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.
