space agency was warned in 1990 that the protective tiles around
the shuttle's wheel wells were particularly vulnerable to damage
and failure, inviting catastrophe because those tiles protect
both fuel tanks and the shuttle's hydraulic system.
The study, conducted by experts at Stanford University and
Carnegie Mellon and financed by NASA, also identified ice that
builds up on the supercold external fuel tank as a major source
of debris that could fall on the tiles and trigger a cascade of
failures that could doom the spacecraft.
These two observations fit the leading theory emerging as
investigators try to discover what destroyed the shuttle
Columbia. If this theory is correct, it would mean that
accidents involving two vulnerable areas — the foam and the
tiles — combined to destroy the craft.
NASA officials said yesterday that they still believed that
the object that hit the underside of the wing on takeoff was
foam insulation, but there is growing speculation that it may
have been mixed with ice. Video images taken about 80 seconds
into the flight show the object to be white or light — the
insulation itself is bright orange — fueling speculation that
NASA engineers may have seriously underestimated its weight when
they concluded that a blow from a block of rigid foam would pose
no safety hazard to the orbiter. The insulation is applied as a
shaving cream-like foam, but turns hard as a brick.
In another indication that NASA is focusing on the likelihood
that foam may have broken loose from an external fuel tank and
damaged the spacecraft, a team of investigators spent a second
day yesterday examining records concerning the construction of
the tank at the agency's Michoud Assembly Facility in New
Orleans. Parts of the factory were shut down on Monday and
records have been secured by a team of about 20 investigators
"The investigative team is on site," said Marion LaNasa,
communications director for Lockheed Martin Space Systems, which
manufactures the tanks at the site. "They are directing the
NASA officials also confirmed yesterday that the Columbia had
sat on its launching pad for 39 days, more than two weeks longer
than usual. That means the Columbia was on the pad for 23 days
in December, when the Cape Canaveral area received four times
the usual amount of rain.
The unusual weather conditions mean the foam insulation
around the 15-story external tank was drenched. If that water
soaked into insulation or cracks around it, it could have
created significant ice when the tank was filled with supercold
hydrogen and liquid oxygen before the launching, experts said
The study by researchers at Stanford and Carnegie Mellon was
particularly significant because it determined that the two
wheel areas were especially vulnerable to damage. They were
often hit by flying debris, became unusually hot during the
shuttle's fiery re-entry through the atmosphere and covered some
of the most vital — and volatile — sections of the orbiter. Its
helium, oxygen and hydrogen are stored there in pressurized
tanks, any of which might explode and rupture the winged
If even a single tile were lost in this area, said an update
on the 1990 study, a "zipper effect" could occur, stripping away
other tiles. The study noted that once several tiles were gone,
the resulting heat "could cause a burn through in the aluminum
skin of the orbiter during re-entry, exposing and possible
crippling some of the critical subsystems and leading to the
loss of the vehicle and crew."
Yesterday, one of the authors of the study, Dr. Elisabeth
Paté-Cornell, a researcher at Stanford, said NASA headquarters
called two days ago to get a copy of its original 1990 study. "I
don't blame them," she said of the space agency's inability to
find the study. "I'm the same way."
But the research does not appear to be widely understood in
NASA. Yesterday afternoon, Maj. Gen. Michael C. Kostelnik, the
deputy associate administrator for the space station and the
shuttle — who oversees safety issues — was asked whether the
tiles around the wheel wells were considered a particular safety
"Not really that I'm aware of," he said. He described the
protective tiles, which shed heat as the orbiter re-enters the
atmosphere, as "a very robust system."
General Kostelnik joined the agency only last year.
General Kostelnik told reporters today that the debris that
hit the shuttle was larger than any seen before. But he said
that it was "in the family" of previous stray pieces of
insulation and that the agency did not believe there was any
reason to be concerned about it.
The two researchers, Dr. Paté-Cornell and Dr. Paul S.
Fischbeck of Carnegie Mellon, analyzed the orbiter's 25,000
tiles and identified which were the most likely to fail
catastrophically. They did their study for NASA in 1990 and
published three academic papers on their work in 1993 and 1994.
Dr. Fischbeck was dismayed about NASA's sudden request for a
copy of the study the agency itself had sought and paid for.
"That was discouraging," he said.
Even so, the researchers agreed that their warnings in the
1990's had helped NASA improve its maintenance and repair of the
shuttle's fragile tiles.
The federal investigation is scrutinizing the left wheel area
of the Columbia because it may have been hit by flying debris
during the shuttle's climb into space. Moreover, several clues
from re-entry, including rising temperatures, suggest that the
ship's end may have started there.
The two scientists, who worked closely with NASA managers,
engineers and technicians to produce their 1990 study, found the
shuttle's two wheel areas were among a half-dozen dangerous tile
zones. Others included the nose area and either side of the
shuttle's central belly.
Fifteen percent of the shuttle's tiles contributed to 85
percent of the risk of catastrophe, they found.
The scientists began their analysis by studying how debris
flying off the shuttle's external fuel tank and booster rockets
— including ice and hardened foam — had damaged orbiter tiles
during the shuttle's first 33 flights. They found high-, medium-
and low-damage zones. Tile damage in the wheel areas was
relatively high, as it was along the whole right side of the
underbelly. That, they found, was because foam insulation had
flown off two long pipes that run alongside the external fuel
With Columbia, NASA officials have said the foam piece that
hit the orbiter's wing came dislodged from around the "bipod,"
an attachment point that goes from the external fuel tank to the
That location has long been a source of trouble. One reason
may be that when the external fuel tank is filled with its
superchilled fuels, its metallic interior shrinks, which can
cause its shell of insulating foam to pull away. The area around
the bipod is particularly vulnerable, one NASA expert said
yesterday, meaning that "it's possible that the foam there can
pull away more easily from the tank."
At the moment that the foam or ice hit the wing, the shuttle
was 80 seconds into flight. That is a period of maximum stress —
the same stresses that contributed to the explosion of the
Challenger, 73 seconds into flight, in 1986.
Dr. Paté-Cornell, who is an engineer, said places where
things like the bipod penetrate the foam "you have a potential
To the data on tile damage, the scientists added temperature
readings of where the shuttle tiles became hottest and were most
likely to burn through. Dr. Paté-Cornell said the wheel areas
got unusually hot during re-entry because their doors created a
"discontinuity" in the tiles that would disrupt the smooth flow
of air over the shuttle's belly.
"It's probably more turbulent," she said.
NASA's heat map that the team obtained, she added, showed
clear heat increases near the wheel doors but did not explain
A final ingredient of their study was to correlate the
high-damage and high-heat areas with critical parts. The fear,
they said, was that flying debris could damage a tile in an
unusually hot area, creating a blowtorch aimed at gear below the
shuttle's aluminum skin.
Dr. Fischbeck of Carnegie Mellon said that in addition to
holding the wheels and landing gear, the wheel wells and
adjoining areas held many fuel and oxygen tanks, as well as
hydraulic lines that control the shuttle's flight surfaces.
NASA, the researchers say, took the warnings seriously.
"They did a lot of the things we suggested," Dr. Fischbeck
said. "Mainly, they inspected the tiles in a different fashion.
We said all tiles are not created equal in terms of risk. And so
they started to inspect the ones in the most critical areas
Dr. Paté-Cornell said one of the 1990 report's
recommendations to NASA was to "make sure that the insulation on
the external tank is applied in a very secure way."
In their report four years later, the researchers estimated
that such improvements could reduce the possibility of shuttle
accidents attributable to tile failure by about 70 percent.