Local Versus Global
Scientifically, it is useful to divide the impact hazard into two types of events: those with local consequences and those with 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). These impacts 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 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, and tsunamis. A Tunguska-like event probably occurs somewhere on Earth's surface once every 1,000 years or so. 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 10,000 years. Looking at it another way, the risk for a Tunguska-sized impact on a lightly or densely populated area is about 1 percent per century.
What distinguishes "local" impacts from "global" impacts are the responses of Earth's ecosystem and inhabitants. 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.
A global event is one where impact fallout (dust lofted into the stratosphere, smoke from wildfires, and so on) causes 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 where on Earth the impact takes place -- inhabited or uninhabited areas, northern or southern hemisphere, land or ocean.
Sizing Up The Threat
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 on the kinetic energy of the impact (equal to half of the mass of the impactor times the square of its velocity), a comet smaller than 1 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 100 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 million years on average.
Perception of Risk -- Low-Probability, High-Consequence
From a sociological standpoint, it is important to consider whether the hazard due to cosmic impacts is worth worrying about at all. Cosmic impacts fall into the category of events that are extremely rare but are of high consequence when they do occur. An airliner crash is an example of an infrequent but high-consequence event that seems to grab international attention. Motor vehicle accidents, on the other hand, kill 200 times more people in an average year, yet these frequent events, with lesser consequences per event, garner comparatively less public attention.
Thus it would seem that we, as a society, are attuned to low-probability but high-consequence events. However, extremely low-probability events such as cosmic impacts are beyond our personal and even historical experience, requiring that we take a long-term view in evaluating the hazard and relating it to everyday life.
One way to examine the cosmic impact hazard is to compare the long-term threat to you as an individual posed by the two categories of collisions: the local Tunguska-like events and the larger, global-consequence events. Tunguska-like events occur on average once every 1,000 years and are likely to directly result in your death only if you happen to be within the approximately 1,000-square-kilometer (400-square-mile) region of devastation. Given the surface area of Earth, it is fortunate that there is only a 1 in 500,000 chance that you would be at the wrong patch of the planet at the wrong time.
Thus, in any given year, there is only a 1 in 500 million chance that you will die from a Tunguska-like impact. Over a human lifetime, which we round up to an even 100 years for simplicity, it would seem there is only a 1 in 5 million chance that a Tunguska-like impact will result in your untimely death. A 1 in 5 million chance may be small enough that most people would give it little practical concern.
What about the comparative hazard from much less frequent global-scale impacts? If we assume that such events occur only once every million years but are so devastating to the climate that the ultimate result is the death of one-quarter of the world's population, this translates to an annual chance of 1 in 4 million that you will die from a large cosmic impact even if you happen to be far removed from the impact site. Integrated over a century, our simple metric for a human lifetime, the chance becomes 1 in 40,000 that a large cosmic impact will be the cause of your death. Such a probability is in the realm that most people consider a practical concern.
NEO scientists Clark Chapman and David Morrison estimated the chances of an individual dying from selected causes in the United States:
- Motor vehicle accident: 1 in 100
- Murder: 1 in 300
- Fire: 1 in 800
- Firearms accident: 1 in 2,500
- Electrocution: 1 in 5,000
- Passenger aircraft crash: 1 in 20,000
- Flood: 1 in 30,000
- Asteroid/comet impact: 1 in 40,000
- Tornado: 1 in 60,000
- Venomous bite or sting: 1 in 100,000
- Fireworks accident: 1 in 1 million
- Food poisoning by botulism: 1 in 3 million
- Drinking water with EPA limit of TCE*: 1 in 10 million
Source: Reprinted from Clark Chapman and David Morrison, Nature, Vol. 367, page 39 (1994).
*EPA, Environmental Protection Agency; TCE, trichloroethylene.
Finding Out What The Threat Is
If we do find the hazard from cosmic impacts to be a matter of concern, what can we do about it? The first step is to gain more knowledge. Currently, we have discovered probably about 60 percent of the near-Earth asteroid and short-period comet population having body sizes of 1.5 kilometers or larger, whose impacts could have global consequences. For smaller asteroids and comets, our knowledge of the population is correspondingly less complete.
Thus, it is providential that a straightforward survey strategy address the greatest hazard (largest objects) first and evolve to evaluate more thoroughly the lesser hazard as time progresses. Through such a stepwise increase in our knowledge, we can prudently evaluate what approach should be taken to mitigate any possible hazard. For any asteroid or short-period comet actually found to be in a menacing orbit, chances are quite high that we will have decades of advance warning before a hazardous encounter would occur.
Long-period comets (those coming in from the outer solar system), however, would likely be detected only a few months before they reached the vicinity of Earth. Through a Spaceguard-like survey evaluation of the proportion of long-period comets crossing Earth's orbit, we can make a rational assessment of the hazard posed by these objects.
An additional area of knowledge that we can advocate gaining access to is the military surveillance satellite data on small-scale impacts into Earth's atmosphere. Meteoroids with energies equivalent to the Hiroshima bomb strike the atmosphere annually. Fortunately, the US Department of Defense has begun to release information on selected recent events. However, a fuller disclosure of the signatures and frequencies of these types of events would help reduce the risk that such a natural event, occurring over an area of political tension, would trigger a martial and possibly nuclear response.
Finally, it is vital to evaluate whether near-Earth objects really are our foes or our friends. Over the next three centuries, there is a 1 in 30 chance that a Tunguska-like impact will result in some human casualties and a 1 in 3,000 chance for a larger, global-scale impact. A Spaceguard survey, however, is certain to find in near-Earth orbits several thousand non-threatening objects that are more accessible than the Moon in terms of rocket propulsion. Over the next three centuries (and hopefully sooner), these objects can provide intermediate mission destinations as we prepare for long-duration human flights to Mars. As we begin to utilize space, the metals and volatiles (chiefly water) we find in these objects may become vital space resources. Thus, in taking a long view of only a few centuries, it is most likely that we will know the near-Earth objects as our friends. The lesson for us now is to keep in mind that all friends need respect.
Richard P. Binzel and his student Cristina Thomas contributed to this article. Binzel is a professor of planetary science at the Massachusetts Institute of Technology. He was a coeditor for Asteroids III, the primary reference book for the field, which was published in 2002.