By Matthew Fordahl
The Associated Press

SAN JOSE, Calif. - As the space shuttle program took shape in the late 1960s, engineers were confronted with finding technology that could withstand the heat of re-entry without being too heavy, unreliable or costly. They settled on the small silica tiles that brought shuttles home safely more than 100 times, but have fallen under suspicion in Saturday's loss of Columbia as it descended over Texas.

The cause of the accident has not been determined, though a temperature rise on the craft's left side just before it was destroyed suggests some tiles may have been missing or damaged, NASA officials said.

Thermal tiles are crucial to the safety of space shuttles. Thousands of tiles are attached to the shuttle's wings, underbelly and nose, protecting the crew from temperatures exceeding 2,000 degrees as they return to Earth.

The decision to use tiles was reached after years of study. And the alternatives simply weren't practical, experts say.

"That was the cheapest way they could deal with dissipating the heat even though it was cumbersome," said Norm Carlson, a former operations chief at launch control at the Kennedy Space Center.

The tiles met the specification that they could survive as many as 100 re-entries. Though they cost as much as $2,000 each, they also met the requirement that the technology not break the shuttle budget.

"It was an enabling technology because it's the best of all heat shield candidates," said Kevin Forsberg, program manager for the tile project at Lockheed Martin in the 1970s.

Researchers considered several heat-protection options in the 1960s, including a metallic shield first used in a manned space-plane,and an active cooling system in which a stream of cold gas is blown at the heated surface during re-entry.

They also explored using the heat shields that were used by spacecraft prior to the shuttles, including the Apollo moon missions.

But none of the systems proved effective.

The silica tiles were based on a material first developed by Lockheed chemist Robert Beasley, who joined Lockheed Missiles and Space Co. in the early 1960s.

The usefulness of his creation wasn't immediately known.

"Lockheed refused to file a patent, saying there was no market for it," Forsberg said. "It was put on the shelf and his research stopped for two years. Then interest from the shuttle program revived it."

Beasley was awarded $90,000 by Lockheed for helping it win the tile contract. He retired from Lockheed in 1977 and died in 1997 at age 70.

Eventually, thousands of the custom tiles would be built at Lockheed's plant in Sunnyvale, Calif. Each had to be carefully placed on the skin like a massive mosaic when the shuttles were built in Southern California.

From the earliest tests, some tiles fell off the orbiter. Even so, using processes Beasley helped develop, production and installation became so refined that the work could be done at Kennedy Space Center and elsewhere.

Despite the advancements, NASA shuttle program director Ron Dittemore said Sunday the tiles are a work in progress.

"We've been spending money for many years to try to develop this technology," he said. "We haven't been successful yet to make an improvement over the existing tile, but we're making progress."