Tiny meteorites found in northern California's Sierra foothills were part of a giant fireball that exploded with about one-third of the force of the atomic bomb dropped on Hiroshima, scientists said.
The rocks each weighed about 10 grams (0.35oz), or the weight of two US nickels, said John Wasson, a long-time professor and expert in meteorites at UCLA's Institute of Geophysics and Planetary Physics.
Experts say the flaming meteor, dating to the early formation of the solar system four to five billion years ago, was probably about the size of a minivan when it entered the Earth's atmosphere with a loud boom early on Sunday. It was seen from Sacramento, California, to Las Vegas and parts of northern Nevada.
An event of that size might happen once a year around the world, said Don Yeomans of Nasa's Near-Earth Object Programme Office at the Jet Propulsion Laboratory in Pasadena, California. But most of them occurred over the ocean or an uninhabited area, he said.
"Getting to see one is something special," he said. He added, "Most meteors you see in the night's sky are the size of tiny stones or even grains of sand, and their trail lasts all of a second or two."
The meteor probably weighed about 154,300lbs, said Bill Cooke, a specialist in meteors at Nasa's Marshall Space Flight Centre in Huntsville, Alabama. At the time of disintegration, he said, it probably released energy equivalent to a five-kiloton explosion. The 1945 Hiroshima bomb was 15 kilotons.
The boom, another expert said, was caused by the speed with which the space rock entered the atmosphere. Meteorites enter Earth's upper atmosphere at speeds of up to 44,000mph - faster than the speed of sound, thus creating a sonic boom.
The friction between the rock and the air is so intense that "it doesn't even burn it up, it vaporises", said Tim Spahr, director of the Minor Planet Centre at Harvard University.
Prof Wasson said one meteorite was found near the town of Coloma, north east of Sacramento. "I'm sure more will be found, I'm hoping, including some fairly big pieces," he said. "The fact that two pieces already have been found means one knows where to look."
Bits of the meteor could be strewn over an area as long as 10 miles, most likely stretching west from Coloma, where James Marshall first discovered gold in California, at Sutter's Mill in 1848.