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Columbia's Final Minutes

The second-by-second account of the shuttle's last minutes

By Michael Cabbage and William Harwood

January 27, 2004

EDITOR'S NOTE: From "Comm Check ... The Final Flight of Shuttle Columbia," by Michael Cabbage and William Harwood, which is being published Tuesday by Free Press, a division of Simon & Schuster. Cabbage is the space editor of the Orlando (Fla.) Sentinel; Harwood is a veteran space reporter for CBS News. Printed by permission.

"The most complicated machine ever built got knocked out of the sky by a pound and a half of foam. I don't know how any of us could have seen that coming. The message that sends me is, we are walking the razor's edge. This is a dangerous business and it does not take much to knock you off."
-- Flight director Paul Hill

Shuttle wings are made of aluminum, the upper and lower surfaces separated by spars and trusses that form a boxlike internal framework. The main landing gear wheel well boxes are located toward the front of each wing, nestled up against the side of the orbiter's fuselage just behind the leading edge.

Behind its protective insulation, the front of a shuttle wing is flat, made up of a panel of aluminum honeycomb material known as the leading edge spar. To give the wing its aerodynamic shape, and to protect it from the most extreme temperatures of re-entry, 22 reinforced-carbon carbon panels are bolted side by side on that flat front surface, creating a smoothly curving leading edge. So-called spanner beams, made out of a heat-resistant alloy called Inconel, provide rigidity. To seal the gaps between RCC panels, thin carbon-composite strips called T-seals are bolted in place to provide a smooth surface along the entire leading edge.

During re-entry, the shuttle's nose is pitched up 40 degrees, which subjects the lower halves of the RCC panels to the most extreme heating. The fittings used to attach the RCC panels to the main spar are protected by heat-resistant insulation that melts at 3,200 degrees.

Whatever happened to Columbia had utterly destroyed this complex system.

Twenty-seven truckloads of wreckage were hauled to Kennedy Space Center between Feb. 5 and May 6. More than 25,000 searchers, who scoured a debris "footprint" that was 645 miles long, found 84,900 individual pieces, about 38 percent of the space shuttle. Each piece or component was cleaned, decontaminated, bar-coded, photographed and entered into a computer database. Wreckage from Columbia's wings, fuselage, and nose section was laid out on a grid in the Reusable Launch Vehicle Hangar near Kennedy's shuttle runway. The most critical RCC panels and attachment fittings -- those numbered 1 through 13 and nearest the fuselage -- were mounted on a full-scale clear plastic mockup of the rounded leading edge that allowed investigators to see each piece in relationship to its neighbors. It also allowed them to map out exactly where the heat went after it entered the leading edge.

The work at KSC was buttressed by analysis by Johnson Space Center engineers of data from the orbiter's Modular Auxiliary Data System, or MADS, recorder and amateur video images of Columbia's disintegration. The inch-wide MADS tape contained information from 570 sensors; it was found by searchers in Hemphill, Texas, on March 19, six weeks after Columbia disintegrated. Ultimately, the Columbia Accident Investigation Board was able to conclude, without qualification, that the foam impact was the root cause of the accident; that the impact had knocked a 6- to 10-inch hole in the lower half of RCC panel 8 on the shuttle's left wing; and that a plume of super-heated plasma entering through that breach had destroyed the wing and triggered the destruction of the orbiter.

The team concluded the foam broke away from the left bipod ramp 81.7 seconds after liftoff and hit the underside of Columbia's left wing two-tenths of a second later. The foam measured 21 to 27 inches long by 12 to 18 inches wide. It was tumbling at 18 revolutions per second. Before the foam separated, the shuttle -- and the foam -- had a velocity of 1,568 mph, about twice the speed of sound. Because of its low density, the foam rapidly decelerated once in the airstream, slowing by 550 mph in that two-tenths of a second. The foam didn't fall on to the leading edge of the left wing as much as the shuttle ran into it from below. The relative speed of the collision was more than 500 mph, delivering more than a ton of force.

On July 7, investigators using a nitrogen-powered cannon fired a 1,200-cubic-inch block of foam weighing 1.67 pounds at RCC panel 8, taken from the shuttle Atlantis. Traveling at 530 mph, the foam blew a ragged 16-inch hole in the RCC panel, vividly demonstrating how much damage foam could do.

***

With the dramatic foam shot at RCC panel 8, all the pieces of the puzzle were finally in place. There was little doubt about what had doomed Columbia and its crew. A second-by-second time line of the final working scenario provided a gripping account of the shuttle's final minutes.

At 8:44:09 a.m. Eastern time on Feb. 1, 2003, Columbia was a half-hour from home. The shuttle had just dropped below an altitude of 76 miles, slipping into the discernible atmosphere 900 miles northwest of Honolulu.

During re-entry, the shuttle compresses the thin air in front of it, creating two shock waves. Those shock waves intersect around RCC panel 9, subjecting panels in that area to the most extreme heating. But the compression of the air in front of the shuttle forms a so-called boundary layer, a region just a few inches thick that resists further compression and acts as a natural insulator. A few inches away from the leading edge, just beyond the boundary layer, molecules are torn apart and temperatures can exceed 10,000 degrees. But the boundary layer keeps temperatures on the leading edge RCC panels at around 3,000 degrees.

A smooth surface is essential for the boundary layer to form and is crucial to a shuttle's survival during the plunge to Earth. If the boundary layer is disturbed for any reason, its insulating effect can be compromised by high-temperature turbulence, subjecting the shuttle's tiles and RCC panels to much more heat than they were designed to handle.

But even as the Columbia astronauts chatted about the light show outside, the hole in Columbia's left wing was disrupting that boundary layer. Ever more air molecules were shooting into the inside of the wing at RCC panel 8 and slamming into the insulation protecting the panel attachment fittings, swirling through the cavity and spreading out to either side. At that altitude, the effect was small. But the shuttle was descending, and the air was getting thicker with each passing second. With Columbia in a 40-degree nose-up orientation, the plume entering the breach in RCC panel 8 was aimed at the upper attachment fittings and insulation. The insulation began melting, and the front face of the left wing's aluminum honeycomb forward spar -- the only barrier between the plume and the interior of the wing -- began heating up.

At 8:48:39 a.m., just four minutes and 30 seconds after Columbia had dipped into the atmosphere, a sensor mounted behind the forward spar, near the point where RCC panel 9 was bolted to the other side, measured an unusual increase in stress. The spar was softening.

About a minute later -- five and a half minutes after entry interface -- the shuttle's flight computers ordered a turn to the right. Up until this point, the shuttle had simply been falling into the atmosphere, wings level, nose up and pointed straight ahead. Now, the ship's flight computers began actively guiding the shuttle toward Kennedy's runway. The shuttle's nose smoothly swung 80 degrees to the right.

Less than 20 seconds after the maneuver, sensors mounted on Columbia's left rear rocket pod measured an unusual change in temperature. Wind tunnel testing would later show some of the hot air blasting into the RCC cavity was exiting through the vents on the upper surface of the wing, carrying thin clouds of metallic vapor from melted insulation.