Clues Point to Shuttle Wing Edge

New evidence in the Columbia accident investigation is pointing to a theory that the space shuttle was destroyed by damage to the leading edge of its left wing.

The leading edge, a curved structure made of reinforced carbon fibers, sustains some of the highest temperatures on the orbiter during reentry and any breach there could have had devastating consequences, experts said.

Although it has always figured in the investigation, the leading edge has not received as much attention as the possibility that the accident was caused by damage to the heat-resistant tiles that cover the wing surfaces and the landing-gear doors. Investigators have not closed the book on any possible cause, but the leading-edge theory is gaining credibility, according to sources close to the Columbia Accident Investigation Board.

Pinpointing where the orbiter failure began is crucial to understanding not only how the shuttle broke up but what could have caused the damage to the wing in the first place.

No one knows yet what could have caused the initial damage to the shuttle, though most speculation centers on either foam falling off the external tank on liftoff or a collision with space debris while in orbit.

As the Columbia soared over Texas on Feb. 1, it struggled to maintain control against aerodynamic forces that were threatening to send the orbiter into a swinging motion known as a yaw. Tests conducted at NASA’s wind tunnels in Hampton, Va., show that the instability could be explained by the loss of some of its leading-edge panels. Those tests at the Langley Research Center, which were forwarded to the investigation board last week, subjected a scale model of the shuttle to high-speed airflows.

The scale model tests checked what would occur with the loss of just one of the 22 panels that make up each wing’s leading edge. A source close to the investigation said the yaw experienced by Columbia could be explained only by the loss of as many as five or six panels.

The Langley tests have focused on a specific leading-edge panel that is located where the left wing changes its angle, canting backward less sharply. That point is known as the No. 6 panel, NASA officials said, and the loss of that panel could explain why higher-than-normal temperatures were recorded on top of the wing along the point where it intersects with the fuselage. The shuttle began experiencing abnormal heating as it crossed the California coastline, and sensors continued to report hot spots until the Columbia broke up.

The Langley tests include two series of data: an examination of aerodynamic forces acting on the shuttle and of the thermal forces that began to build up. Analysts who have seen the data say the tests are helping to explain why the orbiter first experienced a gradual heat buildup and only later a sudden aerodynamic irregularity. The shuttle’s computers were working until the last seconds to counteract the yaw with control surfaces known as elevons and with rocket thrusters.

Investigators are considering a scenario that begin with a hole or breach in a single panel, allowing hot gases to flow into the wing and sequentially tearing off adjacent panels -- a so-called zipper effect across the leading edge, according to the source close to the investigation.

The source close to the investigation board and others in NASA asked that their names not be used because some of the information has not been formally released to the public.

Past studies of the shuttle have demonstrated that a relatively small hole in the leading edge would quickly enlarge as hot plasma gases streamed through into the wing’s interior. The hole would enlarge through a process known as “ablation,” in which the hot gases erode the material.

The theory is supported by several other pieces of evidence. Recovered leading-edge panels now at Kennedy Space Center in Florida show that the attachment points were melted, rather than broken.

The theory is also supported by a photograph taken during reentry by astronomers at Kirt- land Air Force Base in Albuquerque, showing what appears to be a damaged, jagged area of Columbia’s left wing.

“People are focusing on the leading edge,” said a source close to the investigation. “The Kirt- land pictures are getting more credence.”

NASA ground controllers and the astronauts aboard the Columbia would have no way of knowing that the wing had been damaged.

Members of the investigating board caution that they are far from declaring any finding that would explain the accident, and that they have not foreclosed on any theory.

Laura Brown, a spokeswoman for the board, said the investigation is still looking at a broad range of issues and that any finding must be consistent with all the evidence that is being gathered.

So far, investigators have identified a few possible causes of the damage. A 2.6-pound chunk of foam insulation was observed falling off the external tank 80 seconds after launch, leading to speculation that it could have caused serious impact damage to the delicate heat-resistant tiles on the bottom of the wing or the leading edge.

Investigators said they are also considering the possibility that orbiting space debris or a tiny meteoroid may have hit the orbiter during its mission.

A report by National Research Council, an independent group that provides major technical assessments of government programs, cautioned NASA in 1997 to improve the ability of orbiters to take meteoroid hits without sustaining serious damage and warned that a space debris hit could jeopardize the craft.

“It is a low probability, but it is possible,” said George Gleg- horn, the retired chief engineer for spacecraft at TRW who served on the council.

Gleghorn said leading-edge material is not particularly tough and would be vulnerable to either a space debris strike or a foam strike.

The report notes that a 1-millimeter meteoroid can penetrate the leading edge, while it takes a 3-millimeter to 5-millimeter meteor to penetrate the heat-resistant tiles.

According to engineers who helped design and build the shuttle, a collision with a large piece of foam could penetrate or dislodge a leading-edge panel, which is made of a material called reinforced carbon carbon.

NASA recognized the hazards of a strike to the leading edge, and planned in the late 1990s to modify it by inserting insulation that would protect against a .63-centimeter hole, according to the National Research Council report.

The possibility of hitting a meteoroid is far from theoretical. A separate August 1996 NASA report also raised questions about the potential for orbital debris to cause damage to the leading edge, noting that Atlantis sustained two gouges to its leading edge from orbital debris.

And between 1981 and 1996, for example, NASA had to replace 55 windshields in the shuttle fleet that were damaged by debris, much of it in orbit, the report said. Gleghorn said the window impacts were less of a safety issue than an annoyance for astronauts.