Typically, a radar image could contain measurements recorded at 1,600 different frequencies, etching a richly detailed three-dimensional picture. The object leaving Columbia had been recorded in just one wavelength.
From the way the unidentified object had scattered the radar beam, the researchers deduced that it was smaller than the wavelength of that frequency, about 27 inches. Analysis of the way it fell to Earth gave them some ideas about its weight and shape.
In April, a team led by Brian Kent, a 46-year-old expert in radar measurements, began screening materials and objects aboard the space shuttle based on how they scattered radar energy along that one wavelength.
One by one, Kent positioned each target at the top of the pylon and bombarded it with radar waves. The team tested 31 items from NASA's inventory — shuttle tiles, seals, spar insulation, cargo bay blankets and other materials. They also tested four scraps of debris recovered from Texas.
The radar signal and the ballistic measurements matched only one item: a fragment of the curved reinforced carbon- carbon panel from the leading edge of the orbiter's wing.
But what had knocked it loose?
It was the first to be suspected, the first to be denied and the most credible possibility remaining after months of sophisticated second-guessing.
During every launch for 22 years, the foam insulation coating the shuttle's 15-story external fuel tank had flaked off like dandruff.
The debris that struck Columbia's wing 81.9 seconds after liftoff Jan. 16 was the largest piece of foam ever to strike a shuttle.
NASA Administrator Sean O'Keefe had publicly dismissed concerns, comparing the debris to "a Styrofoam cooler blowing off a pickup truck ahead of you on the highway."
More than 2 tons of foam insulated the tank. Most of it was applied precisely by computer-controlled robotics. A few hard-to-get-at areas still had to be done by hand. One such area was the source of the foam that hit the wing.
Trying to understand how NASA used the foam, Nobel laureate Douglas Osheroff, a member of the Columbia Accident Investigation Board, watched a video on his laptop computer.
In it, a technician at Lockheed Martin, maker of the shuttle's external fuel tank, sprayed streams of foam in broad, measured strokes across a tank brace.
Despite careful application, the hand-sprayed foam in the demonstration video expanded unevenly, leaving gaps and hollows that could fracture. When investigators cut into the foam on another tank, they discovered three air pockets near the same crucial point on the tank.
"Voids will be voids," quipped a technician to the camera operator.
He caught himself and spoke again into the video lens. "I hope you had the audio turned off."