Infrasound
Infrasound is sound too low for humans to hear, which large fireballs produce and distant sensors can detect.
Infrasound is sound with frequencies below about 20 hertz, too low for human ears. It travels very long distances through the atmosphere with little loss, sometimes right around the world.
Large fireballs create powerful pressure waves, including infrasound. A worldwide network of infrasound stations, set up to check that nations obey the treaty banning nuclear tests, also records the airbursts of meteoroids. The Chelyabinsk fireball of February 2013 was detected by stations thousands of kilometres away, and its signal was used to estimate an energy of about 440 to 500 kilotonnes of TNT.
Infrasound helps scientists measure how often large meteoroids hit Earth, including over oceans and remote areas where nobody sees them. Records over many years suggest that objects releasing energy equivalent to a kilotonne or more strike our atmosphere a few times a year on average, mostly far from people.
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