HOUSTON Those investigating Columbias lethal dive to Earth continue to sort through recovered wreckage, baffling sensor data, and invaluable video trying to decipher why the space plane broke up at high altitude.

The Columbia Accident Investigation Board (CAIB) held its second public hearing here, asking expert witnesses to focus on the February 1 reentry of the shuttle.

A chilling and precise step-by-step time line of events of space plane troubles was highlighted, also dramatically portrayed by a collection of video film taken from the ground of the space planes high-speed disintegration.

William Ailor, Director, Center for Orbital and Reentry Debris Studies at The Aerospace Corporation in El Segundo, California, advised the CAIB that deciphering exactly what caused Columbias breakup will be challenging.

"The challenge here [with Columbia] is that the heating itself is going to have the potential of masking the heating and loads during the breakup process," Ailor said. The space planes aluminum skeleton when exposed to extremely elevated temperatures can melt, vaporize into droplets, even burn through other structure, he said.

Furthermore, as pieces shed off Columbia, the aerodynamic properties of the vehicle changed, Ailor said. That makes it very difficult to fully understand all the events that led to the crafts ultimate demise and loss of its crew.

Ailor said the search underway for debris cast off during the beginning phases of Columbias atmospheric plunge is "critical to the investigation."

Because of the amount of debris already recovered, Ailor added, lab analysis of wreckage must be tightly focused on top candidate causes of the mishap specifically, comparative looks at left wing debris contrasted to right wing fragments.

A dramatic portrayal of Columbias troubled trajectory from orbit was spelled out by Douglas White, Director for Operations Requirements, Orbiter Element Department for United Space Alliance (USA).

USA is the prime contractor for NASAs Space Shuttle Program and is responsible for the day-to-day operation and management of the space shuttle fleet.

White highlighted Columbias aileron behavior during entry through the atmosphere a first indication that something odd was happening to the space plane. In addition, a series of communication dropouts with the shuttle before first signs of heating continues to puzzle study teams.

The unexpected aileron actions occur while Columbia remains controllable and responding to computer flight commands. "But the rates and the amount of muscle needed to continue flying the vehiclesomething definitely happened at that point to cause the flight control system to need more muscleto fight harder to control the vehicle," White said.

As some sensors began showing rising temperatures in various spots within Columbia, other sensors remained normal, White said. "Whether the change that caused the temperatures to rise is related to what ultimately caused our tragedy we dont know. They may be connected, they may not," he said.

Efforts are still underway to better characterize what was taking place in Columbias left wheel well. Within 40 seconds, wiring bundles leading to some sensors appear to have been melted or shorted out, perhaps blasted by a moving blowtorch-like wave of intrusive heat.

Paul Hill, Space Shuttle and International Space Station Flight Director at the NASA Johnson Space Center, has been leading a space agency "tiger team" piecing together still and video imagery of Columbias final minutes.

Thanks largely to the public who provided video recordings of the space planes reentry across multiple states, Columbias debris-shedding events, flashes, and other phenomenon are being intensely reviewed by photo image analysts.

From California to Texas there was a relatively steady stream of objects coming off Columbia, Hill said. "The public video we have is probably the best thing we have to try and find some debris out west," he said.

Snow covered areas of Nevada and Utah have prevented air and ground searches from locating debris considered highly important to the investigation, Hill said. California also remains a critical search zone for pieces that fell off the shuttle early in its pathway across the United States.

There have been individuals who have turned in imagery that has been falsified, Hill said.

"We did get hoaxes. Cars driving down the road with headlights onit was relatively clear to us it wasnt something in space," Hill said. There have also been people holding onto photographic evidence in the hopes of gaining some kind of fame and a little money, apparently thinking they could ride "the Columbia gravy train," he said.

Hill said, however, that the overwhelming majority of public support has been fantastic. "These people are definitely our heroes," he concluded.