Earth impact hazard, the danger of collision posed by astronomical small bodies whose orbits around the Sun carry them near Earth; when collisions occur, they are known as "impact events." These objects include the rocky asteroids and their larger fragments and the icy nuclei of comets.

Space in the vicinity of Earth contains a great number of solid objects in a range of sizes. The tiniest (millimetre-size and smaller) and by far most abundant ones, called micrometeroids or interplanetary dust particles, hit Earth’s atmosphere continually. They are also the least dangerous: they either burn up in the atmosphere or settle to the surface as dust. Of the somewhat larger objects—i.e., mostly asteroidal in origin—the great majority that reach the ground as meteorites are too small to endanger human life or property on a significant scale. However, there are occasional reports of roughly softball-sized meteorite fragments damaging houses or cars, and in 2013 more than 1,500 people in the Chelyabinsk region of Russia were injured, mostly by flying glass which had been shattered by the shock wave of a meteorite 17 metres (56 feet) wide breaking up in the atmosphere. (The only verified case of a meteorite hitting and injuring a human being occurred in 1954.) Reports of falls of meteorites with masses in the one-ton range are less frequent; when these objects strike the ground, they can excavate craters a few metres across.

It is only the biggest projectiles, those that collide with Earth very infrequently on average, that are acknowledged to pose a great potential danger to human beings and possibly to all life on the planet. Recognition that such a danger might exist dates back at least to the English astronomers Edmond Halley and Isaac Newton and their work on the Great Comet of 1680, whose orbit they showed crossed that of Earth. Modern interest was rekindled in 1980 when the experimental physicist Luis Alvarez of the University of California, Berkeley, and colleagues presented evidence that the impact of an asteroid or comet having a diameter of about 10 km (6 miles) was responsible for the mass extinction at the end of the Cretaceous Period (66 million years ago), in which the dinosaurs and much of the marine life of the day perished.

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Since that time scientists have identified the probable site of the impact, called the Chicxulub crater, off Mexico’s Yucatán Peninsula and have come to suspect that similar catastrophic impacts may have triggered other mass extinctions as well. In addition to causing tremendous immediate devastation and ensuing earthquakes, firestorms, and giant sea waves (tsunamis), collisions of such magnitude are believed to be capable of perturbing Earth’s environment globally by throwing large quantities of fine debris high into the atmosphere. The consequences would include a decrease in the amount of sunlight reaching the surface and a prolonged depression of surface temperatures—a so-called impact winter—leading to loss of photosynthesizing plant life and worldwide starvation and disease.

In the early 1980s astronomers in the United States, followed by those in several other countries, began studies aimed at better defining the risk posed by cosmic impacts, developing programs to detect threatening objects, and determining if anything could be done to protect Earth from the most devastating impacts. One outgrowth of these efforts was the development of a scale for categorizing the potential impact hazard of objects newly discovered to be orbiting near Earth.

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Objects that pose a threat

All objects that can someday cross Earth’s orbit have the potential to collide with the planet. This includes not only objects that regularly approach Earth but also others whose paths may change over time in a way that would make them cross Earth’s orbit. The objects that fall into this category are asteroids and comets in short-period orbits—together called near-Earth objects (NEOs)—and those long-period comets that make their closest approach to the Sun inside Earth’s orbit. Short-period comets complete their orbits in less than 200 years and so likely have been observed before; they generally approach along the plane of the solar system, near which lie the orbits of most of the planets, including Earth. Like short-period comets, most known Earth-approaching asteroids have orbits tilted by less than 20° to the plane of the solar system and periods of less than about three years. Long-period comets have orbital periods greater than 200 years and usually much greater; they can approach from any direction.

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The amount of damage caused by the impact of an object with Earth is determined primarily by two factors: the object’s mass and its relative velocity. These determine the total kinetic energy released. A typical NEO would strike Earth with a velocity of about 20 km (12 miles) per second and a typical long-period comet with a greater velocity, 50 km (30 miles) per second or higher. For objects with diameters less than a few hundred metres, their physical properties are important in calculating how much destruction would result, but for larger bodies only the total energy of the impact is important. Hence, most damage assessments are based on the kinetic energy of an impact rather than the diameter or mass of the projectile. This energy is expressed in millions of tons (megatons) of TNT, the same units used to quantify the energy released by thermonuclear bombs.

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The energy released by an impact falls between about 10 megatons and 1 billion megatons—i.e., between 700 and 70 billion times the energy of the 15-kiloton atomic bomb dropped on Hiroshima, Japan, in 1945. This very wide range corresponds to NEOs with diameters from about 50 metres (164 feet) to 20 km (12 miles) or to long-period comets with diameters about half as large. (Objects smaller than about 50 metres would break up high in the atmosphere; the damage would be limited to less than a few hundred square kilometres around the impact point.) For an object at the lower end of this size range, an ocean impact could cause more damage than one on land because it would result in large tsunamis that would devastate coastal areas for many kilometres inland. The last destructive impact known, called the Tunguska event, occurred at the low end of this range over land. On June 30, 1908, an object thought to be as much as 50 metres (164 feet) in diameter exploded over central Siberia, leveling about 2,000 square km (500,000 acres) of pine forest.