[an error occurred while processing this directive]
Columbia passes above the Owens Valley Radio Observatory north of Bishop, Calif., before dawn Saturday, one of the last images of it intact. I could see little red pieces sort of breaking away -- being left behind, freelance photographer Gene Blevins said. Then I saw a big red flare coming underneath. I thought, that isnt right. He and photographer William Hartenstein were stunned when they learned later what had happened. We were not expecting something like this, Blevins said.
Landing the shuttle
While the takeoff is the most dangerous part of a space shuttle mission, former shuttle crew members say the landing ranks a close second.
"It's hazardous, because the vehicle is undergoing a lot of stress," explained Winston Scott, a former astronaut who flew on Columbia in 1997 after a trip on its sister ship Endeavour in 1996.
It has been 17 years since the Challenger exploded, and the passage of time has made recent flights seem almost routine. But every shuttle crew faces a dangerous ride home, a trip that begins by falling like a meteor and ends by landing like a glider.
Sen. Bill Nelson, D-Fla., who flew on Columbia in 1986, said every detail must be precisely right. "If the nose is too low, you burn up," he said. "If the tail is too low, you burn up."
The return trip begins on the other side of the globe, about an hour before the shuttle is due to touch down in Florida. The crew is already strapped in, and the payload bay doors are closed.
Somewhere over the Indian Ocean, the shuttle turns backward, facing the opposite way from the direction it has been orbiting.
The engines fire for about 21/2 minutes, just long enough to knock the shuttle out of orbit, before shutting down. The on-board computers adjust its position so the nose is raised to about a 40-degree angle, pointed high enough that the crew cannot see the Earth.
About 30 minutes later, about 400,000 feet above Hawaii, the first tendrils of the upper atmosphere brush up against the protective tiles covering the shuttle's underside. As the craft plunges further into the atmosphere, the angle of the craft becomes critical.
"You've got to have the right angle when you re-enter the Earth's atmosphere, or else the heat is not going to be distributed over the thickest tiles," Nelson said. The heat rises to 3,000 degrees, well above the melting point of steel. The nose cone and the leading edge of the wings catch the worst of it.
The crew's space suits and the cockpit cooling system protect the astronauts from sweltering. But the outside becomes "so hot that you can see the bright orangeish-yellowish glow around the windshield," said Scott, now an engineering professor at Florida State University.
Scott's daughter is a reporter for the Times.
The 19,000 interlocking tiles that cover 70 percent of the shuttle's skin are supposed to repulse that intense heat.
When Columbia first flew in 1981, it was covered with more than 24,000 tiles. Subsequent retrofitting reduced that number by more than 5,000, which also reduced the craft's weight.
The ceramic tiles, which can cost up to $2,000 each, are made with fibers of pure white silica refined from common sand. They are later dried in the nation's largest microwave oven at a California plant owned by Lockheed Space & Missile Co.
Some speculation about the Columbia disaster focused Saturday on the tiles. Shortly after Columbia's Jan. 16 launch, a piece of insulating foam on a fuel tank came off and apparently smacked into the left wing.
A gap in the tiles could trigger a catastrophic failure, as the steel beneath the tiles becomes so hot that it loses its strength and buckles.
"What we don't understand is what the impact did to the tile," Ron Dittemore, shuttle program manager, said Saturday afternoon.
Engineers had concluded before the tragedy that any damage to the wing was minor and there was no danger, Dittemore said, but now "we're going to go back and see if there's a connection."
The heat reaches its peak as the craft descends to 265,000 feet, often creating a layer of ionized gas particles that blocks off radio signals between the shuttle and ground control. The blackout can last until the shuttle drops to 162,000 feet.
But Columbia was expected to avoid such a blackout because of extra satellite coverage along its flight path, NASA spokesman Al Feinberg said.
Minutes before it came apart over Texas at an altitude of about 207,000 feet -- about 39 miles -- the crew responded to a radio call about problems with sensors in the left wing, according to chief flight director Milt Heflin.
The subsequent loss of radio contact was the first clue that something had gone wrong. At that point the shuttle was traveling 12,500 mph, or 18 times the speed of sound.
During that part of re-entry, "the G-forces begin to build up," Scott said. Although the pressure is only double that of Earth's gravity, "because you're not conditioned to it you feel about four times your body weight. The suit is pushing down on you, and your helmet is pushing down on you."
The disaster occurred just after the shuttle had begun a maneuver called a roll reversal, designed to dissipate heat and drop speed by dipping the wings one way and then the other so the shuttle "flies like an S-track over the ground," Scott said.
The maneuvers are controlled by the on-board computer, not the crew. They can take control, however.
If Columbia had followed the pattern of previous flights, the craft would have emerged from its heated plunge and coasted across the country to Florida in a matter of minutes.
-- Staff writers Anita Kumar and Mary Jacoby contributed to this report, which also contains information from Times wires.