Sunday, February 24, 2013

Big meteorites may be fairly common

John Goss

John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.

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Our planet's connection with the unknown of outer space certainly became apparent a few days ago when a celestial traveler crashed near Chelyabinsk in central Russia, causing significant property damage. A house-size, 10,000-ton space rock, speeding at 40,000 miles per hour, struck our little world. Some scientists called it a once-in-a-hundred-year event. Was it really?

Rocky and icy debris enter our planet's upper atmosphere every hour of every day. Because most people live in cities with skies fouled by light pollution, they don't readily look up, and they miss what comes from above. They miss many sporadic meteors every night. They miss the fireball that streaks brightly across the sky every few nights. They miss the icy remnants of comet tails that rain down in the form of meteor showers every year. If they saw these things, they would realize that our planet collides with the flotsam of the inner solar system all the time.

Adding to the confusion of that day was the close passing of the small asteroid 2012 DA14 that occurred just 16 hours after the meteorite strike. Many people assumed that the two events were connected (especially after reports surfaced of other, but much smaller, meteors seen over other countries).

Astronomers say that this was mere coincidence, that the asteroid and the Russian meteorite were completely unrelated. The meteor was moving in an east-to-west direction while the asteroid was traveling south to north. They weren't moving together through space and they were always separated by at least 1 million miles.

How often does a meteor the size of the Chelyabinsk rock hit our planet? It was quickly reported that this was the largest incident since the 1908 Siberian Tunguska event, which leveled trees for 800 square miles. Remember that the Earth's surface is nearly 70 percent ocean and that the land area has large, relatively unpopulated expanses. One, two or three giant meteors could go unseen and unrecorded every few years.

U.S. military satellites placed in orbit to detect the flashes from missile launches also catch the flashes from disintegrating meteors. Much of the information, which would help determine the frequency and magnitude of meteor strikes, is classified and, therefore, not completely released to the scientific community.

So, how often does a large meteor plow into our planet? No one knows with certainty.

March brings another space visitor, one that comes no closer than a harmless 100 million miles. In June 2011, Comet PanSTARRS — named after the telescope that discovered it, the Panoramic Survey Telescope and Rapid Response System — was found dimly glowing in the constellation Scorpius and lurking far beyond the orbit of Jupiter.

Today, the comet is swinging around the sun, approaching closest to it on March 9, some 7 million miles closer than the planet Mercury orbits. If we are lucky, the sun's heat will be great enough to cause the comet's primordial icy nucleus to begin to evaporate, forming a long, beautiful tail, but not so intense as to cause the nucleus to fracture and crumble, depriving us of a great celestial show.

Beginning on March 9, look to the west 30 minutes after sunset for the comet's tail rising in the twilight. Three nights later, the comet's head might be glimpsed just above the horizon with the very thin crescent moon to its immediate right. The next night, the moon should be higher, glowing in the comet's tail.

PanSTARRS moves nearly parallel to the horizon toward the northwest until it fades entirely from view before the end of March.

For a clearer view, use binoculars to admire this very welcome space traveler.

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