Several witnesses in north Texas claim to have heard the space shuttle Columbia split apart, despite the fact that the craft was 40 miles above the Earth. Is it really possible to hear catastrophic events that occur at such a high altitude?
It's most likely that the Texans mistook an amalgamation of post-incineration sonic booms for the Columbia's fatal moment. By now, Sun Belters are used to the thunderous cracks caused by a shuttle's re-entry. As the vehicle crosses from space into the atmosphere, its supersonic descent stirs up shock waves by compressing the surrounding air. The Physics 101 analogy is the way a boat causes ripples as it moves through the water—the faster and bigger the boat, the more turbulent the ensuing ripples. The space shuttle is both fast (the Columbia was traveling at 18 times the speed of sound when it broke up) and big (approximately 122 feet long), so the shock waves it creates are mighty. When they finally reach a listener's ears, the resulting sound resembles two sharp booms in sequence: One is caused by the waves emanating from shuttle's nose, the other by those shimmying off the tail.
In the case of the Columbia, however, the craft was split into thousands upon thousands of debris chunks, each of which produced a smaller sonic boom of its own. The rapid series of relatively diminutive booms could easily be mistaken for an explosion. Plus, the blastlike sound of the booms may have been enhanced by an aural effect known as atmospheric scattering, whereby the shock waves disperse in midair and play tricks on the observers' ears. People who've heard shuttle-caused sonic booms often speculate that the reverberation is caused by a reflection off mountains or other geographic features; in fact, these effects are merely caused by the spreading out of the waves as they make their way toward the ground.
It is not impossible that some observers heard the actual rending apart of the Columbia, but the final verdict on the veracity of their claims will have to wait for the investigation to be completed. If an explosion occurred, the blast would indeed release energy in the form of audible shock waves (although they might be hard to differentiate from the debris-caused booms). On the other hand, if the ill-fated shuttle merely disintegrated, the odds of hearing that event from 40 miles below are rather slim.
(Could ejector seats have saved the Columbia astronauts? Click here to find out.)
Explainer thanks Richard Raspet of the University of Mississippi, Ken Plotkin of Wiley Laboratories, and William van Moorehm of the University of Utah.