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Earth's surface undergoes many kinds of environmental changes that affect human life and the evolution of all living things. Some are caused by human beings, and others result from natural processes; some evolve slowly, whereas others are sudden: "accidents" (if caused by humans) and "natural disasters." Since life can adapt to slow changes, the most disruptive changes are sudden calamities. The worst calamity occurs when a large, errant asteroid or comet collides with Earth.

Sizes of Near-Earth Objects

Fragments of asteroids and comets pervade interplanetary space. Modest cosmic impacts occur all the time. On a dark, clear night one can see a flash of light (a meteor or "shooting star") every few minutes as an interplanetary grain of dust or sand strikes Earth's upper atmosphere. More rarely, larger space rocks cause brilliant "fireballs" when they crash to Earth, perhaps leaving meteorites in the ground. Every few years, Earth-orbiting surveillance satellites record multi-kiloton upper atmospheric explosions when a house-size cosmic object impacts. This happened over the Yukon Territory in January 2000, lighting up the night sky ten times more brilliantly than full daylight.

Objects 50 meters (164 feet) across strike Earth every few centuries, causing airbursts that rival the effects of large thermonuclear bombs. The last one exploded over the Tunguska region of Siberia in 1908, toppling trees over a region the size of Washington, D.C. A similar-sized object composed of solid metal rather than rock struck northern Arizona about 50,000 years ago, forming Meteor Crater.*

Far larger asteroids and comets can strike Earth. About 1,000 asteroids larger than 1 kilometer (0.62 mile) in diameter approach within 45 million kilometers (28 million miles) of Earth; any one of these near-Earth asteroids (NEAs) could impact Earth in the next few million years. Most will crash into the Sun, strike another planet, or be flung by Jupiter's gravity into interstellar space. But every 100,000 years or so a kilometer-sized NEA does crash into Earth, exploding with a force approaching 100,000 megatonsmore powerful than all the world's nuclear bombs together.

A few NEAs are much larger than 1 kilometer (0.62 mile). Eros, which was visited by the NEAR Shoemaker spacecraft in the year 2000, is 34 kilometers (21 miles) long. Studies of its orbital path show that Eros cannot hit Earth in the near future, but millions of years from now there is a 5 percent chance that Eros will crash into Earth; the devastation would greatly exceed the impact 65 million years ago of a 10-to-15 kilometer (6 to 9 miles) diameter asteroid or comet that caused 70 percent of all species of plants and animals recognized in Cretaceous fossil beds to suddenly go extinct, including dinosaurs.

Even larger calamities happened early in the planet's history as life tried to gain a foothold on Earth. The circular dark patches on the full Moon are great circular impact basins formed when 100-kilometer-size (60 miles)planetesimals struck the Moon 3.9 to 4.2 billion years ago. Earth is a larger target than the Moon; it was surely bombarded by such projectiles during that epoch. It is unlikely but possible that Earth will be struck by such a large object again. If this were to occur, it could sterilize the world of all life. In 1997 Comet Hale-Bopp came inside Earth's orbit; its diameter was 25 to 70 kilometers (15 to 45 miles).

Risks and Consequences

Impacts do not happen regularly. Earth is in an essentially random, cosmic shooting gallery. Kilometer-size asteroids impact every 100,000 years "on average." However, that means there is a 1 in 100,000 chance that one will hit "next year," or a 0.1 percent chance during the twenty-first century. A much larger, mass extinction impact is a thousand times less likely than a 1-kilometer (0.62-mile) NEA impact, but even that is not inconceivable in the very near future.

The consequences of impacts vary enormously, depending on the size and velocity of the impacting bodies. A Tunguska-like event, which happens somewhere on Earth every few centuries, could happen in the next fifty years. If it exploded unexpectedly over a major city, it would be a catastrophe in which hundreds of thousands might die. However, only a tiny fraction of Earth's surface has urban population densities. A sparsely populated area is a more likely target, such as Tunguska, where only one or two people may have been killed. Even more likely, the explosion would happen harmlessly over an ocean.

A larger body, perhaps 200 meters (124 miles) in diameter, would be catastrophic no matter where it struck. It would certainly penetrate the atmosphere and strike land or water. Indeed, impact into the ocean would be devastating, generating a tsunami (tidal wave) larger than any ever recorded. Such an event might account for some flood myths from ancient times. Astronomers have discovered and tracked only a small fraction of these comparatively small asteroids, and so an impact like this (about a 1 percent chance of happening in this century) probably would occur without warning. Tsunami-warning systems most likely would be ineffective in alerting people to evacuate to high ground. Massive destruction of property along the shores of the impacted ocean would be certain, with an enormous death toll. A similar impact on land would form a crater far larger than Meteor Crater, but the death and destruction would be restricted to within a couple hundred kilometers of ground zero.

As frightening as impacts by bodies tens to hundreds of meters in size are to contemplate, more usual natural catastrophes capable of killing just as many people happen 100 times as often. During the twentieth century a dozen natural catastrophes (floods, earthquakes, and the like) each killed between 100,000 and 2 million people. Thus, these "smaller" impacts represent only about 1 percent of the danger.

Impacts by comets and asteroids over 2 kilometers (1.24 miles) in diameter have consequences that exceed those of a nuclear war. There are upper limits to the effects of earthquakes, storms, floods, and exploding volcanoes, which are restricted to localities or regions of the planet. A 2-kilometer (1.24-mile) asteroid, however, would throw enormous quantities of dust and aerosols high in the stratosphere , darkening the Sun, leading to the failure of global agriculture for a year or more, and resulting in mass starvation. A billion people might die, and civilization would be threatened. Such impacts are rare, having 1 chance in 10,000 of happening in this century. However, the consequences would be enormous, including possible permanent loss of the accomplishments of modern civilization, and the quantitative risk to human life ranks with other hazards (such as airline safety) that society takes seriously.


The impact hazard has a hopeful feature: Human beings (unlike dinosaurs) could avert such a catastrophe if it were about to happen. Less than half of the 1-to 2-kilometer (0.62 to 1.24 miles) NEAs have been discovered, and so one could strike without warning. However, an international astronomical program (so far based mostly in the United States) called the Spaceguard Survey employs modest-size wide-field telescopes equipped with charge-coupled devices to search the skies for NEAs larger than about 1 kilometer (0.62 mile). Within less than a decade the paths of about 90 percent of these NEAs will have been charted and it will be known whether one is headed toward Earth in the next decades. A few NEAs will remain undiscovered, and comets from beyond Neptune's orbit will continue to arrive in the inner solar system with only months of advance warning. Thus, there will always be a small chance that humankind will be caught unprepared.

However, current space technology could in principle save the world from an impact catastrophe. Depending on the warning time and the size of the threatening body, several low-thrust propulsion technologies could be used to nudge the object away from its Earth-targeted trajectory. These schemes include solar sails, ion drives, mass drivers, and chemical rockets . If the warning time were too short or the object too large, nuclear bombs might be required. Specific engineering designs for these technologies (for example, how to couple the devices to the surface of the NEA) have not been worked out. However, there probably would be enough time to study the body and work out the engineering. Care would have to be taken to deflect the body intact rather than break it into pieces because a swarm of fragments might be more destructive than a single object.

National and international agencies and governments are starting to listen to astronomers, who have been trying to raise the awareness of politicians and emergency management agencies to the impact hazard. However, apart from the modest ground-based Spaceguard Survey, little official action or coordination has been undertaken. Comets and small asteroids are being missed in the Spaceguard census, and the major space and military agencies have paid little attention to the impact hazard. Also, there has been no contingency planning by emergency managers to store food supplies or evacuate people from ground zero in the event of a threatening body. This lack of action represents an implicit political decision to largely ignore the unlikely threats from space in favor of dealing with more near-term issues.

see also Asteroids (volume 2); Close Encounters (volume 2); Cmets (volume 2); Environmental Changes (volume 4); Meteorites (volume 2); Movies (volume 4).

Clark R. Chapman


Gehrels, Tom, ed. Hazards Due to Comets and Asteroids. Tucson: University of Arizona Press, 1994.

Internet Resources

"Asteroid and Comet Impact Hazards." NASA Ames Research Center. <>.

"Near-Earth Object Program." NASA Jet Propulsion Laboratory.<>.

"Report of the U.K. Task Force on Near Earth Objects."<>.

"Tumbling Stone." Spaceguard Foundation. <>.

International Regimes See Governance (Volume 4); Political Systems (Volume 4).

An image of Meteor Crater can be found in the volume 2 article "Meteorites."

* An image of Meteor Crater can be found in the volume 2 article "Meteorites."