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In asteroid's aftermath, a sigh of relief

In asteroid's aftermath, a sigh of relief
Using an eclectic mix of crowd-sourced video, information from Google Earth and data from nuclear test-ban sensors, scientists have gotten a much more accurate picture of the small asteroid that exploded near the Russian city of Chelyabinsk last month.

But what is most clear is that while Chelyabinsk did not exactly dodge a bullet on Feb. 15, the city was fortunate to be only grazed by it.

"The people of Chelyabinsk were very lucky," Edward Lu, a former astronaut who now leads the B612 Foundation, a private initiative to detect similar asteroids, said at a congressional hearing last week about the space threats.

The Russian meteor - which, according to the latest estimates, was about 60 feet in diameter and came in undetected at roughly 42,000 miles an hour - was almost 15 miles high when it blew apart. There were no deaths, and most of the 1,500 injuries were from glass as windows shattered when a shock wave hit the city 88 seconds later.

"If it had detonated closer to the ground, it would have been worse," said Margaret Campbell-Brown, a member of a team of researchers at the University of Western Ontario who have analyzed the meteor's orbit and characteristics of the blast. It also helped that the meteor was a stony one - what scientists call an ordinary chondrite - and not a rarer iron-nickel one, in which case it might have reached the ground before exploding.

The blast was the largest fireball since the Tunguska event of 1908, and as videos from surveillance, car-mounted and cellphone cameras quickly began to be posted online, it became apparent that this explosion would be studied like no other, with scientists and amateur bloggers alike rushing to analyze the images.

The Tunguska event involved the detonation of what is generally thought to be a meteor over a remote part of central Siberia. That object may have entered the atmosphere at a steeper angle than the Chelyabinsk meteor - which came in at an angle of less than 20 degrees from horizontal - and exploded about five miles above the ground. The lower altitude, as well as the larger size of the Tunguska rock, help explain why it had a much greater impact, flattening trees over an area the size of metropolitan Washington.

The Canadian team has calculated that the energy released in the Chelyabinsk explosion was the equivalent of about 440 kilotons of TNT, or about 30 times the power of the Hiroshima bomb. That calculation was made with the help of data from a network of acoustic sensors set up to monitor compliance with the treaty to ban nuclear weapons testing. There are about 45 of the sensors worldwide, detecting so-called infrasound at frequencies well below the range of human hearing.

Thomas Muetzelburg, a spokesman for the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, which is based in Vienna, said the explosion was detected by more than 20 of the sensors, including one in Antarctica nearly 10,000 miles from Chelyabinsk. Low-frequency sounds do not dissipate easily, so some of the detectors picked up the explosion more than once as the sound circled the globe several times.

With knowledge of the energy and information about how fast the meteor was traveling, researchers were then able to calculate the mass of the meteor, about 11,000 metric tons.

"It's all about the kinetic energy of that body," which is related to its mass and velocity, said Richard P. Binzel, a planetary scientist at the Massachusetts Institute of Technology.

The meteor has a certain amount of energy as it enters the atmosphere, Binzel said. As it hits the air, it starts to decelerate quickly, and then the stress differential between the superpressurized air in front and the less-pressurized air behind causes the rock to break apart violently.

"All that kinetic energy has to be released," he said. Only small fragments of the meteor reached the ground, and some have been brought to laboratories in the United States, including the Institute for Rock Magnetism at the University of Minnesota, where a Russian scientist is studying their magnetic properties.

Like other teams, the Canadian researchers have used video images of the meteor's east-to-west trail above Chelyabinsk to help calculate its trajectory and orbit. Since the explosion occurred in daylight, Campbell-Brown said they asked a colleague there to take night photographs from the same locations. By superimposing the images of the meteor's path over the night images containing stars, the researchers should be able to plot the path even more accurately.

For now, though, they and other teams, including one from the University of Antioquia in Medellin, Colombia, have established that the meteor originated in the inner part of the asteroid belt, in a regular, harmless orbit around the sun between Mars and Jupiter. How it came to have an irregular Earth-crossing orbit is not known, but like other Earth crossers it was probably affected by the gravity of Jupiter or another planet at some point. It then circled the sun every 18 months in a highly eccentric orbit - more than 2 1/2 times the Earth-sun distance at its farthest, close to the orbit of Venus at its nearest - before it smacked into the Earth's atmosphere.

Jorge Zuluaga, a member of the Colombian group, said he was inspired by work done soon after the event by Stefan Geens, a blogger in Stockholm. Geens used several videos - including one from a camera in Revolution Square in central Chelyabinsk - to triangulate the path. The video does not show the explosion directly, but rather the shadows of lampposts (and of a huge statue of Lenin), which moved like the shadow on a sundial as the fireball crossed the sky.

Using Google Earth, it is easy to pinpoint the coordinates of the Revolution Square camera and others. Zuluaga said that his team, which included Geens as a co-author on its most recent paper, is working to improve its calculations by, among other things, learning about the optics of the various video cameras. "We are trying to understand the distortion we have," he said.

Zuluaga thinks that eventually it will be possible to reconstruct the rock's orbit so precisely that researchers may be able to detect it retroactively, by poring over images from sky-surveying telescopes taken the last time it flew near the Earth. "Then we can assign it a name," he said. "It doesn't have one now, because it's a dead asteroid."

In addition to helping scientists plot the meteor's path, the videos that were posted online contributed to a surge in interest in projects to detect asteroids before they strike Earth. In an interview after his congressional testimony, Lu said his foundation, which wants to put a privately financed telescope in space within five years, had seen an increase in donations since the explosion.

"It made it more real to folks," he said. "There's nothing like a hundred YouTube videos to do that."

© 2013, The New York Times News Service
Story First Published: March 26, 2013 08:25 IST

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