On Sunday 22 April a fireball was seen passing over Nevada and California generating a sonic boom as it went, before finally exploding over central California. Scientists estimate this explosion released energy the equivalent to that from 4-5 kilotons of TNT a height of about 10 km (the Hiroshima bomb was equivalent to 15 kilotons of TNT, but was closer to the ground). This occurred during the Lyrid Meteor Shower, but is thought to have been caused by a large solitary asteroid weighing about 70 tonnes impacting the atmosphere, separate from the Lyrids, which are small fragments from the tail of comet C/1861 G1 (Thatcher).
Since this time meteorite hunters (a meteor is an object observed in the sky, a meteorite a peice of rock on the ground) have been flocking to the area to look for pieces of the object, ahead of heavy rains expected later this week.
The first piece was found by Arizona-based meteorite hunter Robert Ward on Tuesday 24 April, by the side of a road in the town of Lotus, California. It is a piece of carbonaceous chondrite weighing about 10 grams. Carbonaceous chondrites are carbon rich meteorites, thought to have originated early in the formation of the solar system. They often contain amino acids and other organic compounds, and some scientists believe they may have played a role in the origin of life on Earth.
Robert Ward with his fragment of chondrite. AP.
Shortly after this a second chunk of similar size was found in a nearby parking lot by meteorite hunter Peter Jenniskens. Since this there have been several reports of other finds. Experts think that given the size of the explosion, material could be scattered over several kilometers, probably strung out in a rough line extending eastwards from Coloma California.
Map of California showing the area where the meteorites were found. ESRI/AP.
Objects of this size probably hit the Earth several times a year, but most are likely to fall over open oceans or other uninhabited areas. Scientists that study meteorites particularly value fresh specimens, as they have had less time to be altered by the Earth's atmosphere.
See also Fireball over Nevada and California, The Lyrid Meteors, Images of Vesta, What Hayabusa brought back from 25143 (Itokawa), and The composition of Comet C/2009 P1.
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