Celestial population boom

The number of car-to-house-sized meteoroids whizzing through the Earth’s neighborhood is about 10 times higher than Earth-based telescopic surveys suggest, a new study reveals.

That finding, reported online August 28 and in an upcoming Journal of Geophysical Research–Planets, comes from analyses of recently declassified data on infrasonic waves in the atmosphere detected between November 1960 and April 1972. The network of instruments collecting the data was originally designed to detect low-frequency sound waves produced by aboveground nuclear tests, says Elizabeth Silber, a planetary physicist at the University of Western Ontario in London, Canada.

But during the 13-year period, the instruments also detected infrasonic waves produced by 13 meteoroids. Such waves are generated in prodigious amounts as extraterrestrial objects blaze through the upper atmosphere, where many shatter before reaching the Earth’s surface (SN: 7/19/03, p. 36). The waves can travel thousands of kilometers because they aren’t effectively damped by the atmosphere.

The smallest fireball spied by the network released energy equivalent to that packed in about 1,700 tons of TNT when it broke apart in January 1965. The largest detected explosion occurred in August 1963 and measured around one megaton, Silber and her colleagues report.

From their data, the researchers estimate that about once each year, objects large enough to cause an airburst of 11 to 12 kilotons enter Earth’s atmosphere. Once every 15 years or so, a meteoroid large enough to trigger a 1-megaton airburst slams into the atmosphere. The sizes of these explosions suggest that the incoming objects measured between 5 and 20 meters across. Each of these impact rates is around 10 times higher than that inferred from a recently published telescopic survey, but the results are consistent with previous mathematical predictions, the researchers note.