BOULDER, COLORADO -- Once space shuttle Columbia settled into orbit, numerical nomenclature was assigned to the space plane, a routine procedure done by the U.S. Air Force Space Command and NASA.

In this case, Columbia was given an international ID of 2003-003A and catalogued as number 27647.

The space plane joined upwards of 9,000 other human-made objects now adrift in Earth orbit - at least of a size capable of being tracked by a worldwide network of ground-based equipment.

In the early part of Columbia's mission, an object was seen coming off the space plane - spotted by Air Force Space Command radar tracking gear. That piece reentered Earth's atmosphere within three days while the orbiter continued on its science flight.

Joel Williamsen, an orbital debris specialist at the University of Denver Research Institute's Center for Space Systems Survivability in Denver, Colorado, said the puzzling object moving away from Columbia makes it all the more curious of an event.

Whipping through the space environment, a lump of ice should have quickly evaporated, Williamsen said.

It is possible that the space plane was using its maneuvering jets, Williamsen added, causing the mystery piece to float away at whatever speed Columbia itself was steering away from the object.

"I really think the evidence of that object moving away does point to the possibility of an impact event," Williamsen said. "It is going to have to be a fairly large piece in order for Space Command to track it. That means grapefruit size or better. Now you are starting to talk about the size of a tile," he said.

Williamsen said that orbital debris or a meteoroid could produce a small hole on entrance, and a fairly large, shotgun-like kind effect that rippled through other parts of the space plane.

One or two tile penetrations could lead to a "zipper effect", Williamsen speculated. When Columbia started to enter the atmosphere nose high, as all shuttles do, its tail section first received escalating temperatures. Heat then slowly moved forward to the nose area.

"So you are getting flows in all kinds of different directions," Williamsen said. Tiles that may have been struck over the left wheel well could have led to other tiles giving way. That zipper effect might snake its way forward, even compromising the leading edge of Columbia's left wing, he said.

William Bohl, Co-Director of the Center for Space Systems Survivability, said that damage to Columbia's tiles during liftoff remains a leading candidate in his mind for what caused the accident. Foam chunks falling off the shuttle system's huge external fuel tank and striking the underside of the space plane is the most likely situation, he said.

"But it could be other things too," Bohl said. A debris strike or a meteoroid hit of the space plane in orbit could have done similar damage, he added.

Bohl said a damaged tile from liftoff could have been the object seen by ground radar floating away from Columbia. But a force would have been needed to jar it free of the space plane, he noted.

"I'm sure all the options and possibilities are being looked into," Bohl said.

The idea of Columbia being stuck in orbit is downplayed by Robert Culp, a space debris and reentry expert here at the University of Colorado. "When you get hit by space debris of the smallest kind, it rings that space shuttle like a bell," he told SPACE.com .

Culp said there's no evidence for any impact of the space plane while it circled Earth.

"If there was anything big enough to make a hole in the insulation and into the aluminum that they expect now, then everybody would have come unglued. It would have been another 'Houston, we've had an onboard disaster here'like Apollo 13," Culp said.

Regarding the object drifting free from Columbia early in its mission, Culp said that is not uncommon. Over the years, everything from items floating out of the cargo bay to a chunk of ice have been detected near a space shuttle on orbit, he said.

Culp said, however, that free-floating ice should dissipate in space, perhaps fracturing into smaller pieces, and eventually disappearing.

"To me it's up in the 90 percent range that during launchthat's when the damage occurred. There's tremendous noise and vibration. Any astronaut would tell you that they can't hear themselves think during launch," Culp said.

The words "foam" and "tiles" are misleading the public, Culp said. "Everybody thinks you've got shaving foam hitting bathroom tile. But after it's put on [the shuttle's external tank], that foam hardens to concrete. It is more like the density of lava rock or pumice stone that you use in your garden," he said.

Culp said that foam used on the shuttle's external tank was free of Freon, removed in response to environmental concerns.

"Taking the Freon out made that stuff more brittle and created what became known as the 'popcorn effect', with pieces of the insulation popping off and knocking chunks out of the tiles. They did not have problems with that insulation before they made that change," Culp said.

According to NASA, communication and tracking of the shuttle was lost at an altitude of about 203,000 feet above north central Texas while speeding at some 12,500 miles per hour (Mach 18).

That dive through the Earth's atmosphere rapidly slowed Columbia down "like somebody hitting the water after they jump off the diving board," Culp said. That being the case, everything inside the space plane was slowing down too. He estimates that 90 percent or 95 percent of everything that was Columbia reached the ground.

"We had gotten to where we just thought this was a glider coming in over and over again with no problems," Culp said.

"We had forgotten that it's very dangerous dealing with very high speeds, tremendous aerodynamic forces and heating. All that has to be dealt with before you can turn a shuttle into a glider to land," Culp said.

To what extent the U.S. military offered help, or was requested to use its satellites to eye Columbia before or during its fatal plunge is not known.

First of all, any U.S. military space hardware that might have focused on Columbia would have generated classified data. The military is skittish about handing out information that could reveal the capabilities of its spacecraft.

For instance, satellite constellations -- such as Defense Support Program (DSP) series -- are on-duty to detect and track missile launchings around the globe. Other spacecraft monitor weather over areas of military interest.

In some cases, spaceborne equipment is specifically trained on orbiting objects, including the space shuttle. Doing so helps calibrate satellite sensors, as well as teach ground operators the fine art of spacecraft tracking.

However, given political hot spots at the moment, just how available such space assets were to specially target Columbia isn't clear.

The U.S. Air Force is not saying anything about what Air Force Space Command-owned sensors captured of Columbia's flight or reentry. "Since this is an ongoing investigation, it wouldn't be appropriate for us to comment," said Lt. Col. Andy Roake, a spokesman for the Space Command at Peterson Air Force Base, Colorado.

Roake told SPACE.com that all data that may be pertinent to the investigation is being sent through the U.S. Strategic Command (USSSTRATCOM) to those investigating the catastrophe. USSTRATCOM has organized a team that is gathering and giving Department of Defense data to the Columbia Accident Investigation Board, he said.