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Scientists gather this fall to study preventing an asteroid strike on Earth

Monday, June 17, 2002

By Michael Woods, Post-Gazette National Bureau

The federal government is summoning the world's top scientists to plan defenses against an attack that could wipe out an American city or disrupt the whole country's infrastructure.

Don Davis illustration courtesy of NASA

No, it's not global terrorism.

The scientists will map ways to combat an asteroid impact like the collision that killed the dinosaurs 65 million years ago and flattened a Siberian forest in 1908.

While the world's attention is focused on the real threat of terrorism, the theoretical asteroid menace has been garnering behind-the-scenes attention.

It was the topic for an international meeting hosted by Britain's Royal Astronomical Society in December. In March, NASA activated Sentry, an automated system to monitor near-Earth asteroids and assess their threat.

And NASA will gather scientists in September in Washington, D.C., to talk about how an incoming asteroid might be deflected and what sort of research would be required to design and build a system to do that.

"We've had a couple of close shaves during the past few months," said Brian G. Marsden, with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

One asteroid caused jitters when discovered March 12. Named 2002 EM7, it came from the direction of the sun but was hidden by the sun's glare. Astronomers didn't detect 2002 EM7 until four days after it came within 288,000 miles of Earth, which they regarded as a close encounter.

The asteroid was about 200 feet in diameter -- big enough to fill two-thirds of a football field -- and could have flattened a city, unleashing the energy of a 5-megaton nuclear bomb.

How big is really bad?

A NASA analysis concluded that asteroids less than 150 feet in diameter probably would burn up in Earth's atmosphere before they could cause much damage.

Asteroids from 150 to 3,600 feet in diameter would do "tremendous" damage but mostly in a localized area near the air burst or impact.

Objects bigger than 1.2 miles in diameter are the doomsday asteroids. They would strike Earth with the energy of 1 million megatons of TNT and cause catastrophic global environmental damage.

The likely consequence, NASA concluded, would be an "impact winter," in which dirt and other debris thrust into the atmosphere reduces sunlight reaching the surface. Global climatic change would ensue, with colder temperatures and less sunlight causing massive crop failures and widespread starvation.

Astronomers estimate that at least 1 million near-Earth objects are larger than 150 feet in diameter and at least 1,000 have a diameter larger than 3,600 feet. The biggest known objects are about 15 miles in diameter.


"I think Mother Nature has given us yet another wake-up call," said Donald K. Yeomans, manager of NASA's Near-Earth Object Program Office. "Objects the size of 2002 EM7 pass as close as this one did every two weeks or so. We just haven't found them all yet."

A similar scare occurred in January, when a 1,000-foot-diameter asteroid came within 375,000 miles of Earth. Astronomers had detected the mountain-sized rock, named 2001 YB5, only a few weeks earlier.

We can expect more close-encounter stories.

"It is simply a matter of our increasing prowess in detection that objects like 2001 YB5 are now being seen," said Richard P. Binzel of the Massachusetts Institute of Technology.

NASA, the European Space Agency, and universities have been tracking near-Earth objects with great precision.

"The goal is to track NEOs well in advance of any Earth-threatening encounters so that a mitigation plan could be put into effect," said Yeomans, with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "No objects that we know about threaten us, and we're well on the way to finding the majority of the entire population of large NEOs."

Finding the smaller ones, like 2002 EM7, will take years longer and require bigger telescopes than those used in asteroid search-and-tracking efforts.

"That said, NEOs are not something to lose sleep over," Yeomans added.

Gareth Williams, of the Smithsonian center, cited the importance of detecting small asteroids when they're visible -- not hidden in the sun's glare -- so they can be tracked and monitored.

Objects the size of 2002 EM7 make similarly close approaches to Earth several times a month, Williams said. They hit Earth every 30 to 100 years, but usually burn up in the atmosphere.

Such impacts, however, create an air burst, or powerful shock wave, that can cause great damage on the ground.

"The 1908 Tunguska event was an example of the local damage that would occur under and around the air burst of such an object," Williams explained. In that incident near the Stony Tunguska River in Siberia, a mysterious air explosion -- now believed to be an asteroid impact -- leveled a section of forest half the size of Rhode Island. Scientists estimate it caused as much destruction as a 15-megaton nuclear bomb.

"Impacts by such objects are not likely to cause major loss of human life," Williams said. "About 70 percent of the world's surface is water, and much of the land mass is either uninhabited or very sparsely populated."

Using the 1,000-foot diameter 2000 YB5 asteroid as an illustration, Binzel said there is about a 1-in-10,000 chance of an impact with Earth each year or a 1-in-100 chance of an impact sometime during the 21st century.

Binzel said 2001 YB5 and 2002 EM7 were essentially no-risk asteroids.

  On the Net:

Forecasts of known asteroid encounters are easily available on the Internet. One site is the NASA-affiliated www.spaceweather.com.


"Most of these chances are in the 1-in-a-million or 1-in-a-billion range," Marsden said. "And it is very likely that that, as we make further observations, the impact probabilities will become precisely zero."

The only nightmare near-Earth object known today is 2002 CU11, which is about 2,000 feet in diameter and has a 1-in-9,000 chance of hitting Earth on Aug. 31, 2049. It was discovered in February. Scientists think there are at least eight other Earth-impact possibilities between 2032 and 2096.

NASA described its September conference as "urgent" because scientists believe it will take 70 years to develop mitigation technology and learn to use it against an Earth-threatening object.

"The more we know about NEOs, and the longer the advance notice of possible impacts, the better off we are," said Marsden. "We can do it," he added. "Pity the poor dinosaurs, who couldn't."

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