Scientifically, it is useful to divide the impact hazard into two types of events: those having local consequences and those having global consequences. On the low end of the local scale is the fall of meteorites that seem to have a propensity for conking cars (for example, the October 9, 1992, fall in Peekskill, New York, that demolished an old Chevrolet) but are not known to have caused any serious human injuries in modern times. Progenitors for such meteorite falls are probably bodies only a few meters across. Bodies 50 meters (about 160 feet) across having modest strengths are likely to strike the ground intact, creating a crater and a local explosion. The 1908 airburst over the Tunguska River in Siberia was probably due to the atmospheric entry of a comet or weak asteroid about 50 meters across.
Had the Tunguska blast, which leveled 1,000 square kilometers (400 square miles) of forest, occurred over a populated area, the result would have been a devastating disaster with a death toll equivalent to or exceeding such other natural disasters as floods, hurricanes or tsunamis. A Tunguska-like event probably occurs somewhere on the Earth's surface once every three centuries. Estimating that only 10 percent of Earth's surface is lightly or densely populated, a threat to humans from such an impact is likely to occur once every 3,000 years.
What distinguishes a "local" impact event from a "global" one are the responses of Earth's ecosystem and society. While the occurrence of a Tunguska-like or larger event over a major city would be an unprecedented human disaster, the consequences to the worldwide ecosystem and climate would be minimal. Assuming that the cosmic impact is not misinterpreted as a hostile nuclear attack set in motion by a real or imagined enemy, the remaining civilizations of the world would presumably remain stable and would be able to supply aid and comfort to the afflicted area.
In contrast, a global event is one where the impact fallout (dust lofted into the stratosphere, smoke from possible wildfires and so on) causes a global climate change sufficient to disrupt worldwide agriculture and threaten mass starvation. For a global event, all citizens of the world are endangered, regardless of whether the impact occurs in an inhabited or uninhabited part of our planet.
Most estimates suggest that an impacting stony asteroid about 1.5 kilometers (1 mile) across or larger marks the threshold energy for causing a globally devastating event. However, there is much uncertainty associated with making this size estimate, and realistic guesses fall between 0.5 and 5.0 kilometers (0.3 and 3 miles). One part of the uncertainty is the lack of knowledge about how our planet's ecosystem and our society would respond to the sudden and severe stress wrought by an impact. Another area of uncertainty arises from variations in the nature of potential impactors.
For example, asteroids in near-Earth space typically encounter our planet with velocities of about 20 kilometers (12 miles) per second. Comets, however, encounter Earth with much higher velocities, typically 30 to 60 kilometers (19 to 37 miles) per second. Because the damaging effects are dependent upon the kinetic energy of the impact [equal to half the mass of the projectile times the square of its velocity], a comet smaller than one kilometer (0.6 mile) across could pack a punch with sufficient energy to initiate a global climate disaster.
Given their greater numbers in near-Earth space, asteroids probably account for 75 percent of the total hazard. Comets comprise the other 25 percent. From the recent lunar cratering record, from the record of more than a hundred now identified terrestrial craters and from our preliminary reconnaissance of near-Earth space, we can estimate that the impact of a 1.5 kilometer asteroid (or equally energetic comet) probably occurs on Earth once every 500,000 years on average.
Paraphrase of further discussion of probabilities of human death due to impact of comets and asteroids: Although Tunguska-like impacts can be anticipated every 300 years somewhere on the planet, their relatively localized effects make the chances of dying from such a calamity only about 1 in 1.5 million for any individual human being. By comparison, the chances (for U.S. citizens) of dying in a car accident are about 1 in 100; of being accidentally shot, about 1 in 2,500; and of dying in a plane crash, about 1 in 20,000 (these statistics are taken from the journal "Nature," vol 367, page 39, 1994). On the other hand, although much less frequent, the potential global consequences of a major asteroid or comet strike make the odds of dying in such a calamity as low as 1 in 20,000, or about the same as those for dying in a plane crash. For this reason, and with the recent example of the impact of Comet Shoemaker-Levy 9 on Jupiter, the danger associated with such a strike is being taken much more seriously today than at any time in the past.
Original file name: .CNI - Asteroid impacts
This file was converted with TextToHTML - (c) Logic n.v.