Searchers have recovered remains of all seven astronauts and more than 12,000 shards of metal, wires and debris that rained down across two states. But the findings so far have yielded few clues.

The most significant discovery has been a 2-foot section of shuttle wing, including the carbon-covered leading edge designed to protect Columbia's insulating tiles as the spacecraft heats to 3,000 degrees re-entering the atmosphere.

If that section came from the troubled left wing, where temperatures surged in the shuttle's final moments and sensors failed in rapid sequence, it could provide hard evidence of what went wrong.

Investigators hadn't yet determined which wing the fragment belonged to, but should know "in relatively short order," NASA Administrator Sean O'Keefe said Saturday after a memorial service at Louisiana's Barksdale Air Force Base, where pieces of the shuttle are being stored.

In the shuttle's final eight minutes the morning of Feb. 1, temperatures surged in the left landing gear compartment, and the brake lines began overheating one by one. Sensors began showing overheating across other areas of the left wing and adjoining fuselage. Then Mission Control lost all contact and Columbia broke apart.

Investigators are considering every possible scenario, from the impact of a large chunk of hard insulating foam that hit the shuttle seconds after liftoff Jan. 16, to a deadly bull-eye's strike by a piece of space junk, to a lightning-like electrical phenomenon in the upper atmosphere.

The seemingly innocuous piece of foam once NASA's focus, then all but discarded is back at the heart of the mystery.

The 2 1/2-pound chunk of insulation, measuring 20 inches by 16 inches by 6 inches, broke off Columbia's external fuel tank 81 seconds after liftoff and smacked into the left wing, where the sensors later failed during the shuttle's return.

Engineers studied the impact while Columbia was in orbit and concluded it posed no safety threat. Now they're redoing their analyses, in excruciating detail, to see if they might have missed anything.

It's also possible that something more than the foam chunk such as ice or maybe hardware came off the fuel tank or booster rockets and ricocheted into Columbia.

Imagery experts are poring over a high-resolution photo taken by an Air Force telescope a minute or two before Columbia broke apart; some have suggested the leading edge of the left wing looks as if it could be damaged, and the photo shows a gray streak that could be a fiery plume trailing the wing.

Shuttle program manager Ron Dittemore acknowledges confusion, misinformation and "even some second-guessing on all of our parts" in the past week.

NASA is no longer even certain exactly what time temperatures started to rise and sensors started to malfunction during Columbia's final eight minutes of flight.

The sequence of sensor measurements and failures is puzzling, particularly one reading in a part of the mid-fuselage not connected to any of the wire bundles linked to the other sensors that were going haywire.

"Whether that's important or not, we don't know," Dittemore said.

Though the sensor data are being closely examined, it's possible the source of heat might not have been near them.

Up to that point, nothing aboard Columbia or at Mission Control at Houston's Johnson Space Center had indicated severe damage.

Dittemore said investigators were resisting the temptation to draw quick conclusions.

"We have been in this business long enough to know that you go down that merry path of making a judgment or a rush to judgment, and you will be fooled," he said. "You need to go through the process. You need to gather the data. You need to correlate all the data, the time frames, the evidence, the photos, the way the system behaved, and you need to do it under the scrutiny of a microscope for you to get the right answer."

It is now up to a NASA-appointed investigation board which some lawmakers would like to have fewer ties to the space agency to piece together the agonizing puzzle.

In Alabama, NASA was gearing up for foam impact tests on the thermal tiles and insulating panels that cover space shuttles and protect against the intense heat of re-entry.

In California, Boeing Co. a NASA contractor was looking for any hint that a 17-month overhaul of Columbia at the company's assembly plant in Palmdale might have contributed to the accident. Among the modifications to NASA's oldest shuttle were increased protection from space debris and enhanced heat protection for the leading edges of the wings. Columbia flew one successful mission after the overhaul was completed in 2001.

Meanwhile, NASA continues to gather evidence through an extensive debris search, centered primarily in Texas and Louisiana. There have been more than 350 reports of debris west of Texas where NASA believes the most telling evidence will be found but none of those reports had yet been confirmed to be from the shuttle.

The only thing ruled out, definitively and swiftly, is that Columbia was brought down by terrorists. With Israel's first astronaut on board, terrorism had been the major concern for the shuttle's launch and its return to Florida 16 days later. Federal officials said the shuttle was too high when it broke up 207,135 feet to be reached by any surface-to-air missile.

Columbia was just 16 minutes away from landing at Kennedy Space Center in Florida, and the astronauts' families and friends and colleagues and bosses were waiting at the runway, when all contact and seven lives abruptly came to an end.

(China Daily February 9, 2003)