Recordings made by instruments sensitive to sound below the threshold of human hearing may help investigators build a timeline of any uncharacteristic movements made by the space shuttle Columbia minutes before it broke apart, scientists say.
The instruments also captured an explosion high over Texas that one scientist said could have been Columbia's cabin rupturing.
As parts of Columbia began to break off as the shuttle streaked across the West, the flight behavior of the normally streamlined spacecraft would have changed. Those changes would have generated distinctly different patterns of sound waves compared to previous shuttle flights.
The patterns, recorded on the ground by instruments in Texas, Nevada and elsewhere in the West, are now being examined as part of the Columbia disaster investigation.
Any abnormal patterns can help investigators establish the timing of events as the shuttle entered the Earth's atmosphere Feb. 1, said Keith Koper, a geophysicist at Saint Louis University in Missouri.
Investigators already know from sensor data sent from the shuttle in its final minutes -- supported by eyewitness reports, photographs and video footage -- that Columbia's cascade of problems began while the spacecraft was still over the Pacific Ocean.
The sensors indicated increasing heat as well as increased drag on shuttle's left wing, suggesting it was somehow damaged, perhaps from the impact of a chunk of hard foam that broke off the external fuel tank and hit the wing shortly after liftoff Jan. 16.
Investigators have said they suspect that data mean Columbia was already dropping debris over the West, several minutes and hundreds of miles before it broke apart high over Texas. All seven crew members were killed.
One array of the sound-sensitive instruments, located near Big Bend National Park in southwest Texas, recorded sound waves from Columbia as it was over West Texas indicating an explosion equivalent to a few pounds of TNT, said Eugene Herrin, a geophysicist at Southern Methodist University.
"Our guess is that it could have been caused by a rapid decompression, which is what would have happened if you ruptured the crew compartment," Herrin said.
He said an initial analysis of data collected in Columbia's wake by another array of microbarometers, outside Mina, Nev., showed "unusual" patterns when compared to data from other shuttle flights passing overhead. The instruments record minute pressure changes caused by infrasound, or sound waves below about four cycles per second that are inaudible to humans.
"There was something about this one. I am not going to speculate. What we see are oscillations in the shock wave that we don't normally see. Whether that's diagnostic or not, that's a NASA call," Herrin said.
Space agency spokesman William Jeffs said Sunday that NASA would consider the information in the shuttle investigation.
Search crews have yet to find any confirmed shuttle debris more than 20 miles west of Fort Worth. A concentrated search in a canyon east of Albuquerque, N.M., on Saturday turned up nothing from the shuttle, officials there said.
Earthquake instruments throughout the West also picked up vibrations induced by Columbia's supersonic flight overhead, said Andrew Michael, a U.S. Geological Survey geophysicist in Menlo Park, Calif. That data also was sent to NASA.
Such seismic data was used to study the bombing of the U.S. embassy in Nairobi, the sinking of the Russian submarine Kursk and the collapse of the World Trade Center towers.