The infrasound signal (too low in frequency for humans to hear) was much longer, lasting nearly an hour after the sonic boom. “We're still not clear why this is the case,” says Henry Bass, leader of the infrasound team that studied the data and director of the University of Mississippi's National Center for Physical Acoustics. “We're not sure if this indicates something about the fragments or pieces that were actually flying along with the shuttle or not.”

Scientists have analyzed recordings made at 13 sites the morning Columbia broke apart and compared them with observations from three earlier shuttle reentries. Data contains hints of a catastrophic event near the California-Nevada border, but Bass says early reports of an explosion may have been too hasty. Researchers are still analyzing the sound data to figure out exactly what it means. Comparing it 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 the 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 findings show that although the data could be taken to mean an explosion or other catastrophic event, it could also indicate a change in the shuttle's angle. Says Bass, ruling out potential causes is one of the data's biggest contributions. The infrasound data discounted extraneous claims, such as the shuttle being struck by a meteor or lighting.