The conclusion by NASA engineers during Columbia's flight that the spacecraft probably did not suffer serious damage from debris during its Jan. 16 launch was based on two key assumptions that no one could confirm as true, according to details in a pair of reports produced for NASA by Boeing Co. engineers while Columbia was in space. The reports are cited by The Washington Post in its Saturday editions.
Separately, The Associated Press reports that, two years before the Columbia disaster, NASA safety experts fearing similar damage to delicate heat tiles on the space shuttle Atlantis decided it was "prudent" to adjust its return path to lessen danger during the fiery descent, according to internal documents.
NASA has maintained since last week's disaster that there was no recourse after liftoff for the seven astronauts aboard Columbia if there was serious damage to those tiles. But the Atlantis mission, in which the crew was instructed to make a protective re-entry, suggests there were at least options for an ailing shuttle to return safely.
Engineers similarly feared damage to tiles on Columbia's left wing, but they did not instruct Columbia to perform such a protective maneuver.
The Post also reported Saturday that NASA plans to retest some of the exotic materials used to insulate Columbia from the searing heat of re-entry as it continues to investigate whether damage to the space shuttle's left wing contributed to its breakup.
Ames Research Center in California's Silicon Valley will simulate the pressure, aerodynamic stress and heat of re-entry on pieces of reinforced carbon-carbon ceramic, the newspaper says. Engineers hope to determine the effect re-entry might have had on damaged portions of the material that sheathed areas of Columbia's nose and wings, the Post adds.
Meanwhile, critics are suggesting that the NASA-appointed commission conducting an independent investigation into the Columbia accident is too closely linked to the space agency and has too little time to do its work, leading the critics to wonder whether the public can trust the findings.
CBS News correspondent Jerry Bowen reports that a section of Columbia's underside with burned and missing thermal tiles was recovered in East Texas Saturday.
Investigators are considering every possible scenario, from the impact of the large chunk that hit the shuttle seconds after liftoff Jan. 16, to a deadly bull-eye's strike by a piece of space junk, to a lightning-like electrical phenomenon in the upper atmosphere.
In New Mexico, a tracking station that recorded the final seconds of the space shuttle Columbia has only 1 second of data that wasn't recorded elsewhere, but it could be a very important second, the station's director said.
A three-day amnesty period in Texas to turn over looted shuttle debris brought in hundreds of pieces of the shattered Columbia ended Friday, but officials said Saturday that people are still illegally hoarding the debris and at least nine could soon face charges.
And there is also word that studies performed by NASA engineers during the 1990s raised the possibility that tiny pinholes on the space shuttle orbiters' wings could be enlarged by hot gasses during re-entry, but concluded that the problem was unlikely to endanger the spacecraft or their crews.
Yet with increasing focus on the leading edge of Columbia's left wing in the days since the spacecraft broke up over Texas, interest in the pinholes may be renewed.
NASA officials declined to say Thursday whether they considered pinholes a possible cause of the Columbia accident. Engineers who have studied the pinholes for NASA and its contractors said they could not discuss the shuttle.
In Texas, a 2-foot-long section of one of Columbia's wings found near Fort Worth was described as the most significant discovery yet in the search for clues to the shuttle's destruction.
The wing section was recovered at the far western edge of the known debris field, which stretches more than 200 miles from Fort Worth across East Texas and Louisiana.
It wasn't immediately known if it was part of the important left wing, where several heat sensors failed in the shuttle's final moments, but that should be known "in relatively short order," NASA Administrator Sean O'Keefe said Saturday after a memorial service at Louisiana's Barksdale Air Force Base, where pieces of the shuttle are being stored.
"Examining where debris fell and where it was gathered is going to be very important as far as piecing the puzzle together as far as what happened at what altitude," shuttle program manager Ron Dittemore said Friday.
The two assumptions in the Boeing reports -- that the shuttle was hit by a piece of insulating foam and not by ice, and that the leading edge of the wing was not damaged by the impact -- remain unproved, the Post says. Indeed, NASA has explicitly acknowledged that ice damage has not been ruled out and, according to one of the Boeing reports, there was "large uncertainty" as to where the debris hit the spacecraft.
The Boeing analyses obtained by the Post indicate that if either assumption was wrong, the flight could well be in serious jeopardy. Yet the second of the two reports -- the only one to make an overall safety prediction -- concludes with the reassuring words: "safe return indicated."
Paul Fischbeck, a professor at Carnegie Mellon University and coauthor of a NASA-funded report on launch debris and critical mishaps, told the Post the reports left him shaken when he viewed them.
"This says that if only a single tile is lost, you're already on the edge," under the conditions scientists think existed during the debris hit, he said. "And it also shows that, in fact, multiple tile damage is likely. Looking at this did not exactly make my day."
"This looks like a case of people trying to fool themselves into not being worried," said a former shuttle engineer who spoke to the Post on condition of anonymity after looking at the report.
Had the debris that hit the wing 80 seconds into flight been not foam but ice, one of the Boeing reports indicates, it could easily have breached the structural integrity of the wing -- even the super hard reinforced carbon-carbon material used in the shuttle's most heat-exposed areas. That would leave the wing unprotected from the nearly 3,000-degree Fahrenheit temperatures reached during reentry.
And even if the debris were a piece of foam, it might have inflicted serious damage and undermined flight safety if the impact were just a few inches further forward than the engineers assumed, said two independent engineers and a former NASA flight controller who looked at the reports for The Post.
All told, the reports, portions of which have been reported by Florida newspapers, suggest NASA engineers may not have considered a wide enough array of possibilities as they decided whether the crew members were at risk, the experts told the Post. But recognizing that it is always easier to find fault after the ending is known, they added, the reports show at a minimum how difficult it was for engineers to calculate the degree of risk for Columbia and her crew.
NASA confirmed Friday it received photos of Columbia from a powerful Air Force ground camera, but Dittemore - who showed one of the images to reporters - said it wasn't clear whether it showed structural damage to Columbia's left wing.
"All by itself, I don't think it's very revealing," he said.
But CBS News space consultant William Harwood says, "Whatever's happening on the left wing when that picture was taken was very serious, and getting rapidly much more serious, enough so that the spacecraft ultimately broke up."
The shuttle was 16 minutes from a landing in Florida when it broke apart over the Southwest, killing all seven astronauts. Dittemore said Friday that no shuttle debris had yet been confirmed west of Texas.
Michael Kostelnik, a deputy associate NASA administrator, said NASA believes there is "a substantial amount of material" around Fort Worth and up to 150 miles west and northwest of the city.
In all, more than 12,000 individual pieces of debris have been found and their locations cataloged, officials said.
The pieces could arrive as early as Tuesday at Kennedy Space Center in Florida, where they will be laid out in a storage hangar on a grid marking off the entire shuttle, NASA spokeswoman Lisa Malone said.
The critics of the panel named by NASA cite the perception of conflicts of interests - a NASA official is a commissioner, a NASA engineer is the executive secretary, the board is relying on NASA staff and using NASA facilities.
The commission has 60 days to complete its mission, a timeframe perhaps insufficient for a thorough probe, says one lawmaker who notes the presidential commission investigating the shuttle Challenger explosion got 120 days.
Acknowledging congressional skepticism, NASA's O'Keefe has changed the commission's charter and may add more members.
Despite those moves, "I'm still concerned that the credibility and the independence of the commission can be challenged," Tennessee Rep. Bart Gordon, ranking Democrat on the House Science space and aeronautics subcommittee, said Saturday.
Some members of Congress want a presidential commission to investigate Feb. 1 shuttle Columbia disaster, much like the one President Reagan sent to look into the Challenger's explosion 17 years ago. Only a few days into that investigation, NASA was accused of secrecy and cover-ups.
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.