Tiny meteorites found in the Sierra foothills of northern California were part of a giant fireball that exploded over the weekend with about one-third the explosive force of the atomic bomb dropped on Hiroshima in World War II, scientists said Wednesday.
The rocks each weighed about 10 grams, or the weight of two nickels, said John T. Wasson, a longtime 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 4 to 5 billion years ago, was probably about the size of a minivan when it entered the Earth's atmosphere with a loud boom early Sunday. It was seen from Sacramento, Calif., 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 Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. "But most of them occur over the ocean or an uninhabited area," he said.
"Getting to see one is something special," he said.
"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," he added.
The meteor probably weighed about 154,300 pounds, said Bill Cooke, a specialist in meteors at NASA's Marshall Space Flight Center in Huntsville, Ala. At the time of disintegration, he said, it probably released energy equivalent to a 5-kiloton explosion. The Hiroshima bomb was 15 kilotons.
"You don't often have kiloton rocks flying over your head," he said.
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 somewhere between 22,000 mph and 44,000 mph -- faster than the speed of sound, thus creating a sonic boom.
Wasson said one meteorite was found near the town of Coloma, about 35 miles northeast of Sacramento. "I'm sure more will be found, I'm hoping, including some fairly big pieces," he said.
Robert Ward, who lives in Arizona and has been hunting and collecting meteorites around the world for more than 20 years, said he found the first piece about 10 a.m. Tuesday between a baseball field and park on the edge of the town of Lotus.
Ward said he "instantly knew" it was a rare meteorite known as "CM" -- carbonaceous chondrite -- based in part on the "fusion crusts from atmospheric entry" on one side of the rock.
"It was just, needless to say, a thrilling moment," he said.
Wasson suspected that hundreds of dealers and collectors already have joined the search for meteorites. He said it was important to recover them soon because any rain will cause them to degrade, losing their sodium and potassium. "From my viewpoint as a meteorite researcher," he said, "I'm hopeful some big pieces are found right away."