Earlier shuttle adjusted flight amid fears of tile damage

WASHINGTON -- Two years before the Columbia disaster, NASA safety experts fearing similar damage to delicate heat tiles on the space shuttle Atlantis decided it was "prudent" to adjust its return path to lessen danger during the fiery descent, according to internal documents.

NASA has maintained since last week's disaster that there was no recourse after liftoff for the seven astronauts aboard Columbia if there was serious damage to those tiles. But the Atlantis mission, in which the crew was instructed to make a protective re-entry, suggests there were at least options for an ailing shuttle to return safely.

Experts during the Atlantis mission, in May 2000, feared damage to tiles on the wing, like Columbia, and over a slightly smaller area than the damage suspected to the shuttle that disintegrated over Texas last weekend, documents show.

In both cases, engineers believed the damage was not a threat to safety. But NASA took the extra precaution in the Atlantis case, the documents show.

NASA spokesman Michael Braukus said Friday the space agency did not consider ordering Columbia to make the same maneuver, and believes now that it would have interfered with efforts to warm Columbia's landing gear tires for a safe landing.

The tires were unusually cold because Columbia orbited the planet for days with its landing gear aimed toward deep space, he said.

The maneuver, called "thermal-conditioning," was described as far back as 1990 for NASA by researchers. It involves pivoting the shuttle slightly left or right before it speeds through earth's atmosphere so that damaged tiles on one side might be exposed to lower temperatures.

It's akin to a football player turning his body to favor an injury as he's tackled.

Since Columbia's fiery breakup Saturday, NASA officials have maintained there was no recourse for any shuttle that reaches orbit with damaged tiles.

"We couldn't do anything about it anyway," shuttle program manager Ron Dittemore said earlier this week. "Our only effective action is to prevent the loss of tiles through design and through test. And that has been perfectly adequate up to this point."

Atlantis adjusted its return flight because experts noticed a 6-inch ice chunk from its external fuel tank shattered about 8 feet from one of the hinged, flap-like devices, called "elevons," along the back of the wing that help the pilot steer.

"It was prudent to take some precautions," according to NASA documents. The maneuver to pivot Atlantis to protect its right wing "increased the temperature margin and therefore reduced the potential for structural damage."

Engineers similarly feared damage to tiles on Columbia's left wing, but they did not instruct Columbia to perform such a protective maneuver. NASA officials believe the shuttle disintegrated over Texas shortly after it performed a routine turn to slow down. The first sign of problems was the loss of temperature readings from systems controlling Columbia's left-wing elevons.

The damage in May 2000 to Atlantis' right wing "was not considered a safety of flight issue," NASA's documents show. Inspectors later found a gouge in thermal tiles there, measuring about 5.25 inches by 1.5 inches by one-half inch.

NASA officials have said they also concluded that possible damage to Columbia's insulating tiles didn't threaten the shuttle's safety. They based their conclusions on scientific models showing possible damage over an area about 7 inches by 32 inches -- larger than the damage to Atlantis years earlier.

Outside experts said it was impossible to know yet whether flight adjustments by Columbia could have prevented its destruction.

"You can yaw the vehicle to the side, you can roll the vehicle a little bit," said Steven P. Schneider, an associate professor at Purdue University's Aerospace Sciences Laboratory. He said some shuttle surfaces, such as near the fuselage or the back edges of the wings, could be better shielded during such maneuvers than others. "You can't change the trajectory too much."

A 1990 study for NASA by outside researchers said threats to the shuttle from damaged thermal tiles -- which protect against temperatures that can reach 3,000 degrees -- could be lessened by rerouting important internal systems or changing the shuttle's re-entry profile. Using that technique, researchers wrote, "it may be possible to reduce the temperature of some weak, vulnerable areas."