COLUMN

Floating In A Cosmic Shooting Gallery

Sometimes, it's comforting to know that none of us are really in charge. There but for the grace of the great roulette wheel in the sky go I.

I refer to the planetary stray bullets called asteroids, one of which streaked into the Russian atmosphere last week to produce a meteor brighter than the daytime sun. When this stony mass about 50 feet wide exploded at a height of 12 to 15 miles, it sent out a shock wave 10 times more powerful than the atomic bombs dropped to end World War II. It damaged many urban buildings and injured hundreds in the region of Chelyabinsk, mostly from flying shards of glass. A shower of meteorite fragments fell near Chebarkul Lake. They're now being gathered and sold as mementos.

The last time something like this happened in Russia was the much more powerful 1908 Tunguska event. Its shock wave splintered the forest over an area of 830 square miles. This is equivalent to a radius of 19 miles surrounding the Capitol dome in Hartford, large enough to blast all of its suburbs.

On the one hand, this was a very remarkable event. On the other hand, it was business as usual for planet Earth. Understanding this false dichotomy and what it means is my main purpose here. The key concept is what scientists call a log-log frequency distribution.

First, a quick review. Using powers of 10, the log of 1,000 is 3 because it's the same as 10 raised to the third power. Similarly, the log of 100 is 2, and that of 10, 1. This conversion provides an easy way of putting very large and very small numbers on the same scale.

The extraterrestrial objects that bombard us arrive continuously in a range of scales from microscopic dust to asteroids measuring miles across. Exceptionally powerful collisions happen very rarely. Technically, magnitude and frequency are inversely related, plotting as a straight line on a log-log graph. The same is true for earthquakes and the events in our lives. Statistically, big events like a marriage or divorce are more rare than birthdays by some power of 10, which are more rare than breakfasts by some power of 100.

Aside from gravitational effects due to Earth's orbit, the rain of extraterrestrial objects is continuous, and approximates a log-log distribution. We take no notice whatsoever of the billions of dust-sized fragments; no public notice of "falling stars" at night; only short-term notice of decade-scale events such as the Chelyabinsk meteor; and historic notice of century-scale events such as Tunguska. To these three reported events, we can add three more reconstructed by geology that put the Chelyabinsk explosion into its proper perspective.

The first created Meteor Crater in Arizona about 50,000 years ago. A nickel-iron asteroid about 160 feet across slammed straight down at a speed of at least 25,000 mph. The resulting crater, about 4,000 feet across, is now a major tourist attraction. The log of its age is 4. The second event created the now buried Chicxulub crater on the Yucatan Peninsula in Mexico about 66 million years ago. An asteroid 6 to 9 miles across collided obliquely with Earth to take out an estimated 75 percent of all species, including the dinosaurs. The log of its age is 6. The third event was the collision of a Mars-sized object with the newborn earth about 4.5 billion years ago. This melted the whole planet and created Earth's moon. The log of its age is 9.

NASA estimates that there are approximately a million potentially threatening asteroids comparable to those that caused the Chelyabinsk and Meteor Crater events. We've spotted and named some of the larger ones. The smaller ones still come out of nowhere and blindside us.

Each day of our lives is a spin of an orbital roulette wheel. We must learn to never take a day for granted. And we must learn to take out lumps in stride, at least until a really big one takes us out.

Robert M. Thorson is a professor of geology at the University of Connecticut's College of Liberal Arts and Sciences. His column appears every other Thursday. He can be reached at profthorson@yahoo.com.

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