quarta-feira, 25 de abril de 2012
Meteor Produces Sound and Fury
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.
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.
Publicado por Jorge M. Gonçalves às 2:13 da tarde