The infrasound signal was much longer than those associated with other shuttle reentries to Earth's atmosphere, lasting almost an hour after the sonic boom, the scientists reported today while meeting at the University of Mississippi.

"We're still not clear why this is the case," said Henry Bass, leader of the infrasound team that studied the data.

"We're not sure if this indicates something about the fragments or pieces that were actually flying along with the shuttle or not. Given all the other things that affect the received signal, we really don't have enough experience with this to sort it all out."

Bass is director of the National Center for Physical Acoustics at UM. His team reported its findings to the U.S. Department of Defense at the end of March. The report is being released to the public this week.

Infrasound is the part of the audio spectrum too low in frequency for humans to hear. From major events such as explosions or volcano eruptions, infrasound can travel thousands of miles, making it an effective detection tool for nuclear tests.

Scientists have analyzed recordings made at 13 sites the morning of Feb. 1 and compared them with observations during three earlier shuttle reentries.

Data contains hints of a catastrophic event near the California-Nevada border, but early reports that stations heard an explosion aboard Columbia were premature, said Bass.

"It's really not clear to us if there was an explosion," he said. "The breakup may or may not have been explosive in nature."

More analysis will be necessary to determine exactly what the sound data means, Bass said. Comparing data from Columbia with earlier reentries is difficult because most of the infrasound stations are relatively new and because Columbia returned to earth on a trajectory last used by a shuttle in 1998.

In addition to Columbia's lengthy reentry sound, researchers are analyzing spikes in the sound signal near the time when NASA lost radio contact with the shuttle.

The report concludes that although the data could be interpreted as indicating an explosion or other catastrophic event, it also could indicate a change in the shuttle's angle.

Ruling out potential causes is one of the data's biggest contributions, Bass said. The infrasound data discounted "extraneous claims," such as the shuttle's being struck by a meteor or bolide, or lightning.

Bringing together a diverse group of people to examine the data, with a high level of cooperation between agencies, bodes well for the future, Bass said. "We now know how to pull this team together to get definitive results."

Five of the locations recording Columbia's infrasound are part of the International Monitoring System, a 60-station network to monitor compliance with the Comprehensive Test Ban Treaty.

Data also was obtained from stations funded by the U.S. departments of Defense and Energy, the National Oceanic and Atmospheric Administration, and the U.S. Army Research Laboratory.

Besides NCPA, team members also included representatives from BBN Technologies Inc., the Center for Monitoring Research, Los Alamos National Laboratory, NOAA Environmental Technology Laboratory, U.S. Naval Research Laboratory, the Laboratory for Atmospheric Acoustics at the University of California-San Diego and the University of Hawaii's Infrasound Laboratory.

IMS stations providing data for the study are in Pinon Flat, Calif.; Fairbanks, Alaska; Kona, Hawaii; Newport, Wash.; and Lac du Bonnet, Manitoba. Data also was from stations in St. George, Utah; Los Alamos, N.M.; Pinedale, Wyo.; Mina, Nev.; Lajita, Texas; Boulder, Colo.; and White Sands Missile Range in New Mexico. Except for Alaska and Hawaii, all locations recorded clear signals from the shuttle reentry.

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