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Most bodies in the solar system with a visible solid surface exhibit craters. On Earth, we see very few because geological processes such as weathering and erosion soon destroy the obvious evidence. On bodies with no atmosphere, such as Mercury or the Moon, craters are everywhere. Without going into detail, there is strong evidence of a period of intense cratering in the solar system that ended about 3.9 billion years ago. Since that time, cratering appears to have continued at a much slower and fairly uniform rate. The craters were caused by the impacts of comets and asteroids. Most asteroids follow sensibly circular orbits between the planets Mars and Jupiter, but all of these asteroids are perturbed, occasionally by each other and more regularly and dramatically by Jupiter. As a result, some find themselves in orbits that cross that of Mars or even Earth. Comets, on the other hand, as noted in Section 2, follow highly elongated orbits that often come close to Earth or other major bodies. These orbits are greatly affected if they come anywhere near Jupiter. Over the eons every moon and planet finds itself in the wrong place in its orbit at the wrong time, many times, and suffers the insult of a major impact.
Earth's atmosphere protects us from the multitude of small debris, the size of grains of sand or pebbles, thousands of which pelt our planet every day. The meteors in our night sky are visible evidence of bodies of this type burning up high in the atmosphere. In fact, up to a diameter of about 10 meters (33 feet), most stony meteoroids are destroyed in the atmosphere in a terminal explosion. Obviously, some fragments do reach the ground, because we have stony meteorites in our museums. Such falls are known to cause property damage from time to time. On October 9, 1992, a fireball was seen streaking across the sky all the way from Kentucky to New York. A 12-kilogram (27-pound) stony meteorite (chondrite) from the fireball fell in Peekskill, New York, punching a hole in the rear end of an automobile parked in a driveway and coming to rest in a shallow depression beneath it. Falls into a Connecticut dining room and an Alabama bedroom are other well documented incursions in this century. A 10-meter (33-foot) body typically has the kinetic energy of about five Hiroshima fission bombs, however, and the shock wave it creates can do considerable damage even if nothing but comparatively small fragments survive to reach the ground.
Many fragments of a 10-meter (33-foot) iron meteoroid will reach the ground. The only well studied example of such a fall in recent times took place in the Sikhote-Alin Mountains of eastern Siberia on February 12, 1947. About 136 metric tons (150 tons) of fragments reached the ground, the largest intact fragment weighing 1,741 kilograms (3,839 pounds). The fragments covered an area of about 1 to 2 square kilometers (0.4 to .8 square miles). within which there were 102 craters greater than 1 meter (39 inches) in diameter, the largest of them 26.5 meters (87 feet), and about 100 more smaller craters. If this small iron meteorite had landed in a city, it obviously would have created quite a stir. The effect of the larger pieces would be comparable to having a supersonic auto suddenly drop in! Such an event occurs about once per decade somewhere on Earth, but most of them are never recorded, occurring at sea or in some remote region such as Antarctica. It is a fact that there is no record in modern times of any person being killed by a meteorite.
It is the falls larger than 10 meters (33 feet) that start to become really worrisome. The 1908 Tunguska event described in Section 7 was a stony meteorite in the 100-meter (330-foot) class. The famous meteor crater in northern Arizona, some 1.2 kilometers (4,000 feet) in diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 meters (197 feet) in diameter. It probably survived nearly intact until impact, at which time it was pulverized and largely vaporized as its 6- 7 4 1016 joules of kinetic energy were rapidly dissipated. An explosion equivalent to some 15 million tons of TNT creates quite a bang! Falls of this class occur once or twice every 1,000 years.
There are now over 100 ring-like structures on Earth recognized as definite impact craters. Most of them are not obviously craters, their identity masked by heavy erosion over the centuries, but the minerals and shocked rocks present make it clear that impact was their cause. The Ries Crater in Bavaria is a lush green basin some 25 kilometers (15 miles) in diameter with the city of Nordlingen in the middle. Fifteen million years ago a 1,500-meter (5,000-foot) asteroid or comet hit there, excavating more than a trillion tons of material and scattering it all over central Europe. This sort of thing happens about once every million years or so. Another step upward in size takes us to Chicxulub, described in detail in Appendix B, an event that occurs once in 50-100 million years. Chicxulub is the largest crater known which seems definitely to have an impact origin, but there are a few ring-like structures that are two to three times larger yet about which geologists are suspicious.
There are now more than 150 asteroids known that come nearer to the Sun than the outermost point of Earth's orbit. These range in diameter from a few meters to about 8 km. A working group chaired by D. Morrison estimates that there are some 2,100 such asteroids larger than 1 kilometer (.6 miles) and perhaps 320,000 larger than 100 meters (330 feet), the size that caused the Tunguska event and the Arizona meteor crater. An impact by one of the latter in the wrong place would be a great catastrophe, but it would not threaten civilization. An impact by an 8-kilometer (5-mile) object is in the mass extinction category. In addition, there are many comets in the 1- to 10-kilometer (.6 to 6-mile) class, 15 of them in short-period orbits that pass inside Earth's orbit, and an unknown number of long-period comets. Virtually any short-period comet among the 100 or so not currently coming near to Earth could become dangerous after a close passage by Jupiter.
This all sounds pretty scary. However, as noted earlier, no human in the past 1,000 years is known to have been killed by a meteorite or by the effects of one impacting. (There are ancient Chinese records of such deaths.) An individual's chance of being killed by a meteorite is ridiculously small as compared to death by lightning, volcanism, earthquake, or hurricane, to say nothing of the multitude of human-aided events. That small probability was unlikely to have been any consolation to the dinosaurs, however. For this reason, astronomers today are conducting ever-increasing searches for all of the larger asteroids that could become dangerous. Once discovered, with a few years of warning, there is every reason to believe that a space mission could be mounted to shove them aside.
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