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Posted at 11:23 AM ET, 04/30/2012

A Lyrid meteor shower look back: Exploding fireball scatters debris over central California


Via NASA: A meteor in the sky above Reno, Nevada on April 22, 2012. Image credit: Lisa Warren
Those who saw the Lyrid meteor shower were treated to a spectacular show of celestial fireworks during the pre-dawn of Sunday, April 22. And in the central valley of California, a brilliant fireball exploded in full daylight Sunday morning!

(Unfortunately, sky watchers in most of the eastern U.S. missed the relatively rare opportunity to view the shower without the interference of moonlight due to clouds.)

The sonic boom generated by the explosion in California led to a flood of calls to 911. Some residents reported it caused some buildings to shake. Robert Lunsford of the American Meteor Society told the Associated Press soon after the event, “If you hear a sonic boom or explosion, that’s a good indication that some fragments may have reached the ground.”


Robert Ward displays two pieces of a meteorite he found at a park in Lotus, Calif., Wednesday, April 25, 2012. The rocks came from a meteor, believed to between 4 to 5 billion years old. Ward, who has been hunting and collecting meteorites for more than 20 years, said they are believed to be "one of the oldest things known to man and one of the rarest types of meteorites there is.” (Rich Pedroncelli - AP)
Two days later small remnants of the exploded meteor - weighing about a third of an ounce each - were found in the towns of Lotus and Coloma, California.

Bill Cooke, a specialist in meteors at NASA’s Marshall Space Flight Center in Huntsville, Alabama said that the meteor weighed about 154,300 pounds, about the “size of a minivan”.

The meteor appears to have remained intact until about 5 miles above the Earth’s surface, where it disintegrated releasing the energy equivalent to a 5-kiloton explosion – about a third of the size of the atomic bomb dropped on Hiroshima at the end of World War II.

Fireballs bright enough to be seen in daylight are relatively rare. Only two daylight fireballs are sighted per year on average, though (coincidentally or otherwise) two were seen on April 2, one over San Antonio, TX and the other over New Zealand. However, it appears neither of them exploded and likely either skipped through the upper atmosphere. Or, they completely vaporized without showering remnants that reached the ground. (Note: of course, they may be more daytime unobserved fireballs over the oceans or uninhabited regions.)

The most spectacular occurrence in recent history of a fireball seen during daytime was the Great Daylight 1972 Fireball. The meteor passed within 35 miles of the surface on August 10, 1972 sometime between entering the atmosphere over Utah and leaving the atmosphere over Alberta, Canada. The size of the Great Daylight 1972 Fireball was about the same size (9.8 feet) as the April 22 fireball.

What if either fireball had encountered the atmosphere head on rather than at the much steeper angle observed?

It’s most likely that atmospheric friction would dissipate the speed of the meteor form maybe 20,000 mph to near zero (except for normal gravitational fall velocity) as the meteor vaporized in the atmosphere.

What else could happen?

* It could explode as occurred April 22 but closer to the surface and cause severe damage locally from the effects of a pressure air blast.

* Large rock size fragments might reach the surface. For example, in southern Peru, the Carancas impact event of 2007 created a 50 foot wide crater . You may have also heard about the grapefruit size Hodges meteorite that crashed through the roof of Ann Hodges home in Alabama in 1954. The rock struck Hodges on the hip leaving a bad bruise and left her the only known person to be injured by a meteorite. More information.

While the chances of a populated area being affected by an exploding meteorite or large rock reaching the surface is exceedingly small, they are excluded from NASA’s list of near Earth objects tracked for possible impact with Earth. In other words, once one is identified, it’s too late to do anything about it.

“They’re too small to image unless they’re right up on top of you” said Don Yeomans of NASA’s Near-Earth Object Program Office. “There are millions of objects of that size that we don’t know about.”

Finally, it turns out the April 22 fragments reaching the ground confirmed the meteor was of a relatively rare type that contains water and carbon. As reported by the Los Angeles Times, the meteorite rocks are important to scientists for studying how life began on Earth.

The meteorite rocks are also of great interest to collectors of meteorites who are scouring the region of impact more for profit than science. On the open market they can be worth $1000 or more per ounce, about the same as the price of gold nuggets.

By  |  11:23 AM ET, 04/30/2012

Categories:  Astronomy, Space, Latest

 
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