Friday`s meteor and asteroid `once in 1,000-years event`

Last Updated: Sunday, February 17, 2013 - 13:04

Washington: As scientists were gearing up to witness an asteroid`s closest ever approach to Earth on Friday, a sizeable meteor exploded over Russia, causing thousands of injuries and major damage to buildings.

As these two rare events occurred the same day, one may think they should be connected, but scientists say they are not, CCN reported.

The meteor that fell Friday near Chelyabinsk, Russia, was pretty big, maybe 50 feet across. In 1908, a slightly larger meteor-perhaps three times larger in diameter, or 27 times larger in mass-flattened a thousand square miles of forest near Tunguska, Russia, downing some 80 million trees.

Now, NASA scientists have estimated that meteors as large as Friday`s might hit the Earth every decade or two, while Tunguska-like events are estimated to occur once every 1,000 years.

The close fly-by of an asteroid like DA14, like the Tunguska meteor, is a once-in-1,000-years event.

With the two rare events happening at approximately the same time, Friday was an extremely unusual day.

Mathematically, if the events are not associated, the probability of this coincidence comes from multiplying the individual probabilities.

First of all, in the time between the two events, the Earth moved roughly 300,000 miles, meaning the asteroid and the meteor were in completely different places. Moreover, they traveled in completely different directions, so they couldn`t have been associated.

So there is no way the meteor and the asteroid are connected. It has to be a coincidence that the two events happened on the same day.

Yet this would seem to be at odds with our instinct that two very rare things would not happen at the same time.

How can we reconcile these two opposite thoughts: the impossibility of an association based on the physics of trajectories, and the improbability of coincidence (lack of association) that the math suggests?

According to CCN, the answer is that we need to rethink the probability calculation. If asteroids as big as DA14 pass close to Earth once every decade or two, and meteors as large as the Chelyabinsk one impact once every 100 years (a similar meteor having caused the Tunguska event in 1908), the chance of both events happening on any one day are indeed very small: 1 in 3,650 days times 1 in 36,500 days, or about 1 in 100 million-not odds you would bet against.

But think again: The Earth has been around for 4.5 billion years-which is 1.6 trillion days. So the chance that these two events would happen on a day sometime in the earth`s history is actually larger than we first thought-it ought to have happened about 12,000 times already.

Of course, during most of that 4.5 billion year history, the earth was not populated by intelligent life-human beings who might have noticed the two events happening on the same day.

So what is the probability that the meteor hits and the asteroid passes Earth on the same day when someone could record it on video? That`s probably been possible for about 50 years, or only about five years if we have to do it on a smartphone or dashboard camera. That`s 1,825 days, which means the chance of someone filming the event is only about one in 70,000 -- and that`s if people blanketed the Earth.

Given how sparsely the Earth is populated, we should correct this number downward by a (large!) geographical factor. It`s also unlikely that this event would happen within 3,000 miles of the Tunguska impact.

The rough calculation says a large meteor impact on the same day as closest passage of the DA14 asteroid is really improbable.

But it did happen. Something in our assumptions could be wrong. For example, the frequency of meteor impacts could be much larger and our estimates too low because we don`t notice most of them.


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First Published: Sunday, February 17, 2013 - 13:04

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