Earth benefits from the life-giving light of the Sun. For millennia, mariners have sailed with the tides caused by the Moon. However, other interactions within the inner solar system neighbourhood are occasionally not so benign. There is an ever-present danger to Earth from collisions with small bodies whose orbits around the Sun carry them near Earth. 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. Occasionally there are reports of roughly softball-size meteorite fragments damaging houses or cars. (The apparently 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
The impact of a near-Earth object 65 million years ago in what is today the Caribbean region, as depicted in an artist's conception. Many scientists believe that the collision of a large asteroid or comet nucleus with Earth triggered the mass extinction of the dinosaurs and many other species near the end of the Cretaceous period. NASA; illustration by Don Davis
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 65 million years ago, in which most species of dinosaurs and much of the marine life of the day perished.
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 photosynthe-sizing 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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