HOUSTON Experts looking into the Columbia accident hope that several ground tests may offer clues as to why the space plane broke up during reentry Feb. 1 as it sped through Earths atmosphere.

Two key tests involve insulating foam used on the shuttle systems large external fuel tank, and trying to identify an unknown object that may have floated away from the space plane on day two of Columbias 16-day orbital mission.

Video of Columbias liftoff shows something flaking off the external tank -- three distinct pieces -- perhaps insulating foam, ice, a combination of the two, or even ablator material used underneath the foam.

All the debris appears to fly underneath the wing, striking the lower half of Columbias left wing and impacting panels of reinforced carbon-carbon designed to take the high heating of reentry.

Initial foam impact experiments have already yielded a surprising result.

A preliminary finding suggests that a striking blow from high-speed foam may harm aluminum structure in the wing, but the thermal protection covering might not show visible damage.

Still to be recovered by ground teams, according to a source here, is photographic gear that should have recorded the jettisoning of the external tank from Columbia as the space plane continued onward into orbit.

That camera equipment is on the underside of the space plane. If the gear survived the fall to Earth, imagery that was taken might reveal more about what came off the large external tank and from where. The fuel tank itself is destroyed as it tumbles through the air and crashes into a remote ocean area.

"We are narrowing down the part of the geography of the orbiter where the assault seems to have taken place," said Columbia Accident Investigation Board (CAIB) chairman, retired Admiral Hal Gehman.

"The Board has some testing initiatives that we want to get started. Just laying the debris out on the floor [at the Kennedy Space Center] is not enough to find the answer to this mystery," Gehman told reporters here at a March 11 meeting.

"We need to do metallurgic testing, heat testinga whole series of tests need to be conducted," Gehman said. "At the same time we have to be careful that we dont, in any way, disturb or remove anything that might be evidence."

Major General John Barry, a CAIB member, said that work is underway to identify an object detected by ground radar floating away from or near Columbia early in its 16-day mission.

Barry said that experts at Wright Patterson Air Force Base in Ohio are studying radar signatures of test items that could have come off Columbia.

Several radar systems are being used, he said, in the hopes of finding a match with radar scans of the flotsam seen floating by the orbiter on day two of the crafts lengthy stay in space.

In addition, reflectivity and luminosity tests comparing those with known items -- may help classify the nature of the object.

Spotting an object floating away from a shuttle is not unusual, Barry told SPACE.com.

Ice has come off, screws, washers, thermal blankets, and other items can be set free, much of this associated with an orbiters cargo doors being open, he said.

"The question is, was that [object] something that was off the left wing? It was not sighted by the astronauts or reported. So we have to go back to the detective story on radar signatures that might help us find out what that object was," Barry said.

Gehman would not state a date on when he thought the CAIB will conclude its work. "If we dont know anythingwe cant say anything," he said.

"We cant possibly work any harder than nowseven days a week4,000 to 5,000 people every day of the week searching for debris with every one of them covering about three acres at a shotalong with divers, pilots, helicopters," Gehman said.

"The energy level is still very high, both among those finding debris, the debris analysts, and this Board. I would not put any kind of time frame on it. Itll be a lot of weeks," Gehman concluded.