Meteor a once-in-a-century display

The meteor that streaked across the sky on Friday, creating a powerful shock wave when it exploded more than 12 miles above the Earth’s surface, was a once-in-a-century occurrence. It was the largest recorded meteor since the 1908 Tunguska event, when a larger one exploded over Siberia.

NASA scientists said the meteoroid that came down over Russia was a space rock about 50 feet in diameter. They said the resulting explosion was equivalent to detonating about 300 kilotons of TNT — making it roughly 20 times more powerful than the atomic bomb dropped on Hiroshima, although the bomb was detonated near the Earth’s surface.

The meteor exploded over Chelyabinsk, Russia, where it broke windows and collapsed some walls, causing damage throughout the city and injuring 1,200 people. If a similar-sized meteor were to explode above Boston — a remote possibility — it could cause similar destruction in an area roughly stretching from the State House to Fenway Park, MIT planetary scientist Richard Binzel estimated.

The Earth is under constant assault by dust and rocks flying through the solar system — meteoroids — most of which burn up in the atmosphere before reaching the ground. Events the size of the one in Russia are uncommon.

Every day, 100 or so tons of rocks and dust bombard the atmosphere, Binzel said. A basketball-sized object hits the Earth about once a day on average, NASA scientists said. Such objects become meteors — flashes of light created as they fly through the atmosphere and burn up.

Fragments of a meteoroid left a hole in the ice on a lake near Chelyabinsk, Russia.

Larger events like the one in Russia probably occur every few decades, Binzel said; a team of NASA scientists said during a news conference Friday that they occur every 50 to 100 years. But because so much of the Earth is ocean, only about once a century does something comparable occur over an inhabited area.

“Objects falling over the oceans could be almost completely missed,” Timothy Spahr, director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics, wrote in an e-mail. “Only the Air Force and Department of Defense have information on a lot of the airbursts that are unobserved from the ground.”

The atmosphere largely protected the Earth this time, Binzel said, heating up the meteoroid and causing it to break apart. Scientists believe it hit the atmosphere at more than 40,000 miles per hour, but broke apart about 12 to 15 miles above the Earth. What reached the ground were fragments of rock, known as meteorites, and a pressure wave and sonic boom knocked out windows.

Binzel said if the original space rock had been slightly smaller, it likely would have exploded farther from the Earth’s surface and the pressure wave would not have made it to the ground.

Binzel said scientists in Russia are already making efforts to collect meteorites that landed. He hopes to examine some himself.

“These typically would break into fragments, hand-sized or smaller, and there could be hundreds or thousands of them,” Binzel said. “We hope there are, because scientifically we would hope to pick them up and bring them into laboratories.”

It is a coincidence that the meteor appeared within a day of a close call with the asteroid known as 2012 DA14, which had long been predicted to come within 17,200 miles of Earth — the closest approach to ever be predicted for an object of its size.

“We’re looking at it carefully. It turns out they almost certainly are not related, which is amazing because of the coincidence,” Binzel said. The object that hit Russia, Binzel said, was traveling north to south, whereas the asteroid was moving south to north.

The asteroid, about half a football field in diameter, is estimated to weigh 287 million pounds. An asteroid that size would cause regional devastation if it slammed into the atmosphere, according to NASA, and would be comparable to the Tunguska event, when a slightly smaller asteroid flattened about 825 square miles of forest in Russia. The space agency estimates there are about half a million asteroids that size near Earth.

Scientists closely monitor near-Earth asteroids that could pose a risk to the planet, but are most focused on identifying and tracking large objects that could cause considerable harm.

“We are focusing on the larger asteroids first,” said Paul Chodas, a research scientist in the Near Earth Object Program Office at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “They are the ones that are the most hazardous. The object that entered over Siberia this morning was a moderate explosion, and frankly was nowhere near the devastation you would get from a larger asteroid hitting the Earth.”

Representative Lamar Smith, Republican of Texas and chairman of the House’s Science, Space, and Technology Committee, said in a statement that in the coming weeks there would be a hearing to examine how to better detect asteroids and protect people from them.

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