Rare Daytime Fireball Created by Minivan-Size Space Rock
A meteor lights up the daytime sky above Reno, Nevada on April 22, 2012 in this amazing photo by skywatcher Lisa Warren. Scientists recovered fragments from the so-called Sutter's Mill meteorite in California after an intense search.
Credit: Lisa Warren

A surprise space rock explosion that lit up the daytime sky over parts of California and Nevada this week was no tiny meteor. NASA scientists say the fireball was created by a minivan-size asteroid that triggered a loud sonic boom as it broke apart and streaked through Earth's atmosphere.

The fireball was spotted —and heard — on Sunday (April 22) when a space rock slammed into the Earth's atmosphere and ignited into a dazzling fireball at around 8 a.m. PDT. According to NASA's meteor expert Bill Cooke, the light show was created by the disintegration of a rock that weighed about 154,300 pounds (about 70 metric tons).

When the fireball broke apart, it released energy equivalent to a 5-kiloton explosion, Cooke explained. The powerful event was witnessed from as far north as Sacramento, Calif., and as far east as North Las Vegas, Nev.

A study by astronomer Elizabeth Silber, a scientist with the Meteor Group at the Western University of Canada in Ontario, found that the Sunday fireball likely exploded in the planet's upper atmosphere, high over California's Central Valley. Silber used sound detectors to map the location where the meteor exploded. [See an eyewitness photo of the fireball]

The red bullseye indicates the location where a meteor exploded over California's Central Valley on April 22, 2012. The yellow triangles mark infrasound arrays, which were key in determining the location of the meteor's explosion.
The red bullseye indicates the location where a meteor exploded over California's Central Valley on April 22, 2012. The yellow triangles mark infrasound arrays, which were key in determining the location of the meteor's explosion.
Credit: WUC/Google

While daytime fireballs like Sunday's event are relatively rare, what makes this week's fireball even more unique is the apparent large size of the space rock that spawned it.

"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," Don Yeomans, of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif., said in a statement. "Fireballs you can see relatively easily in the daytime and are many times that size – anywhere from a baseball-sized object to something as big as a minivan."

But actually seeing a fireball in the daytime sky is a special skywatching treat, Yeomans said.

"An event of this size might happen about once a year," said Yeomans. "But most of them occur over the ocean or an uninhabited area, so getting to see one is something special."

And while Sunday's fireball occurred just after the peak of the annual mid-April Lyrid meteor shower, it was likely not a Lyrid meteor, according to Cooke, who leads NASA's Meteoroid Environments Office at the agency's Marshall Space Flight Center in Huntsville, Ala.

"[W]ithout a trajectory, I cannot rule out a Lyrid origin, but I think it likely that it was a background or sporadic meteor," Cooke said.

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