At 8:18 a.m. EST, the shuttle fired its engines to leave orbit.

There was no turning back. Columbia was a glider. Once it entered the atmosphere, it flew by force of gravity alone in an artful computer-controlled fall.

Just after 8:44 a.m., the orbiter skimmed into the upper reaches of the atmosphere over the central Pacific at 24 times the speed of sound.

Normally, the ship was protected by a smooth aerodynamic flow that swept dangerous gases around it, with a thin boundary layer insulating it from the most severe heat.

The hole in the wing disrupted that layer. It exposed the wing to extreme superheated gases that could melt a 5-inch hole in an aluminum plate in 13 seconds.

Columbia inhaled a blowtorch.

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The first imperceptible signs of trouble came quickly.

A gauge about 15 inches from the spot where the foam debris had struck 16 days earlier registered the earliest signs of stress. The information went no further than the flight data recorder.

Twenty seconds later, a well-insulated temperature sensor nearby started to detect an unusual rise in heat. But no one knew.

Inside the crew compartment, the astronauts stowed the trash and worked through pre-landing checklists.

Columbia began the first in a series of computer-controlled rolls designed to slow it for a safe landing.

Almost immediately, the blazing plume blew through the hollow behind the left wing's heat shielding and began spraying gas out through a thin vent that ran the length of the wing's leading edge.

The wing was burning from the inside out.

Five times in quick succession, beginning at 8:50 a.m. EST, communications between Columbia and Mission Control in Houston flickered on and off rapidly, as if someone were fanning his fingers in front of a light.

Sprays of molten metal erupted from the wing and enveloped the fuselage, interfering with electronic transmissions, like handfuls of metallic chaff dumped from a fighter jet to confuse enemy radar.

Over the next three minutes, the aluminum spars supporting the wing softened and began to sag.

Flight control responses were still normal. Telemetry measurements received at Mission Control were normal.

In the tumult of reentry, the crew sensed nothing amiss.

There was a small but noticeable increase in the temperature of a hydraulic brake line running through the left wheel well.