A falling space rock that terrified Russians on Friday was clearly strange, but the frequency of such strikes might surprise you.
Thousands of rocky bits of space debris, from dust-size to the size of small boulders, tumble through our atmosphere every day, experts say. Most disintegrate before they hit the ground.
What was most unusual in Russia was that a rock that large -- maybe the size of a small house -- fell near people, said Geoff Chester, an astronomer with the U.S. Naval Observatory in Washington.
"It's kind of a wakeup call that this is a tangible threat that we have to be aware of," Chester said.
A space rock that size probably hits the Earth every two years or so, but usually in oceans or remote places, Chester said.
Space rocks are asteroids left over from the beginning of the solar system, Chester said. The falling rocks are called meteors when you see their light and meteorites if they strike the Earth.
Chester said the Russian rock had nothing to do with a much-larger asteroid that passed Earth on Friday afternoon.
The two rocks were traveling different paths.
A direct hit from a rock the size of the larger asteroid could wipe out a midsize city, Chester said.
The two rocks are dwarfed by some of history's top space objects to hit Earth. David Hagan, a staff scientist at the Science Museum of Virginia, said those big collisions include:
--an asteroid or comet that knocked down trees across about 800 square miles in Siberia in 1908;
--another object that struck what is now the Chesapeake Bay 35 million years ago, an impact that still contributes to a settling of the ground in that area; and
--an apparent asteroid that struck the Yucatan Peninsula area 65 million years ago, a monstrous event that apparently wiped out the dinosaurs.
NASA and other countries' space agencies track so-called near-Earth objects.
Should a huge space rock be headed for Earth, scientists hope to ward it off, perhaps by someday developing a way to attach thrusters to it, Hagan said.
"Our hope is with a good lead time, we would be able to deflect an object that would be a major disaster," Hagan said.
News accounts say the Russian meteor "exploded" over the Ural Mountains. What probably happened, Chester said, is that the falling rock caused a sonic boom that fragmented it, and friction from the atmosphere caused the falling rocks to burn.
Such fireballs are common -- they are occasionally seen over Virginia -- but they are usually caused by much smaller rocks.
The Russian rock fell at an angle almost parallel to Earth. That means the Earth received just a glancing blow from its pieces, Chester said.
If the rock had fallen straight down, "it probably would have leveled an area that would have been several city blocks," Chester said.
"To the Earth, this was kind of like a mosquito bite, but to the people affected by it, it was a very scary event," he said. "It caused damage and hurt people."
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