CAPE CANAVERAL, Fla -- A new photographic analysis of Columbia's launch shows that foam debris falling off the external tank slammed into the orbiter's vulnerable leading edge, rather than the underside of the wing as NASA earlier had believed, investigators said Wednesday.

Although such an impact already was under examination, the analysis is the strongest evidence to date that the shuttle burned up on reentry Feb. 1, killing the seven astronauts aboard, because of a breach in some part of its leading edge that allowed super-heated gas into the wing.

Scott Hubbard, a member of the Columbia Accident Investigation Board, acknowledged that the probe now is focusing on the possibility that foam debris damaged parts of the leading edge. Still, no conclusions have been reached, the board said, and a number of possible causes of the accident remain under consideration.

But evidence supporting the leading-edge theory appears to be growing every day.

The board also disclosed Wednesday that defects may exist in the foam used to insulate the external fuel tanks. When investigators recently cut into the foam of another tank at a Lockheed Martin plant in Michaud, Miss., they discovered three air pockets near a critical attachment point, called a bipod ramp.

Investigators have suspected foam fell off near or at the attachment point during the Columbia liftoff. The photographic analysis made public Wednesday also confirms that the foam indeed did come off at the bipod ramp. The voids in the foam may explain why it was vulnerable to becoming detached.

A number of tests in the coming weeks should confirm whether the foam-leading edge explanation is credible. The board has contracted with the Southwest Research Institute, a nonprofit organization, to conduct tests during which chunks of foam will be shot at leading-edge components.

The tests will use a high-powered nitrogen gun to bombard leading-edge parts to simulate the event that occurred on the Columbia liftoff. Two sizes of foam -- a 1-pound chuck measuring 6 inches by 14 inches by 24 inches, and a 2-pound chunk measuring 3 inches by 11 inches by 24 inches -- will be used, the larger one delivering the energy equal to a 70-pound sack of concrete hitting a car on a freeway.

The tests will include a leading-edge panel that has flown on another orbiter, so that investigators can determine whether age and use have made the panels more vulnerable to foam collisions.

The foam strike on Columbia's launch could have damaged one of three parts of the leading edge: the panels made of reinforced carbon carbon; a so-called t-seal that fills the gap between the leading edge panels; or a part known as a carrier panel that covers the joint between the leading edge panels and the wing's surface.

Each wing of the shuttle has 22 leading-edge panels that can withstand reentry temperatures of 3,000 degrees. But investigators have discovered that the panels degrade with age, forming surface fractures, pinholes and internal voids that may have weakened the structures. A sample panel -- a large U-shaped structure that is just a quarter-inch thick -- was shown at a news conference Wednesday. Adm. Stephen Turcotte, a board member, said all but two of Columbia's 44 leading-edge panels were the ones originally installed when the orbiter was built in the late 1970s. They had been extensively damaged in flight and repaired, he said; it remains to be determined whether those repairs left the panels weakened.

In addition to the foam-shooting tests, investigators plan to cut into the foam of another external tank that more closely resembles the one used on Columbia.

The latest photographic analysis of Columbia's launch was conducted by more closely examining and combining pictures taken by two long-range cameras at Kennedy Space Center. Those photographs previously showed foam breaking away from the external tank about 80 seconds into the flight, but it was difficult to determine where the foam came from or where it struck the orbiter.

During Columbia's final mission, NASA and Boeing Co. engineers concluded that it most probably hit the underside of the wing near the landing-gear door.

But in a more careful, frame-by-frame, analysis after the accident, investigators determined that the foam fell off the bipod area. As it tumbled at about 500 mph relative to the wing, it struck with a footprint about two feet in diameter, Hubbard said. It struck the wing's leading-edge panels number five, six and seven -- a key area of the wing where it angles out less sharply.

Aeronautical engineers at NASA's Langley Flight Research Center in Virginia have been conducting wind tunnel tests that examine what could happen if a panel were destroyed in that region of the wing. The tests indicate such an event could explain some of the aerodynamic problems the orbiter was experiencing in its final minutes of flight.

Hubbard said he could not fault NASA for its original photographic analysis, saying the board had made a strong effort to make its more-precise determination of where the foam struck.

Debris evidence is providing important clues about what may have happened to the leading edge. NASA officials have laid out the Columbia's wing and fuselage parts on a precise grid. Much of the left wing is gone, presumed to have burned up.

Scientists are attempting to conduct both chemical and metallurgical tests on the wreckage, said Gregory Kovac, an associate professor of electronics.

So far, they have found that the left side leading-edge panels are more badly damaged than those on the right side. The panels on the left side also contained heavy deposits of melted metal. Panel number six, where the foam struck, is missing, Steve Altemus, a NASA engineer overseeing the reconstruction, told the board Wednesday.
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