Heat built up in Columbia just before explosion

Dittemore said that initially, NASA’s experts thought the glancing blow was insignificant, but they now are looking at the possibility that it might somehow be connected to the crash. He said that no one saw the piece of insulation strike the wing on January 16, and it wasn’t until technicians reviewed film of the take off a day later that they realised it had happened.

‘‘The technical experts believed the degree to which that hit the orbiter was inconsequential,’’ Dittemore said. He said NASA realised that there was nothing that could be done if the experts were wrong.

Columbia’s crew, told what had happened, had no way to examine the outside of their spacecraft and make repairs, Dittemore said, and the shuttle already was scheduled to fly a path ‘‘that minimises wear and tear’’ on re-entry.

A witness in eastern California saw something fall from the shuttle, Dittemore said, adding that the report was an important part of the investigation.

On Saturday, the agency had downplayed California Institute of Technology astronomer Anthony Beasley’s observations, suggesting that he had seen only the hot plasma that routinely builds up around a space shuttle during re-entry.

The fragile thermal tiles that protect the space shuttle on its fiery re-entry to Earth’s atmosphere are extremely effective in dissipating heat, but they also have a history of problems that have made them a constant concern.

Black tiles known as high-temperature reusable surface insulation are bonded to the bottom of the orbiter. Made out of very pure silica — a missing or damaged tile can allow heat to penetrate to the aluminum structure of the shuttle.

‘‘Surface heat dissipates so quickly that a tile can be held by its corners with a bare hand only seconds after removal from a 2,300-degree oven, while the centre of the tile still glows red with heat,’’ a NASA briefing paper said.

Damaged tiles can also change the aerodynamic profile of the wing, increasing drag and heat and possibly precipitating further damage.

The problem repeatedly emerged on recent launches.

In 1988, the shuttle Atlantis suffered what was described as ‘‘minor damage’’ when insulation peeled away from the external tank 85 seconds after liftoff.

In 1997, Columbia’s tiles suffered ‘‘multiple divots’’ after the external tank’s extensively patched insulation cover came apart during the launch.

Most recently, the Endeavor lost insulation on ascent out of Cape Canaveral in February 2000. Remains of the seven Columbia astronauts have been recovered and identified, officials said, but they declined to disclose the identities. (LATWP)