The green flash seen by observers throughout the Southland on Thursday evening was almost certainly a meteor, astronomers said Friday, but scientists have no idea where--or even whether--it reached the ground.
That it was observed over such a broad area--from San Francisco to Arizona and Mexico--indicates that it was at a very high altitude, said astronomical observer Tony Cook of the Griffith Observatory.
Because it was so high, he added, it is "extremely unlikely" that the object landed anywhere near Los Angeles.
Scattered reports suggest that the object might have landed in Kern County, but researchers say those reports are probably wrong because they present an inaccurate picture of what happens when a meteorite touches down.
People in desert or open areas often report seeing a meteor pass across the sky and appear to strike the ground in the distance. "But what they are actually seeing is the meteor passing over the horizon," said cosmochemist John Wasson, a meteorite researcher at UCLA. When a meteor does reach the ground, it is no longer glowing, he said.
A meteor is essentially a chunk of rock that strikes Earth's upper atmosphere at what Wasson calls "cosmic speed," about 40,000 mph. But it loses speed rapidly as it falls, and at about 18 miles above Earth's surface--about three times as high as jets normally fly--it has slowed to its final velocity of about 600 mph.
At that speed, there is not enough friction with the atmosphere to heat the rock and it will seemingly disappear--often in a flash of light--the familiar "shooting star" effect. "At that point, it will more or less fall straight down unless there are high winds," Wasson said.
To someone who is near the point where the meteor strikes Earth, the shooting star would be accompanied by a sonic boom.
Only about 10% of meteors reach Earth's surface. Most disintegrate before they pass through the atmosphere.
Wasson said the flash seen Thursday was probably not caused by space debris because any item large enough to produce it "would certainly be large enough to be tracked by NASA."
The space agency did not report the fall of any debris Thursday.
The green color is typical of meteors, said astronomer John A. Russell, who is retired from USC. It is probably caused by the burning of magnesium, a major component of most rocks in space, he said.
Wasson and colleague Lori Leshin are attempting to find the meteor's landing point, if there was one, and would like to talk to anyone who observed the phenomenon. He can be reached by regular mail at UCLA or by e-mail at: firstname.lastname@example.org