage. "This is now and has always been very serious," Michael Kostelnik, NASA's deputy associate administrator for the shuttle, told the AP.
The report of Discovery's wing damage, along with other internal documents about the mysterious corrosion suggest NASA had previously recognized yet another existing design problem with its shuttles that may have contributed to the deaths of Columbia's seven astronauts.
Other agency documents released previously showed that before the shuttle mission preceding Columbia's, NASA flagged as a major concern a loss of insulating foam in the same area on fuel tanks where investigators believe debris broke away and smashed against Columbia's left wing.
There have been at least four flights since 1983 where foam broke off that part of shuttle fuel tanks, most recently on a 10-day mission by Atlantis in October 2002. But NASA concluded damage from such breakaway foam was not a safety threat even though it considers any debris striking the shuttle unacceptable.
Collectively, these two flaws — breakaway tank foam and weakened wing panels — figure into the leading theory among board members investigating Columbia's destruction. But NASA apparently never tied together its reports on these problems to anticipate such a disaster.
NASA Administrator O'Keefe acknowledged that NASA has done a poor job studying trends to identify safety risks over decades of flights, a failure he said "has come screaming home to me" since Columbia's breakup.
The investigating board believes wing panel corrosion probably has been caused by a zinc-based paint primer leaching onto shuttles from the nearby launch tower amid rainstorms — the subject of NASA's studies in the late 1990s.
NASA has suggested that rainwater reacting with the wing panels over many months produces sodium carbonate, which aggressively corrodes the protective silicon carbide layer above 900 degrees.
The agency in early 2001 found corrosive white carbonate residue on Columbia, Atlantis and Endeavour and noted the three shuttles were "all exposed to an excessive amount of rain." More recent testing has focused back on the primer as the likely suspect.
Inspectors studying Discovery's wing in 2001 were convinced the damage would not endanger the shuttle even if a patch they had applied fell off during the next fiery re-entry, which they deemed a "remote possibility." They repaired the wing panel, rather than replace it, because there were not enough spare parts, according to the damage report.
Their conclusion: Discovery would survive and any future damage "would be readily apparent." Discovery safely completed its subsequent flight in August 2001, its last flight before Columbia was destroyed.
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.