Earth's Closest Neighbors

The dinosaurs became extinct because they didn't have a space program.

— Larry Niven

Michelle Knapp and Her 1980 Chevrolet Malibu

Let me introduce Michelle Knapp of Peekskill, New York, and her 1980 Chevy Malibu sedan. On a rainy Friday night, October 9, 1992, just before 8:00 PM, Michelle, an eighteen-year-old high school senior, heard a loud crash in her driveway and raced outside to discover the rear end of her automobile had been completely destroyed by a football-sized rock. The twenty-seven-pound projectile had punched completely through the trunk, just missing the gas tank.

As unlikely as it sounds, a fragment of a near-Earth asteroid that had collided with Earth destroyed Michelle's car. The fiery trail of the initial, Volkswagen-sized, near-Earth asteroid was first seen over West Virginia appearing with a greenish hue and brighter than the full Moon. Due to the forces of the atmospheric resistance, the asteroid fragmented into more than seventy pieces while traveling northeast for more than forty seconds over Pennsylvania and then New York. The only known surviving fragment came to a full stop underneath Michelle's Chevy Malibu. Countless people, many of whom were watching high school football games that Friday evening, observed the fiery train of fragments in the Pennsylvania and New York skies. Although Michelle's automobile insurance company refused to pay for her car's damages, claiming that it was an act of God, she got the last laugh by selling the so-called Peekskill meteorite and the twelve-year-old Chevy to a consortium of three meteorite collectors for $69,000.

On a daily basis, at least one hundred tons of interplanetary material rain down upon the Earth's atmosphere, but most of it is in the form of very small dust particles or very small stones. Much of this dust and sand grain–sized material, the debris from active comets, can be seen on almost any clear, dark night as meteors or shooting stars. Larger basketball-sized rocks rain down upon the Earth daily and while they can cause impressive fireball events, our atmosphere prevents almost all of them from reaching the ground intact. Volkswagen-sized asteroids, like the one that fragmented and caused the Peekskill meteorite, strike the Earth's atmosphere every six months or so on average. At this point, you may be incredulous because you may not have ever seen a fireball and probably not a major fireball like the Peekskill event. But the vast majority of the Earth's surface is either ocean or unpopulated and besides, how often do you monitor the skies all night? Department of Defense satellites do, however, continuously monitor the skies and detect fireballs, as they look downward, twenty-four hours each day, to provide alerts of possible missile launch events and nuclear explosions.

Mr. S. B. Semenov Gets Blown off His Porch

Allow me to introduce Mr. S. B. Semenov, who was an eyewitness to a larger Earth-impacting near-Earth asteroid on June 30, 1908, in a remote region of Russian Siberia called Tunguska.

Mr. Semenov, a farmer, was sitting at a trading post when he noticed what appeared to be a fire high and wide over the local forest. A loud and strong shock ensued, blowing him a few meters off the trading post porch. He noted that the heat from the blast felt like his shirt was on fire, even though he was located about sixty-five kilometers south of ground zero. Although a wide variety of suggestions have been made to explain the Tunguska event, including the absurd notions that the blast was due to a UFO crash or an overzealous signal greeting by aliens, by far the most likely cause of the Tunguska blast was an atmospheric impact by a near-Earth asteroid. Most likely, a thirty-meter-sized asteroid entered the Earth's atmosphere and reached an altitude of about eight kilometers before the atmospheric pressure in front of the stony object "pancaked" the rock, causing it to explode above the forest floor. A tremendous blast wave then continued to the surface, and an area that spanned some two thousand square kilometers of forest, involving millions of trees, was leveled. But since the stony asteroid itself disintegrated in the air blast explosion and did not reach the ground, no crater was evident on the forest floor and no sizable meteorites were left near ground zero. Current estimates place the energy of the event at about four million tons (four megatons) of TNT high explosives. Considering there are more than a million of these asteroids, thirty meters and larger, in the Earth's neighborhood, one would expect an asteroid of this size or larger to strike the Earth every few hundred years. Thirty meters, or Tunguska-sized, is about the minimum diameter of an Earth impactor that could cause significant ground damage. In general, smaller stony objects would not be expected to survive passage through the Earth's atmosphere.

The Dinosaurs Check Out Early

At the upper end of the near-Earth object sizes are about one thousand asteroids larger than one kilometer in diameter, and an Earth impact by one of these asteroids would be capable of causing global devastation. Fortunately asteroids of this size would not be expected to strike Earth but every seven hundred thousand years on average, and NASA's ongoing search programs have already found more than 90 percent of this population. None of them represents a credible threat during the next century. The very largest near-Earth objects are as large as ten kilometers in diameter. Sixty-five million years ago, one of them killed much of the land and sea flora and fauna along with almost all of the large vertebrates on land and at sea. Most species were exterminated. Crater evidence for this major extinction event has been found near Chicxulub on the edge of the Mexican Yucatán peninsula. A ten-kilometer impactor could cause a so-called extinction event because it would subject the Earth to global firestorms, severe acid rain, and the darkening of the skies with soot and impact-created debris. The resultant loss of photosynthesis would cause plants to die along with the animals and marine life that depend upon these plants for food. After more than a 160-million-year run, the large land dinosaurs could not survive this impact event because of the complete disruption of their food chain. An Earth impact by a ten-kilometer-sized near-Earth object would create an impact of unimaginable energy —...