Just after 10 a.m. during the second of Columbia's 16 full days in orbit, something drifted away from the shuttle at the speed of a brisk walk.
Columbia was flying at 17,500 mph, tail-first and upside down, its open cargo bay pointing toward the planet below.
The object, measuring no more than 1 square foot, began to spin faster and faster. It took 2 1/2 days to fall out of orbit and burn up in the atmosphere.
It twirled and twinkled as it fell to Earth.
Apparently unseen by human eyes, it was recorded by military radar and stored in a computer.
There the reading stayed until after the accident, when Air Force technicians discovered it among thousands of automatic radar sweeps of Columbia in space.
In the surveillance files of the Air Force 21st Space Wing, the electronic traces of the mysterious object amounted to little more than squirts of static.
Had something drifted out of the open cargo bay on Jan. 17?
NASA investigators checked the lost-and-found log at Kennedy Space Center and discovered seven missing tools -- an Allen wrench, screwdrivers, a flashlight and pliers.
Had the shuttle been hit by something in orbit?
Investigators checked mission records to see if Columbia had shuddered from the impact of a meteoroid or orbital debris.
They found nothing out of the ordinary.
Only one place had a chance of deciphering the faint radar signal -- the Hog Works, a windowless laboratory behind two sets of locked security doors at Wright-Patterson Air Force Base near Dayton, Ohio.
Inside, signs warned against classified discussions in the hallways. A red security light flashed in warning when the janitor came by to mop the floors.
When a visitor entered the building, a researcher made a timeout gesture with his hands like a football referee, indicating that conversations should stop.
Technically, the radar range was called an occult chamber.
The sealed room was the size of a strip mall cinema. Its walls, floor and high ceiling were covered with thick layers of blue sculpted baffles that resembled the sound-scattering acoustic clouds in a concert hall.
In the center was a 40-foot-high pylon. Researchers secured targets at the top and bombarded them with radar waves.
The technical trick of the range was to allow researchers to see how objects appeared on radar at distances of hundreds or even thousands of miles.
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.
In essence, it was a drawing that consisted of a single jagged line.
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.