James Hallock discovered just how little it takes to bring down a space shuttle.

He did it by playing with pencils.

As a member of the Columbia Accident Investigation Board, the pear-shaped, bewhiskered expert on flight safety had a New Englander's flinty skepticism and a physicist's distaste for untested accident theories.

On this day, Hallock, 62, scowled at specifications for the reinforced carbon panels that shielded the leading edge of Columbia's wings from the heat of reentry.

If one of the $800,000 panels had cracked, it might have been the flaw that on Feb. 1 caused the $1.8-billion spacecraft and its crew of seven astronauts to plummet in a shower of molten debris across six states.

Hallock brooded over a simple question: What would it take to break one?

Engineers gave him the original 25-year-old NASA specifications, which said the panels must withstand an impact equal to the kinetic energy of 0.006 foot-pounds. What did that mean? Hallock twiddled a yellow No. 2 pencil between his fingers. How far, for instance, would he have to drop the pencil to generate that kind of impact?

In the mailroom at the board's makeshift headquarters in Shirlington, Va., he found a box of pencils and a postal scale.

He weighed the pencils, then calculated the mass of the average pencil. With that number, he worked out how much punch each pencil would pack.

"The answer," he would say later, "is that a No. 2 pencil dropped from about 6 inches equals the kinetic energy number they had."

The panels, in actual manufacture, were much stronger. But that remarkably low standard, Hallock believed, was a tangible measure of how confident NASA engineers were that nothing would hit the leading edge of the wing.

"The number didn't matter to them," Hallock said, "because they assumed nothing would ever hit the shuttle."

Call it forensic engineering or, more plainly, detective work.

The Columbia accident investigation was the most exhaustive scientific inquest ever undertaken.

Suspicion led down a hundred blind alleys. Investigators quarreled. Mission insiders tried to control the probe. Outsiders railed about secrecy.

The investigators mustered the most sophisticated techniques that science could devise — X-ray scanners, neutron beam machines, hypersonic wind tunnels.

They also used red food coloring, a bicycle pump, a hobby-shop hacksaw and a steam iron.