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Danger from the Sky
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By Stephen Browne
Stephen Browne
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By Stephen W. Browne
Feb. 21, 2013 11:19 a.m.

Note: This is my weekly op-ed column.
Last week the world learned of a meteor that exploded over the Ural mountains on Friday, shattering windows over a 2,000,000 square foot area and injuring roughly 200 children and 1,000 adults.
The meteor broke up in the atmosphere creating damage through the shock wave, but at least one piece fell to earth and broke through the ice of a local lake.
It could have been worse. In the Tunguska event of June 30, 1908, a meteor exploding in the air over Krasnoyarsk Krai, Russia, knocked down an estimated 80 million trees over an 830 square mile area. If it had hit anywhere but out in the Siberian boonies, it would have been a Hiroshima-level event.
To give you an idea of the difference, the energy from Friday’s event has been compared to a nuclear explosions measured in kilotons of TNT. The Tunguska event is measured in megatons.
Estimates of the size of the Tunguska meteor vary, but it was probably on the order of 330 feet.
If that hasn’t disturbed your sleep yet, on Friday asteroid 2012 DA14 flew by the earth within 17,150 miles. That’s within the orbit of the moon, in fact closer than the orbit of some communications satellites.
Estimates of the size of 2012 DA14 vary between 130 and 160 feet, and most accounts just call it “football field” sized. You could call it half a Tunguska.
Now what’s really scary is, the two events are not related. Astronomers say the two objects were going in different directions, the fact they happened the same day is, “just a coincidence.”
Let that sink in for a moment. Purely by chance we had one hit and one near-miss on the same day.
A bit of background, highly simplified. The solar system used to have a lot more sky junk zipping around. Much of it has been hooverd up by great Jupiter, but quite a lot of it has impacted the inner planets over time.
The surface of the other major rocky bodies: the moon, Mars, and Mercury are pitted with craters like a very bad case of acne.
Venus and the earth have thick atmospheres that are highly erosive, so evidence of past strikes is worn away over time. Yet even on earth there remains evidence of past giant impacts.
The extinction-level event that wiped out the dinosaurs, and an estimated 90 percent of the species living on earth at the time is now generally accepted to have been a meteor strike.
The Manson Crater in Iowa, now buried under glacial till, is evidence of the ancient impact of an asteroid more than a mile across. It was once a prime candidate for the dinosaur extinction event, until proven to be too old.
Meteor Crater in Arizona is 4,000 in diameter and 570 feet deep, after 50,000 of erosion.
Getting nearer historical times, about 14,000 years ago a meteorite (when one actually hits the ground it’s called a meteorite) hit northern Canada and caused a mini-ice age.
And about 5,000 years ago one landed in the Indian Ocean, causing a tsunami thought by some to be the origin of the Great Flood legends.
However, these are very rare. Thousands of meteors hit the earth every day, most ranging in size from a grain of sand to a basketball. They are the ones which burn up in the atmosphere, creating the glorious “shooting stars.”
Occasionally, on the order of once a week, a meteor the size of a car will hit the earth, about once a month one the size of a house. Recently one of those exploded over Indonesia, causing some panic but no damage.
There are a couple of things we ought to take into consideration. One is that there are far more densely populated areas around the world than ever before. A Tunguska-sized event in just the right place, or another Indian Ocean event would have far more disastrous consequences today than at any time in the past.
Another is, “rare” does not mean “never will happen.” Wait long enough and it’ll happen again.
For the first time in history we’re able to keep track of sky junk that passes close enough to give cause for alarm, and we’re getting better at it every day. Forewarned is forearmed as they say. Given enough warning we have the capability to smack an asteroid out of the way.
As often happens, a profound observation was recently expressed as a joke going around.
“That asteroid was God’s way of asking, ‘How’s that space program going?’”

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