When it comes to small space rocks blowing up in Earth’s atmosphere, not all days are created equal. Scientists have found that, contrary to what they thought, such events are not random, and these explosions may occur more frequently on certain days. Large objects can survive a trip through Earth’s atmosphere relatively intact, but many smaller bodies break up at high altitude, sometimes in an immense burst of energy. Researchers used data from sensors designed to detect clandestine nuclear tests, among other sources, to identify airbursts with an energy equivalent to or larger than that released by 1 kiloton of exploding TNT. Between 2000 and 2013, they identified 33 such events (including the meteoroid that blazed into the atmosphere and detonated over Chelyabinsk, Russia, in February 2013, see image; the large blip in the meteor trail at right denotes where the 500-kiloton airburst occurred). Of those events, there were nine pairs of explosions—or airbursts—that occurred within one calendar day of each other, the researchers will report in January in the Monthly Notices of the Royal Astronomical Society. There’s a less than 2% chance of finding nine such pairs in a random sample, the researchers note. The data set also sports 16 pairs of events with three or fewer calendar days’ difference, which for a random sample could be statistically expected only about 2.2% of the time—a number of coincidences that is simply too high to be the result of chance alone, the researchers contend. Rather than random occurrences, many large airbursts might result from collisions between Earth and streams of debris associated with small asteroids or comets. The new findings may help astronomers narrow their search for objects in orbits that threaten Earth, the researchers suggest.

http://news.sciencemag.org/earth/2014/11/space-rock-impacts-not-random