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 cause of the craters is impacts by comets and asteroids. Most asteroids follow simple 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 follow highly elongated orbits that often come close to Earth or other major bodies to begin with. 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 and suffers the insult of a major impact.
The 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 this small debris 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 thermal explosions. 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 fire ball was seen streaking across the sky all the way from Kentucky to New York. A 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 well documented incursions in this century. A 10-meter body typically has the kinetic energy of about five nuclear warheads of the size dropped on Hiroshima, 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 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 150 US tons of fragments reached the ground, the largest intact fragment weighing 3,839 pounds. The fragments covered an area of about 1 x 2 kilometers (0.6 x 1.2 miles), within which there were 102 craters greater than 1 meter in diameter, the largest of them 26.5 meters (87 feet), and about 100 more smaller craters. If this small iron meteoroid 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 car suddenly drop in at supersonic speeds! 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 that start to become really worrisome. The 1908 Tunguska event was a stony meteorite in the 100-meter class. The famous meteor crater in northern Arizona, some 1219 meters (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 in diameter. It probably survived nearly intact until impact, at which time it was pulverized and largely vaporized as its 6-7 x 1016 joules* of kinetic energy were rapidly dissipated in an explosion equivalent to some 15 million tons of TNT! Falls of this class occur once or twice every 1000 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 1500-meter (5000 feet) asteroid or comet hit there, excavating more than a trillion tons of material and scattering it all over Europe. This sort of thing happens about once every million years or so. Another step upward in size take us to Chicxulub, 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 2-3 times larger yet about which geologists are uncertain. 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 up to about 8 kilometers. A working group chaired by Dr. David Morrison, NASA Ames Research Center, estimates that there are some 2,100 such asteroids larger than 1 kilometer and perhaps 320,000 larger than 100 meters, the size that caused the Tunguska event and the Arizona Meteor Crater. An impact by one of these larger meteors in the wrong place would be a catastrophe, but it would not threaten civilization. However, the working group concluded that an impact by an asteroid larger than 1-2 kilometers could degrade the global climate, leading to widespread crop failure and loss of life. Such global environmental catastrophes, which place the entire population of the Earth at risk, are estimated to take place several times per million years on average. A still larger impact by an object larger than about 5 kilometers is damaging enough to cause mass extinctions. In addition there are many comets in the 1-10 kilometer class, 15 of them in short-period orbits that pass inside the 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 the Earth could become dangerous after a close passage by Jupiter.
This all sounds pretty scary. However, as noted earlier, no human in the past 1000 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 small, but the risk increases with the size of the impacting comet or asteroid, with the greatest risk associated with global catastrophes resulting from impacts of objects larger than 1 kilometer. NASA knows of no asteroid or comet currently on a collision course with Earth, so the probability of a major collision is quite small. In fact, as best as we can tell, no large object is likely to strike the Earth any time in the next several hundred years. To be able to better calculate the statistics, astronomers need to detect as many of the near-Earth objects as possible. It's likely that we could identify a threatening near-Earth object large enough to potentially cause catastrophic changes in the Earth's environment, and most astronomers believe that a systematic approach to studying asteroids and comets that pass close to the Earth makes good sense. It's too late for the dinosaurs, but today astonomers are conducting ever-increasing searches to identify all of the larger objects which pose an impact danger to Earth.
* joule: a unit of measurement, the amount of energy corresponding to one watt
acting for one second.