Earth safe after asteroid flyby
As predicted, a 150-foot-wide asteroid streaked safely past Earth Friday, making a record close approach just 17,200 miles above the Eastern Hemisphere, well inside the orbits of geostationary communications and weather satellites.
The close encounter came on the heels of a spectacular fireball over western Siberia earlier in the day, a 10-ton meteoroid that broke up in the atmosphere with a supersonic air blast that set off car alarms, shattered windows and sent hundreds of people to area doctors and hospitals with injuries from flying glass and debris.
In a cosmic coincidence, the meteor upstaged asteroid 2012 DA14, a much larger, unrelated body that passed above Indonesia at 2:24 p.m EST. The asteroid passed so close to Earth, in fact, that the planet's gravity was expected to bend its trajectory slightly, putting it in a slightly different orbit and reducing the chances of additional close encounters in the foreseeable future.
"What an exciting day!" said Paul Chodas, a scientist with NASA's Near-Earth Object program. "It's like a shooting gallery here, we have two rare events of near-Earth objects approaching the Earth on the same day."
The asteroid was not visible to the unaided eye, but observers from Eastern Europe to Indonesia had a chance to spot the rocky body with binoculars or telescopes, weather permitting.
A video feed from a telescope in Australia that was carried live on NASA's satellite television channel showed DA14 as a quickly moving truncated streak of light as the body's 5-mile-per-second velocity caused a slight smearing in sequential time-exposure photographs.
The much smaller Russian meteor was much more dramatic -- and much more dangerous -- as the shock wave racing in its wake shattered windows along its path and injured hundreds.
Video cameras in and around Chelyabinsk, Russia, captured the fiery meteor as it streaked across the sky, flaring brilliantly and casting sharp shadows as sonic booms rocked buildings along its path.
It was not immediately clear if any debris from the fireball made it to the surface in the form of meteorites, but Russian authorities were investigating reports of at least one small crater and another circular opening in an ice-covered lake that may be related.
Scientists believe the meteor was a body about 50 feet or so across, or about one third the presumed size of 2012 DA14.
Even so, "you can see what sort of destruction and shock wave a smaller asteroid can produce," Chodas said. "It's like Mother Nature is showing us what a tiny one, really, can do. And DA14 is only a small asteroid on that scale."
Speaking on NASA's satellite television channel, Chodas said the meteor and asteroid 2012 DA14 were on different trajectories and "it's simply a coincidence they happened to hit and come near the Earth the same day."
But the meteor and the asteroid flyby highlighted the threat posed by debris left over from the formation of the solar system 4.6 billion years ago.
While the odds of a major asteroid impact in any given year are low, the consequences could be extreme, prompting ongoing work by NASA and other agencies to identify near-Earth asteroids that could one day pose a threat to the planet.
Since dedicated surveys began some 15 years ago, astronomers have catalogued nearly 10,000 near-Earth asteroids, including about a thousand big enough to cause global damage in a collision.
2012 DA14 is believed to measure about 150 feet across. It was moving at nearly 5 miles per second when it streaked past Earth.
A similar but slightly smaller rocky asteroid is believed to have crashed into the atmosphere above Siberia in June 1908, disintegrating in an air blast known as the Tunguska Event that leveled millions of trees over more than 800 square miles.
A denser, 150-foot-wide nickel-iron asteroid blasted through the atmosphere above Arizona 50,000 years ago, excavating Meteor Crater, a mile-wide impact basin similar in appearance to those on the moon.
Don Yeomans, an asteroid expert at NASA's Jet Propulsion Laboratory, said 2012 DA14 likely would have broken up in the atmosphere had it hit the planet, releasing some 2.4 megatons of energy in the resulting air blast. That's roughly 150 times the 16-kiloton atomic bomb blast above Hiroshima in 1945.
Asteroid collisions with Earth are not uncommon, but most of the 100 tons of debris that hit the atmosphere every day is made up of small objects, burning up unseen at high altitude. Objects the size of basketballs impact daily, with car-size objects hitting every few weeks.
Yeomans said asteroids the size of 2012 DA14 could be expected to impact the planet once every 1,200 years on average. He said bodies large enough to trigger a global catastrophe, like the six-mile-wide asteroid believed to have wiped out the dinosaurs, hit every hundred million years or so.
