By Irene Klotz
BOSTON (Reuters) - An asteroid half the size of a football field passed closer to Earth than any other known object of its size on Friday, the same day an unrelated and much smaller space rock blazed over central Russia, creating shock waves that shattered windows and injured 1,200 people.
Asteroid 2012 DA14, discovered just last year, passed about 17,200 miles from Earth at 2:25 p.m. EST (1925 GMT), closer than the networks of television and weather satellites that ring the planet.
"It's like a shooting gallery here. We have two rare events of near-Earth objects approaching the Earth on the same day," NASA scientist Paul Chodas said during a webcast showing live images of the asteroid from a telescope in Australia.
Scientists said the two events, both rare, are not related -the body that exploded over Chelyabinsk, Russia, at 10:20 p.m. EST Thursday (0320 GMT Friday) came from a different direction and different speed than DA14.
"It's simply a coincidence," Chodas said.
NASA has been tasked by the U.S. Congress to find and track all near-Earth objects that are .62 miles in diameter or larger.
The effort is intended to give scientists and engineers as much time as possible to learn if an asteroid or comet is on a collision course with Earth, in hopes of sending up a spacecraft or taking other measures to avert catastrophe.
About 66 million years ago, an object 6 miles in diameter smashed into what is now the Yucatan Peninsula in Mexico, leading to the demise of the dinosaurs, as well as most plant and animal life on Earth.
Scientists estimate that only about 10 percent of smaller objects, such as DA14, have been found.
"Things that are that tiny are very hard to see. Their orbits are very close to that of the Earth," said Paul Dimotakis, a professor of aeronautics and applied physics at the California Institute of Technology in Pasadena.
Asteroid DA14, for example, was discovered last year, and it was found serendipitously by a group of amateur astronomers.
"This is a shot across the bow," Dimotakis said. "It illustrates the challenge of the observation campaign which is now in progress."
The planet is regularly pelted with objects from space, adding up to about 100 tons of material per day, said astronomer Donald Yeomans, with NASA's Jet Propulsion Laboratory in Pasadena, California.
Rocks the size of basketballs come in every day. Things the size of a small car arrive every couple of weeks. Larger meteors are less common, so the frequency of hits decreases, Yeomans added.
DIFFICULT TO SEE AHEAD OF TIME
The rock that broke apart over Russia was believed to be a tiny asteroid, estimated to be about 49 feet - more than twice the size of a small car - and traveling at 11 miles per second, NASA said.
"These things are very faint until they get close enough to the Earth to be seen. An asteroid such as this, which approaches the Earth from the daytime sky, is virtually impossible to see ahead of time because telescopes have to look in the night-time sky to discover asteroids," Chodas told reporters on a conference call.
The asteroid weighed about 7,000 tons, and created a fireball trail visible for 30 seconds - in daylight - as it plummeted through the atmosphere.
Shock waves from the blast shattered thousands of windows and damaged buildings. Many of the 1,200 people injured were hit by flying glass, Russia's Interior Ministry said.
"You can see what sort of destruction and shock wave that a smaller asteroid can produce. It's like Mother Nature is showing us what a tiny one can do," Chodas said.
The Russian fireball was the largest space rock to hit Earth's atmosphere since the 1908 Tunguska event when an asteroid or comet exploded over Siberia, leveling 80 million trees over 830 square miles (2,150 sq km), NASA said.
Asteroid DA14 blazed past the planet at about 8 miles per second. At that speed, an object of similar size on a collision course with Earth would strike with the force of about 2.4 million tons of dynamite, the equivalent of hundreds of Hiroshima-type bombs.
"It's a good thing it's not hitting us, because truth be told there's nothing we could do about it except possibly evacuate, which is not going to be easy given the uncertainty about where the impact would take place," Dimotakis said.
"We would essentially take the hit," he added.
(Editing by Kevin Gray and Mohammad Zargham)