The meteor that exploded over Russia set off the largest explosion of its kind in more than a century, at least according to Western scientists. And, even more unusual for a meteor, it caused numerous injuries, mostly from flying glass, as the shock wave violently jolted the city of Chelyabinsk.
It hit on the same day that astronomers were watching another, larger, rock miss Earth by just seventeen thousand miles, and experts scrambled to understand the two rare cosmic events. Some initially speculated that Earth was passing through a swarm of asteroids, but the Russian meteor came from the other direction. “There is no relation there,” said Paul Chodas, a scientist with NASA’s Near Earth Object Program. “It seems like we’re in a cosmic shooting gallery here. There were two very rare events happening on the same day. Pure coincidence.”
Based on preliminary calculations, the solar system interloper that shook Chelyabinsk weighed about seven thousand tons and was about fifty feet in diameter when it entered the atmosphere at forty thousand miles per hour at about 9:20 a.m. local time, said Peter G. Brown, a professor of physics at the University of Western Ontario. Dr. Brown based his calculations on low-frequency sound waves that traveled as far as Alaska. A worldwide network of such sensors listens for these vibrations, too low to be heard by humans, to verify the ban on nuclear tests. Russian experts estimated a much smaller size, of just ten feet across and ten tons.
The asteroid streaked across the sky for about half a minute, unleashing thunderous sonic booms. (Asteroids are orbiting rocks, usually found between Mars and Jupiter; they are called meteors when they enter the Earth’s atmosphere and begin burning up.) The meteor was likely made of stony materials and not solid metal, so as pressure from the atmosphere increased, it exploded at an altitude of twelve to fifteen miles above the surface. It released the equivalent energy of three hundred thousand tons of TNT, Dr. Brown said, although he said that estimate could be revised higher, perhaps to a half million tons of TNT. The atomic bomb that destroyed Hiroshima during World War II was the equivalent of fifteen thousand tons of TNT.
“This is the largest recorded event since the 1908 Tunguska event,” Dr. Chodas said. On 30 June 1908, the explosion of a meteor, believed to be an asteroid, flattened millions of trees over eight hundred square miles in a remote, largely unhabited area of central Siberia about twelve hundred miles away from the Chelyabinsk event.
At a NASA news conference, Bill Cooke, who leads the Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Alabama, said the orbital path of the meteor showed that it was an asteroid, not a comet. At its farthest point from the Sun, it was 2.5 times as far away as the Earth, before its elliptical path took it closer, and the Earth got in its way.
Other large meteors could have splashed unnoticed in an ocean in decades past, but nowadays military satellites, on the watch for missile launches, would certainly notice an incoming meteor this bright. The asteroid was impossible to detect, Dr. Cooke said, because it was approaching from the day side. “As you know, telescopes can’t see things during the daytime,” he said.
Clark R. Chapman, a senior scientist at the Southwest Research Institute’s Department of Space Studies, based in Boulder, Colorado, said this was the first time a crashing meteor is known to have injured a large number of people. “That’s basically never happened before,” he said.
Within the emptiness of space, billions of rocks, from dust to those miles across, zip around the inner solar system, and collisions with the Earth’s atmosphere are frequent.
Most are mere grains, burning up high in the atmosphere. Larger ones— the size of peas— can be spotted streaking across the night sky as “shooting stars,” but those also never reach the ground.
“Small objects the size of a basketball hit the Earth almost every day, once a day on average, and car-size objects hit every month or two,” Dr. Chodas said. “Little stuff hits the Earth all the time. We just don’t know about it. It’s all burning up in the atmosphere.”
Scientists estimate that these tiny meteoritic bits add up to eighty tons of material falling on Earth from outer space each day.
Giant impacts have changed the course of life on Earth, notably 65 million years ago when an object several miles wide slammed off the coast of Mexico and killed off the dinosaurs. Collisions with objects the size of the Russian meteor or 2012 DA14, the asteroid that did not hit Earth on Friday, occur perhaps once a century.
NASA surveys had cataloged 95 percent of near-Earth asteroids 0.6 miles wide or wider, but fewer of the smaller ones. A year ago, a telescope in Spain discovered 2012 DA14, a 150-foot-wide asteroid just after it made a somewhat close pass to Earth, missing by 1.6 million miles. With an orbital period of 366 days, it made a return visit on Friday, at a much closer distance of 17,200 miles. That is closer than many telecommunications satellites. But, despite needless anxiety that it would hit Earth, its passing was uneventful, and it will not come that close again until at least 2080.
If it had collided with Earth, it would have released the energy of two-and-a-half million tons of TNT, NASA scientists estimate, or nearly ten times as much energy as the meteor in Russia. Some estimates put the Tunguska event even bigger, perhaps twenty million tons of TNT. The makeup of an asteroid can vastly change the damage.
With Tunguska, the asteroid also exploded in the air. But, in Arizona, an asteroid of similar energy, but made of nickel and iron, slammed into the ground about fifty thousand years ago, leaving a crater four thousand feet wide.
Occasionally, astronomers do find the very small asteroids. Astronomers had fortuitous luck in 2008 when they discovered an asteroid, smaller than the Russian one, and were able to track it as it entered the atmosphere over the Sudan twenty hours later. But most of the time, near misses of objects that small are missed. “It is detectable, but only if we are looking at the right place at precisely the right time,” Dr. Chodas said.
Rico says we got lucky...
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