Tuesday, 24 April 2012

Meteor Streaks Across Nevada Sky


A fiery meteor created a thundering explosion and traced a rare daylight fireball seen for about 600 miles across Nevada and California on Sunday, before apparently breaking up harmlessly at high altitude, astronomers said.
A meteor that caused a loud boom and fireball over the West Sunday apparently burned up and caused no damage. Stefanie Ilgenfritz has details on The News Hub. Photo: Robin Loznak/ZUMAPRESS.com.
NASA researchers at the Jet Propulsion Laboratory said the midair explosion, centered over California's Central Valley east of the San Francisco Bay area, was the equivalent of the detonation of about 3.8 kilotons of TNT—about one quarter the energy released by the atomic bomb dropped on Hiroshima, Japan, in 1945.
"The meteor was probably about the size of an SUV," said Donald Yeomans, manager of NASA's Near-Earth Object Program Office at the laboratory, in Pasadena, Calif. "This was a big one. An event of this size might happen about once a year, but most of them occur over the ocean or an uninhabited area."
There were no reports Monday that any fragments of the object had reached the ground or caused any damage. No major telescope in the region tracked the early-morning fireball. NASA astronomers said the explosion might have been five to 10 miles high, which was high enough to let the sound spread widely.
Each day, countless meteors reach Earth's atmosphere. Most are smaller than a grain of sand, according to the American Meteor Society, and usually burn up before they hit Earth's surface.
Sunday's eye-catching event occurred at the height of the annual Lyrid meteor shower, which happens every April as Earth plows through the dust and debris trailing a comet called Thatcher. People have been observing its annual shower of shooting stars for more than 2,600 years. Astronomers usually expect about 20 meteors per hour during the Lyrid shower, with outbursts as high as 100 meteors per hour.
Generally, comet debris can hit Earth's atmosphere at speeds as fast as 110,000 miles per hour. The heat from the friction of its descent into the denser air can ignite the dust and debris in a display of astronomical fireworks. Skywatchers have reported dazzling fireballs, like Sunday's, during Lyrid showers in previous years.
In the far distant past, immense meteorites—meteors that slam into Earth—likely contributed to the demise of the dinosaurs. The largest meteorite found weighs nearly 60 tons. Called Hoba, it is an iron boulder thought to have landed about 80,000 years ago, in present-day Namibia.
On rare occasions, the falling bits of space debris do hit now-populated areas. There is no record of anyone ever having been killed by a meteorite, but in recent years, there have been verified accounts of a meteorite hitting a bedroom in Alabama, a dining room in Connecticut and a parked car in Peekskill, N.Y.


NASA Calls Meteor Eruption "Huge Event"


Chris Ciarlo
Channel 2 News
 
If you heard a large boom just before 8am Sunday, you weren't alone.
Dr. Bill Cooke with the Lead, NASA Meteroid Environments Office said the meteor eruption caused quite the commotion.
Cooke said the meteor's energy was a whopping 3.8 kilotons of TNT. That's about one fourth of the "little boy" bomb, which was dropped on Hiroshima during WWII.
"This meteor was about the size of a minivan and had a mass somewhere around 70 metric tons. So if you can imagine a boulder about the size of a minivan with mass of 70 tons, you get a pretty good idea what the meteor was like."
NASA believes the meteor exploded in the atmosphere over California, somewhere between Sacramento and Fresno.
Cooke said the meteor was probably moving at about 33,000 miles per hour.
He said it probably came from the asteroid belt and exploded in the upper atmosphere and spread meteorites in eastern California, near the Nevada border.
I asked Cooke what was the loud boom everyone heard?
"When they get low in the atmosphere, the pressure in front of them builds up to the point where the meteor simply breaks apart and it does so violently it creates an explosion. The sound you heard was probably build up from that over pressure and the shock wave generated from that explosion."
Cooke said you would think they are dangerous, but in all of recorded history, only one person has ever been hit a meteorite


A Canadian scientist who specializes in meteor sound waves said Monday the infrasonic signal from Sunday’s event felt over a wide swath of Northern Nevada and California was “very strong” and lasted more than 18 minutes at a monitoring station 400 miles away.
Infrasound is low-frequency sound below the threshold of the human hearing range of 20 Hertz.
Elizabeth Silber of the Meteor Physics Group at the University of Western Ontario said the meteor – also called a bolide – exploded just before 8 a.m. on Sunday above the western slope of the Sierra, south and west of Reno and roughly halfway between Sacramento and Fresno.
Witnesses from Elko County in Nevada to Riverside County in Southern California reported seeing the bolide streaking across the sky. The sonic boom caused by the meteor exploding in Earth’s atmosphere was heard – and felt – from Reno and Lake Tahoe to Bakersfield, Calif.
The event was recorded by two infrasound monitoring stations of the Comprehensive Nuclear-Test-Ban Treaty Organization’s International Monitoring System, one in Washington State and the other near the California-Mexico border. The IMS stations “listen” for illicit explosions, Sibler said.
The monitoring stations can pick up sound down to the natural frequency of the atmosphere, >0.01 Hertz. Other sources of infrasound include lightning, volcanoes, severe storms, mining activities, jet planes and chemical and nuclear explosions, Sibler said.
She said it’s likely the meteor disintegrated before reaching the ground, however, it was a powerful blast. She cautioned that she is continuing to analyze data from the blast, but has made some preliminary findings.
“Based on the signal characteristics, the source seems to be fairly energetic for a fireball, but not at all uncommon,” she said. “My preliminary analysis are indicative of energy yield of approximately 4 kilotons of TNT equivalent. That’s equivalent to a very small yield nuclear blast — except in nuclear explosions, half of the energy comes from thermal and ionizing radiation, while the other half is from the blast wave. In bolides, there is no radiation part.”
Silber said the bomb dropped on Hiroshima was about 15 kT and the Oklahoma City bomb was about 0.002 kT.
“To give you a little bit more perspective about energies surrounding fireballs, for example, there was a bolide over Indonesia in October 2009, which produced estimated energy of 50 kT,” she said. “It disintegrated in the atmosphere, producing infrasound on much of the globe.”