"I dont want to get into a specific scenario because which ever one I mention will become the favorite of the day. We dont have a favorite scenario," he said.

For instance, hot gases eking into the space planes left wing remains a much-discussed possibility. However, whether or not damaged tiles, compromised or improperly attached Reinforced Carbon-Carbon (RCC) panels on Columbias left wing caused that heat intrusion remains unknown.

"One of the scenarios is that this had nothing to do with tiles or RCCthat heat got in there another way," Gehman said. Wreckage recovered on the ground shows that the space planes left wing, in general, shows more trauma than the pieces of the right wing, he said.

Aerodynamic mis-arrangement

Gehman also said that Columbia could have suffered some "physical mis-arrangement" of an aerodynamic surface, long before reentry. "The fact that the orbiter was fighting an aerodynamic mis-arrangement earlier than we previously had thought[is] very intriguing," he said.

The board is focusing on what tests are absolutely needed to either prove or disprove any scenario that led to Columbias tragic end, he said.

One such test series will be held in early April and is eagerly awaited by both CAIB and NASA study teams probing into Columbias destruction during reentry. These experiments will look into the role that falling shuttle external tank foam may have played in damaging Columbias left wing.

Structural damage

Preliminary test shots of foam hurled at target materials indicate that the foam is capable of inflicting damage to the shuttles aluminum substructure. However, launch photography does not show gross damage immediately after the strike.

According to a source close to the testing, there will be a series of foam firings on different targets over many days next month. The most prominent tests will be one firing at each of two structures that simulate Columbias wing section with good fidelity.

Experts believe that the upcoming tests, done with great care and involving select shuttle materials, may well show that impacting foam can cause structural damage underneath the tiles.

Still learning

Gehman told SPACE.com that the shuttle program works close to the margin of what is known.

"This is a research and development program. We are operating in regimes for which we are still learningthis hypersonic, low atmospheric regime. We dont have a lot of knowledge about bringing things back under controlled flight into the atmosphere. Weve done it 113 times with a winged vehicle. So its a good thing theres a lot of smart people working in thisbecause it is a research and development field," Gehman said.

Second public hearing

Today the independent Columbia Accident Investigation Board (CAIB) begins its second public hearing here at a local hotel, outside the gates of NASAs Johnson Space Center.

Those invited to address the board include: William Ailor, Director, Center for Orbital and Reentry Debris Studies at The Aerospace Corporation; R. Douglas White, Director for Operations Requirements, Orbiter Element Department for United Space Alliance; and Paul Hill, Space Shuttle and International Space Station Flight Director at the NASA Johnson Space Center.

The CAIB will continue the public hearing on Tuesday, requesting expert advice primarily from individuals at the NASA Johnson Space Center (JSC)

Those asked to appear before the board include: Stephen Labbe, Chief of NASA JSCs Applied Aeroscience and Computational Fluid Dynamics Branch; Christopher Madden, Deputy Chief of NASA JSCs Thermal Design Branch; Jose Caram, NASA JSC Aerospace Engineer of the Aeroscience and Flight Mechanics Division; and John Bertin, Professor of Aeronautics, United States Air Force Academy in Colorado Springs, Colorado.