Cumberland Times-News

Bob Doyle - Astronomy

August 31, 2013

Are these near-Earth objects a threat to us?

There is a new book from Princeton Press by Donald Yeomans with the title ”Near Earth Objects: Finding Them Before They Find Us.” Its ISBN is 978-0-691-14929-5 (hard cover).

Yeoman, who works at the Jet Propulsion Lab in California focuses on asteroids: those in the inner solar system (that could hit us), belt asteroids (between the orbits of Mars and Jupiter) and other asteroids farther out.

Yeoman is frequently interviewed on science television programs about objects that could strike us.

What are Near-Earth Objects? A Near-Earth Object is usually an asteroid that approaches the Earth within 0.3 of the Earth-sun distance or about 28 million miles.

An asteroid is a mostly rocky object, lacking enough mass or gravity to have compacted itself into a roughly spherical shape. Like planets and comets, asteroids orbit the sun. But the orbits of asteroids are not as stable as the planetary orbits.

For instance, if an asteroid is moving around the sun in the same direction as Jupiter and behind Jupiter. Jupiter’s pull will increase the asteroid’s speed, causing the asteroid to go into a bigger orbit around the sun.

Or if the asteroid moves in the same direction as Jupiter but is in front of Jupiter, the asteroid will slow down and its altered orbit will take the asteroid closer to sun. (This is how some belt asteroids can become inner solar system objects that can approach Earth.)

As a result of a number of large telescope sky surveys that continually scan the sky every night, there have been many “finds.”

Statistical analysis allows an estimate the likely number of objects of given size and the frequency that they will impact us. The rate of finding new asteroids is now about 3,000 per month, where most “finds” are in the asteroid belt.

But 20 of these “finds” are asteroids in the inner solar system. The number of known Near-Earth Objects is now approaching 10,000.

The amount of asteroid and comet debris falling Earthward is about 100 tons per day. Most debris is the size of a pea and its fiery descent into the atmosphere is seen as a meteor or “falling star.”

Once a day, a basketball sized object will impact the Earth, producing a fireball as bright as the moon. But most of this object will burn up in our atmosphere.

On the average, an asteroid chunk about the size of a Volkswagen Beetle will impact the Earth about every 6 months.

One such object made a fiery descent on a rainy night on Oct. 9, 1992 on Peekskill, N.Y. The 27-pound asteroid fragment demolished the back end of a 1980 Chevy Malibu. The owner, Michelle Knapp was refused coverage by her car insurance company but sold her car and the meteorite for $69,000.

The last big impact to the Earth was in the region of the Tunguska region of Siberia on the morning of June 30, 1908. The stony object was 30 meters or about 100 feet across.

At an altitude of five miles, the object exploded, sending a pressure wave Earth ward that flattened nearly 800 square miles of forests. The energy of this shock wave was equivalent to 4 million tons of TNT. (The 15-meter wide stony asteroid that shattered in the lower stratosphere over the Ural Mountains in Russia last February 15 had an energy of 400,000 tons of TNT.

Most of this energy didn’t reach the ground.) There are a million of these size objects in the Earth’s vicinity so the frequency of such an impact to Earth is about once a century.

The impact of a 1-kilometer wide object (33 times wider than the Tunguska object) would have an energy release about 100,000 times greater, enough to cause extinctions of animals and plants and alter the course of human civilization. The frequency of such an impact would be once in 700,000 years.

My next column will deal with sky sweeps by robotic telescopes to detect such impactors and what could be done to deflect them away from us.

SKY SIGHTS AHEAD: Tomorrow morning, the crescent moon will appear below and to the west of the planet Mars in the southeastern dawn (try 5:30 a.m.) On Sept. 5, the moon will swing from the morning to the evening side of the sun.

On Sept. 8, there will be a line up of the crescent moon and the brilliant planet Venus low in the western dusk (best time to view is 7:35 p.m. from a place with a flat western horizon).

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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