Nasa was warned at least nine years ago that space shuttles could fail catastrophically if debris hit the vulnerable underside of their wings during liftoff - the very scenario that may have brought down the Columbia space shuttle.
According to Paul Fischbeck, an engineering professor at Carnegie Mellon University who conducted the 1994 analysis, Nasa made changes in materials and flight rules after receiving the warning, in order to lessen the risk of debris breaking loose. Mr Fischbeck and his colleagues had made an initial report to Nasa on their findings in 1990.
"There are very important tiles under there. If you lose the tiles on those stretches ... it can cause the shuttle to be lost," Mr Fischbeck said. He added that Nasa "took a lot of our advice to heart" and made improvements such as changing the foam insulation on the top of the booster rockets and allowing less ice to be present on the fuel tanks before liftoff.
Mr Fischbeck and a colleague at Stanford University studied the damage caused by debris during the first 50 shuttle launches and concluded that on average, 25 thermal tiles per flight sustained damage of at least one inch. He said his risk analysis showed that the most vulnerable spots on the shuttle were the undersides of both wings close to the fuselage, and immediately under the crew compartment.
The underside of Columbia's left wing is the focus of Nasa's investigation into Saturday's disaster, in which seven astronauts died.
Yesterday, the space agency sent teams to investigate reports of shuttle debris found as far west as California and Arizona - material that could shed light on the earliest stages of Columbia's break-up. Later in the day, authorities in Texas said a 6-7ft section of what they believe to be part of a shuttle wing was found in a pond east of Nacogdoches, Texas.
At midday, President George Bush vowed that the space programme would go on as he led a memorial service to the lost astronauts. Thousands of grieving space workers and their families, friends and neighbours listened outside the private memorial on the lawn of the Lyndon Johnson space centre in Houston.
"Their mission was almost complete and we lost them so close to home," Mr Bush said, as his wife Laura wiped away tears.
In Vernon Parish, Louisiana, a woman yesterday found a small, ragged piece of fabric with a star of David on it. The symbol, in a circle between blue and silver bars, is the flag of the Israeli air force and is presumably from the suit of Israeli astronaut Ilan Ramon.
It is not clear exactly where under the wing Columbia was hit, but just before the shuttle break-up temperature spikes were detected around the left wheel well, which is close to the fuselage, and on the left side of the fuselage itself.
The investigation into the disaster is focusing on the possibility that a 1kg, 20in chunk of foam insulation from the shuttle's external fuel tank fell off during lift-off and hit the left underside of the wing, causing fatal damage to the thermal tiles.
Mr Fischbeck stressed that no one knows with absolute certainty yet whether it was this debris that brought down Columbia, or where precisely that debris hit.
The foam and the tiles have been a source of concern at Nasa practically from the start. Over the years, foam insulation often damaged the tiles. In fact, soon after Nasa stopped using Freon in the foam for environmental reasons, Columbia sustained significant tile damage during a 1997 liftoff because of flyaway foam, according to a report by Nasa engineer Gregory Katnik.
"Almost since day one, the insulation has been a pain. Pieces break off," said Seymour Himmel, a retired Nasa executive who served two decades on an aerospace safety panel and looked into the potential dangers of the foam.
The foam is a lightweight, polyurethane, spray-on material that hardens like plastic foam. Given the speed at which shuttles hurtle into space during takeoff, flyaway pieces can have a devastating effect. Moreover, the black, silica glass fibre tiles that cover the underside of the shuttle are famously fragile, so much so that even a bump can cause cracks or impressions.
Nasa has said that during Columbia's mission, its engineers carefully analysed the potential damage done by the foam and concluded that the crew was in no danger.
On Monday, Michael Kostelnik, deputy associate administrator for Nasa spaceflight, said that the chunk that fell from the tank and hit Columbia's tiles was "probably the largest piece we've had" of any shuttle mission.
The tiles protect shuttles against the searing heat of atmospheric re-entry. If some are missing in an especially vulnerable area, it could set off a chain reaction that could destroy the shuttle.
Shuttles almost always return to Earth with marred or missing tiles, the result of debris crashing into them during liftoff.
