NASA Ames Space Science Division
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In the following discussion, we examine the risks posed by impacting objects of various sizes. These projectiles could be either cometary or asteroidal. In terms of the damage they do, it matters little whether they would be called comets or asteroids by astronomical observers. We term these objects collectively NEOs (Near Earth Objects).
Every few centuries the Earth is struck by an NEO large enough to cause thousands of deaths, or hundreds of thousands of deaths if it were to strike in an urban area. On time scales of millennia, impacts large enough to cause damage comparable to the greatest known natural disasters may be expected to occur (Pike 1991). Indeed, during our lifetime, there is a small but non-zero chance (very roughly 1 in 10,000) that the Earth will be struck by an object large enough to destroy food crops on a global scale and possibly end civilization as we know it (Shoemaker and others 1990).
As described in Chapter 3, estimates of the population of NEOs large enough to pose a global hazard are reliable to within a factor of two, although estimates of the numbers of smaller objects are more uncertain. Particularly uncertain is the significance of hard-to-detect long-period or new comets, which would generally strike at higher velocities than other NEO's (Olsson-Steel 1987), although asteroids (including dead comets) are believed to dominate the flux. However, the resulting environmental consequences of the impacts of these objects are much less well understood. The greatest uncertainty in comparing the impact hazard with other natural hazards relates to the economic and social consequences of impacts. Little work has been done on this problem, but we summarize the consequences -- to the degree they are understood -- in this chapter.
FIGURE 2.1. On August 10, 1972, an alert photographer in Grand
Teton National Park recorded the passage of an object estimated at 10 m
diameter and weighing several thousand tons. The object narrowly missed
colliding with Earth's surface, although it burned in our atmosphere for
101 seconds as it travelled over 1,475 km at about 15 km/s.
The threshold size of an impacting body for each category depends on its
density, strength, and velocity as well as on the nature of the target. The
threshold for global effects, in particular, is not well determined.
For the 10-m projectiles, only rare iron or stony-iron projectiles reach the ground with a sufficient fraction of their entry velocity to produce craters, as happened in the Sikhote-Alin region of Siberia in 1947. Stony bodies are crushed and fragmented during atmospheric deceleration, and the resulting fragments are quickly slowed to free-fall velocity, while the kinetic energy is transferred to an atmospheric shock wave. Part of the shock wave energy is released in a burst of light and heat (called a meteoritic fireball) and part is transported in a mechanical wave. Generally, these 100-kiloton disruptions occur high enough in the atmosphere so that no damage occurs on the ground, although the fireball can attract attention from distances of 600 km or more and the shock wave can be heard and even felt on the ground.
With increasing size, asteroidal projectiles reach progressively lower levels in the atmosphere before disruption, and the energy transferred to the shock wave is correspondingly greater. There is a threshold where both the radiated energy from the shock and the pressure in the shock wave can produce damage. A historical example is the Tunguska event of 1908, when a body perhaps 60 m in diameter was disrupted in the atmosphere at an altitude of about 8 km. The energy released was about 12 megatons, as estimated from airwaves recorded on meteorological barographs in England, or perhaps 20 megatons as estimated from the radius of destruction. Siberian forest trees were mostly knocked to the ground out to distances of about 20 km from the end point of the fireball trajectory, and some were snapped off or knocked over at distances as great as 40 km. Circumstantial evidence suggests that fires were ignited up to 15 km from the endpoint by the intense burst of radiant energy. The combined effects were similar to those expected from a nuclear detonation at a similar altitude, except, of course, that there were no accompanying bursts of neutrons or gamma rays nor any lingering radioactivity. Should a Tunguska-like event happen over a densely populated area today, the resulting airburst would be like that of a 10-20 megaton bomb: buildings would be flattened over an area 20 km in radius, and exposed flammable materials would be ignited near the center of the devastated region.
An associated hazard from such a Tunguska-like phenomenon is the possibility that it might be misinterpreted as the explosion of an actual nuclear weapon, particularly if it were to occur in a region of the world where tensions were already high. Although it is expected that sophisticated nuclear powers would not respond automatically to such an event, the possible misinterpretation of such a natural event dramatizes the need for heightening public consciousness around the world about the nature of unusually bright fireballs.
FIGURE 2.2. On June 30, 1908, at 7:40 AM, a cosmic projectile exploded in the sky over Siberia. It flattened 2,000 square kilometers of forest in the Tunguska region. If a similar event were to occur today, hundreds of thousands of people would be killed, and damage would be measured in hundreds of billions of dollars.
Toward the upper end of this size range, the megaton equivalent energy would so vastly exceed what has been studied in nuclear war scenarios that it is difficult to be certain of the effects. Extrapolation from smaller yields suggests that the "local" zones of damage from the impact of a 1-km object could envelop whole states or countries, with fatalities of tens of millions in a densely populated region. There would also begin to be noticeable global consequences, including alterations in atmospheric chemistry and cooling due to atmospheric dust -- perhaps analogous to the "year without a summer" in 1817, following the explosion of the volcano Tambora.
Comets are composed in large part of water ice and other volatiles and therefore are more easily fragmented than rocky or metallic asteroids. In the size range from 100 m to 1 km, a comet probably cannot survive passage through the atmosphere, although it may generate atmospheric bursts sufficient to produce local destruction. This is a subject that needs additional study, requiring a better knowledge of the physical nature of comets.
What happens when an object several kilometers in diameter strikes the Earth at a speed of tens of kilometers per second? Primarily there is a massive explosion, sufficient to fragment and partially vaporize both the projectile and the target area. Meteoric phenomena associated with high speed ejecta could subject plants and animals to scorching heat for about half an hour, and a global firestorm might them ensue. Dust thrown up from a very large crater would lead to total darkness over the whole Earth, which might persist for several months. Temperatures could drop as much as tens of degrees C. Nitric acid, produced from the burning of atmospheric nitrogen in the impact fireball, would acidify lakes, soils, streams, and perhaps the surface layer of the oceans. Months later, after the atmosphere had cleared, water vapor and carbon dioxide released to the stratosphere would produce an enhanced greenhouse effect, possibly raising global temperatures by as much as ten degrees C above the pre-existing ambient temperatures. This global warming might last for decades, as there are several positive feedbacks; warming of the surface increases the humidity of the troposphere thereby increasing the greenhouse effect, and warming of the ocean surface releases carbon dioxide which also increases the greenhouse effect. Both the initial months of darkness and cold, and then the following years of enhanced temperatures, would severely stress the environment and would lead to drastic population reductions of both terrestrial and marine life.
Death by starvation of much of the world's population could result from a
global catastrophe far less horrendous than those cataclysmic impacts that
would suddenly render a significant fraction of species actually extinct, but we know only very poorly what size impact would cause such mortality. In addition to all of the known variables (site of impact, time of year) and the uncertainties in physical and ecological
consequences, there is the question of how resilient our agriculture,
commerce, economy, and societal organization might prove to be in the face of
such an unprecedented catastrophe.
These uncertainties could be expressed either as a wide range of possible consequences for a particular size (or energy) of impactor or as a range of impactor sizes that might produce a certain scale of global catastrophe. We take the second approach and express the uncertainty as a range of threshold impactor sizes that would yield a global catastrophe of the following proportions:
A catastrophe having one, or all, of these traits would be a horrifying thing, unprecedented in history, with potential implications for generations to come.
To appreciate the scale of global catastrophe that we have defined, it is important to be clear what is not. We are talking about a catastrophe far larger than the effects of the great World Wars; it would result from an impact explosion certainly larger than if 100 of the very biggest Hydrogen bombs ever tested were detonated at once. On the other hand, we are talking about an explosion far smaller (less than 1 percent of the energy) the the K-T impact 65 million years ago. We mean a catastrophe that would threaten modern civilization, not an apocalypse that would threaten the survival of the human species.
What is the range of impactor sizes that might lead to this magnitude of global catastrophe? At the July 1991 Near-Earth Asteroid Conference in San Jaun Capistrano, California, the most frequently discussed estimate of the threshold impactor diameter for globally catstrphic effects was about 2 km. An estimate of the threshold size was derived for this Workshop in September 1991 by Brian Toon, of NASA Ames Research Center. Of the various enviromental effects of a large impact, Toon believes that the greatest harm would be done by the sub-micrometer dust launced into the stratosphere. The very fine dust has a long residence time, and global climate modeling studies by Covey and others (1990) imply significant drops in global temperature that would threaten agriculture worldwide. The quanity of sub-micrometer dust required for climate effects equivalent to those calculated for nuclear winter is estimated at about 10,000 Teragrams (Tg) (1 Tg = 1012g). For a 30 km/s impact, this translate to a threshold impacting body diameter of between 1 and 1.5 km diameter.
The threshold for an impact that causes widespread global mortality and threatens civilization almost certainly lies between about 0.5 and 5 km diameter, perhaps near 2 km. Impacts of objects this large occur from one to several times per million years.
Because the risk of such an impact happening in the near future is very low, the nature of the impact hazard is unique in our experience. Nearly all hazards we face in life actually happen to someone we know, or we learn about them from the media, whereas no large impact has taken place within the total span of human history. (If such an event took place before the dawn of history roughly 10,000 years ago there would be no record of the event, since we are not postulating an impact large enough to produce a mass extinction that would be readily visible in the fossil record). But also in contrast to more familiar disasters, the postulated impact would produce devastation on a global scale. Natural disasters, including tornadoes and cyclones, earthquakes, tsunamis, volcanic eruptions, firestorms, and floods often kill thousands of people, and occasionally several million. But the civilization-destroying impact exceeds all of these other disasters in that it could kill a billion or more people, leading to as large a percentage loss of life worldwide as that experienced by Europe from the Black Death in the 14th century. It is this juxtaposition of the small probability of occurrence balanced against the enormous consequences if it does happen that makes the impact hazard such a difficult and controversial topic.
For the globally catastrophic impact:
Average interval between impacts for total Earth: 300 years
Average interval between impacts for populated area of Earth: 3,000 years
Average interval between impacts for world urban areas: 100,000 years
Average interval between impacts for U.S. urban areas only: 1,000,000 years
We see from this simple calculation that even for a large country such as the U.S., the Tunguska-class impacts on urban areas occur less often than the globally catastrophic impact, emphasizing the fact that the large impacts dominate the risk. This point is also made in Figure 2.5, which plots the expected fatalities per event as a function of diameter (and energy) of the impacting object. The figure shows schematically the transition in expected fatalities per impact event that takes place as the global threshold is reached for objects between 0.5 and 5 kilometers in diameter.
For the globally catastrophic impact:
Average interval between impacts for total Earth: 500,000 years
Annual probability of impact: 1/500,000
Assumed fatalities from impact: one-quarter of world population
Probability of death for an individual: 1/4
Annual probability of an individuals death: 1/2,000,000
For the Tunguska-class impact:
Assumed area of devastation and total mortality from impact: 5,000 sq km (1/10,000 of Earth's surface)
Annual probability of an individual's death: 1/30,000,000
Thus we see that the annualized risk is about 15 times greater from the large impact than from the Tunguska-class impact.
For the globally catastrophic impact:
For the Tunguska-class impact:
These figures can be compared with the mortality rates from other natural and man-made causes to obtain a very rough index of the magnitude of the impact-catastrophe hazard. For example, the U.S. numbers can be compared with such other causes of death as food poisoning by botulism (a few per year), tornadoes (100 per year), and auto accidents (50,000 per year).
Finally, because of the higher frequency and nonetheless significant consequences of impact of objects with diameters in the range of 100 m to 1 km, the survey should include bodies in this size range as well. There are wide differences among people in their response to hazards of various types. We have concentrated on the globally catastrophic case because of its qualitatively dreadful nature. But some people consider the threat of the more frequent Tunguska-like events to be more relevant to their concerns, even though the objective hazard to human life is much less. In order to protect against such events (or at least mitigate their effects), impactors as small as 100 m diameter would need to be located with adequate warning before impact to destroy them or at least evacuate local populations. Fortunately, as will be described in Chapter 7, the survey network designed to detect and track the larger asteroids and comets will also discover tens of thousands of Earth-approaching objects in the 100-m to 1-km size range.
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