Space scientists say meteorite monitoring remains hit and miss

February 16, 2013

THE ASAHI SHIMBUN

A meteorite that exploded over central Russia and fell to earth, injuring hundreds of people, has fueled discussion of whether science is able to predict such events. The answer: not often enough.

"More than 90 percent of rock bodies smaller than 150 meters in size have yet to be noticed," said Noritsugu Takahashi, board chairman of the Japan Spaceguard Association, referring to the asteroids that might approach Earth within the next few years.

Recently, the space agencies of various nations have begun trying to monitor approaching celestial bodies, but the idea of making them dodge the Earth remains largely pie in the sky.

News of the Feb. 15 incident above Chelyabinsk Oblast, a prefecture-like administrative region in the Russian Urals, spread quickly worldwide.

"Oh, it happened at long last!" was the first thought of Seitaro Urakawa of the Bisei Spaceguard Center in Ibara, Okayama Prefecture, where staff work in shifts to monitor the trajectories of asteroids. The center is operated by the Japan Spaceguard Association.

Urakawa said he checked out videos online showing how the meteorite fell. He said he had seen many similar images before, but never one quite like this.

"I did think, in my mind, that such an event was conceivable," Urakawa said. "But I had never seen something of that brightness before."

The U.S. National Aeronautics and Space Administration (NASA) on Feb. 15 estimated the Chelyabinsk meteorite measured 17 meters in diameter and weighed 10,000 tons before it entered Earth's atmosphere.

"It is nearly impossible to detect small rock bodies of 40 meters or less as they approach," said Shin-ichiro Okumura, another member of staff at the Bisei Spaceguard Center.

When asteroids collide they shatter into masses of small rock fragments, some of which fall to Earth. Most of them burn up in the atmosphere, but the larger fragments can survive and hit the ground. These become known as meteorites.

The meteorite in Chelyabinsk exploded shortly before it hit the ground. Meteorites heat up as they rip through Earth's atmosphere and their weaker parts tend to fracture. This can result in explosions.

Meteorites are surprisingly common. Somewhere between several thousand and 10,000 are thought to fall to Earth every year, said Takehiko Akabane of the Ibaraki Nature Museum.

However, most pass unnoticed, falling either in the sea or in uninhabited areas.

Worldwide, 40,000-50,000 meteorites have so far been identified, including 50 in Japan. The world's oldest documented fall of a meteorite is an event in Nogata, Fukuoka Prefecture, in 861.

Scientists believe most meteorites of 1-10 meters in size burn up on entering Earth's atmosphere. But a meteorite measuring 50 meters in diameter when it hits the ground would have an impact of about 1,000 times that of the atomic bomb dropped on Hiroshima in 1945 and would result in a crater several hundred meters across.

Most scientists believe that dinosaurs became extinct 65 million years ago because a meteorite measuring more than 10 kilometers across hit the Yucatan Peninsula in Central America. Sand and debris thrust into the atmosphere blocked out sunlight around the world for a prolonged period, causing the mass extinction of living creatures.

Among the largest events on record is the 1908 explosion of a comet-like object measuring 100 meters across above Tunguska in Siberia. The blast leveled trees in the surrounding forest for an area of about 2,000 square kilometers.

"Meteorites seldom injure people," said Shigekazu Yoneda, a meteorite expert at the National Museum of Nature and Science in Tokyo. "It is surprising that the latest event caused extensive damage."

In 1992, a 6.4-kilogram meteorite hit a private house in Mihonoseki, currently part of Matsue, in Shimane Prefecture. The rock smashed the roof and penetrated the floor.

Then, in 1995, a 400-gram meteorite hit the hood of a car in Neagari, part of present-day Nomi, in Ishikawa Prefecture.

And in 1996, an aerial explosion above Tsukuba, Ibaraki Prefecture, resulted in pieces of space rock falling to the ground. Twenty-three fragments, with a combined weight of 800 grams, were recovered.

Alert to the risks, in the 1990s space organizations in the United States and Japan began setting up systems to monitor asteroids that might come Earth's way.

Since 2010, a group of organizations, including NASA, have been operating Pan-STARRS, an international monitoring project based at an astronomical observatory in Hawaii.

NASA said 9,697 such asteroids had been identified by Feb. 12. But the goal of Pan-STARRS is limited: It aims only to identify within the coming decade 90 percent of all asteroids sized 1 km or more in diameter that could come near.

It remains difficult to identify asteroids of only tens of meters in size because observation can be hindered by factors such as sunlight.

THE ASAHI SHIMBUN
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A hole in the ice at Chebarkul Lake, where a meteor fragment apparently landed Feb. 15. The lake is about 1,500 kilometers east of Moscow. (AP photo)

A hole in the ice at Chebarkul Lake, where a meteor fragment apparently landed Feb. 15. The lake is about 1,500 kilometers east of Moscow. (AP photo)

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  • A hole in the ice at Chebarkul Lake, where a meteor fragment apparently landed Feb. 15. The lake is about 1,500 kilometers east of Moscow. (AP photo)
  • A meteorite that hit a home in Mihonoseki, Shimane Prefecture, in 1992 is on display in the Meteor Plaza amusement facility in Matsue. (Asahi Shimbun file photo)
  • A fragment of a meteorite that fell on Tsukuba, Ibaraki Prefecture, in 1996 is on display at the Geological Museum of the National Institute of Advanced Industrial Science and Technology in Tsukuba. (Asahi Shimbun file photo)

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