Just hours after a spectacular meteor exploded in a shower of fiery debris over western Russia, a much larger 150-foot-wide asteroid streaked safely past Earth Friday in an unrelated close encounter, making a record-setting flyby some 17,200 miles above the Eastern Hemisphere.
In what amounts to an exceedingly close shave by astronomical standards, 2012 DA14 sailed well inside the ring of geostationary weather and communications satellites that orbit 22,300 miles above the equator, passing so close to Earth that the planet's gravity was expected to bend its trajectory enough to put it in a slightly different orbit.
As a result, the chances of additional close encounters in the foreseeable future are considered remote.
The dramatic asteroid flyby came on the heels of an unexpected fireball over western Siberia earlier in the day when a previously undetected 7,700-ton meteoroid 50 feet across slammed into the atmosphere at some 40,000 mph, more than twice as fast as 2012 DA14.
The small asteroid broke apart 30 seconds later with a supersonic blast that set off car alarms, shattered windows and sent hundreds of people to area hospitals with injuries from flying glass and debris.
Bill Cooke, the lead researcher in NASA's Meteoroid Environments Office at the Marshall Space Flight Center in Huntsville, Ala., said the blast was the equivalent of 300,000 tons of TNT, nearly 20 times more powerful than the 16-kiloton atomic bomb that destroyed Hiroshima in 1945.
"This rock was about 15 meters in diameter, it was a weight of about 7,000 metric tons, it was moving at 18 kilometers per second," he said. "That's about 40,000 mph. So it hit the atmosphere above Russia, moving at that speed. It penetrated at a shallow angle, less than 20 degrees, it lasted over 30 seconds in our atmosphere before breaking apart about 20 to 25 kilometers, which is 12 to 15 miles above Earth's surface.
"When it broke apart, this produced a violent explosion ... in the vicinity of 300 kilotons of energy, which produced a shock wave that propagated down as well as through the atmosphere. And when it propagated down, this shock wave struck the city below causing large numbers of windows to be broken, some walls to collapse and minor damage throughout the city."
More than 1,000 people were injured by flying debris, according to Russian authorities.
Paul Chodas, a scientist with NASA's Near-Earth Object program office at the Jet Propulsion Laboratory in Pasadena, Calif., said an analysis of the fireball's trajectory and velocity showed it was unrelated to 2012 DA14. In short, a cosmic coincidence.
"What an exciting day!" he said. "It's like a shooting gallery here, we have two rare events of near-Earth objects approaching the Earth on the same day."
He said the Russian meteor represented the largest impact since 1908 when a 150-foot-wide asteroid slammed into the atmosphere and detonated above Siberia. The so-called Tunguska Event leveled millions of trees over more than 800 square miles.
Researchers were able to roughly calculate the mass of the Russian meteor that hit Friday, the energy release and details about its trajectory from multiple videos of the fireball and data from four infrasound stations, part of a global network used to "listen" for nuclear blasts.
"There are nuclear test ban treaties forbidding surface nuclear tests," Cooke said. "So international agencies established a network of infrasound stations all over the globe designed to detect big explosions in the atmosphere. ... As you can guess with an energy of 300 kilotons, this is similar to a nuclear explosion in magnitude."
Unlike the Russian meteor, which came from the dayside and was not detected before impact, DA14 was discovered last year by Spanish astronomers and its trajectory was carefully plotted. It 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, collapsed walls and injured more than 1,000 people.
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 the way.
It was not immediately clear how much debris from the fireball might have 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.
Even though the Russian meteor was small by asteroid standards, "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."
But the coincidental flyby and impact 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.
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 as powerful as the Hiroshima blast.
Asteroid collisions with Earth are not uncommon, but most of the 80 to 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.