"Although NASA is taking steps to protect the shuttle from orbital debris such as spent rocket bodies, satellite fragments, and paint chips, there still is a real risk that a collision could cripple the shuttle or threaten the safety of the crew," said Frederick Hauck, president and chief executive officer of AXA Space, upon release of the 1997 report. "NASA needs to obtain a more precise picture of the potential dangers and assess additional methods for reducing these threats."

For some missions, the report concluded, adding the debris threat to estimates of potential risks -- such as the risks during launch or re-entry -- almost doubles the likelihood of major damage or harm to the crew.

"Did we take some hit? That's a possibility," Milt Heflin, the flight director, told The Los Angeles Times. "Something was breached."

Heflin and other NASA officials have said repeatedly that they are not ruling out any possibilities for what caused Columbia to break apart. Further, there are no indications that an in-flight hit is being considered as any sort of prime candidate for the cause.

"Every spacecraft is being hit by small debris every day," Nick Johnson, the chief scientist for orbital debris at NASA's Johnson Space Flight Center, told SPACE.com Wednesday. He could not comment on what might have happened to Columbia.

More than 4 million pounds of junk from previous missions -- gloves, bolts, tools and shards of disintegrated satellites -- orbit the planet. There are at least 110,000 pieces of junk larger than 1 centimeter (0.4 inches).

Meteoroids as small as a marble, moving at several thousands of miles per hour relative to a spacecraft in orbit, can shoot right through unreinforced parts of a craft.

NASA has extensive evidence of pits and even one hole on the Hubble Space Telescope, from pictures taken during servicing missions.

On average, every square meter of Hubble gets hit by about 5 sand-grain-sized bits each year, says Johnson, the debris expert. Most of the pockmarks are less than 3 millimeters across, about the size an "o" in your newspaper.

Such debris is too small to see and avoid. Hubble also has a -inch hole in its high-gain antenna, caused by a larger impact and photographed during a 1993 servicing mission.

Post-flight examinations of previous shuttle missions have revealed meteoroid damage, too.

A meteoroid is any naturally occurring rock in space, down to and including dust particles. Upon entering Earth's atmosphere, where they typically vaporize, they are called meteors.

The NRC report in 1997 found that an object as small as -inch could punch a hole through the crew cabin wall, causing a loss of air pressure. An object puncturing a hole in the shuttle wing "could cause structural failure when the shuttle re-enters the atmosphere," the report concluded.

Engineers cannot possibly locate, let alone track, individual small pieces of space debris. Natural rocks are not tracked and only become apparent when they hit something or generate a streak of light or an explosive fireball in the night sky.

Each November in recent years, during the annual Leonid meteor shower, satellite controllers have taken steps to avoid strikes by small meteoroids. They point the crafts into the oncoming stream of material to create the thinnest profile possible. They also shut down all but crucial electronics, effectively putting some satellites to sleep.

Crewed spacecraft are vastly better protected against strikes by small objects. The shuttles and the International Space Station are designed, in theory, to withstand hits by marble-sized objects.

But a shuttle's aluminum skin is covered in ceramic tiles designed to protect the craft against the heat of re-entry. These tiles are brittle.

The leading idea for what went wrong suggests that a chunk of hardened foam insulation fell from the shell of the rocket booster used to lift Columbia into space. At more than twice the speed of sound, that foam appeared in a launch video to strike Columbia's left wing, possibly damaging one or more tiles.

But officials are not sure how much damage the foam caused. Heflin said a bit of space junk or small meteoroid might have caused or contributed to the damage.

Large space debris is tracked, and officials know of more than 9,000 pieces.

In mid-December of 2001, flight controllers made slight changes to the Space Shuttle Endeavour's departure from the International Space Station. The added time allowed a small jet firing by the shuttle to boost the station's future path away from a spent Russian rocket upper stage set a drift since the 1970s.