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Asteroids are relatively small, rocky chunks of matter that shine like small stars, but orbit the Sun like planets. Most asteroids, or minor planets as they are sometimes called, are made of carbon-rich rock, while others (those farthest from the Sun) contain iron, nickel, and a few other elements. One asteroid, Pholus, is coated with a red material that may be organic compounds similar to those that form living material.

Asteroids vary in size from 580 miles (940 kilometers) in diameter (Ceres, the first asteroid discovered) to 33 feet (10 meters) in diameter. Most are so small their size cannot be measured directly. They are generally irregular in shape and vary in brightness as they rotate.

Scientists once thought asteroids were remnants of exploded planets. This theory was soon discarded, however, because asteroids are so small. All the known asteroids combined would form an object much smaller than Earth's moon. Scientists now believe that asteroids are planetesimalsancient chunks of matter left over from the formation of the solar system that never came together to form a planet, perhaps because of the strong gravitational influence of Jupiter. Asteroid-like objects were among the first structures to form in our solar system some 4.6 billion years ago. For this reason, asteroids can provide valuable information about the beginnings of our solar system.

The discovery of asteroids

For years, astronomers theorized about a hidden planet between Mars and Jupiter. In 1766, German astronomer Johann Titius devised a formula for calculating distances between planets. The formula predicted the existence of a planet between Mars and Jupiter. (Although Titius devised the formula, it did not receive much attention until German astronomer Johann Bode restated it in 1772. By then, Titius had been all but forgotten, and the formula became known as Bode's Law.)

On the night of January 1, 1801, Italian astronomer Giuseppe Piazzi discovered the first asteroid. He observed a starlike body that was not listed in star catalogues. It was moving faster than Mars yet slower than Jupiter, so he deduced it must lie between the two. He named the asteroid Ceres, the Roman goddess of agriculture. Later, Ceres was found to be in the right place to satisfy Bode's Law.

In the mid-1800s, with the improvement of telescopic equipment and techniques, many new asteroids were discovered. There are now more than 5,000 tracked and documented asteroids, an additional 13,000 identified, and an estimated total of 1,000,000. About 100,000 asteroids are bright enough to be photographed from Earth. Only one, Vesta (the fourth one discovered), is bright enough to be seen with the naked eye.

Words to Know

Apollo objects: Group of asteroids that crosses Earth's orbit.

Bode's Law: Simple yet flawed mathematical formula establishing the distances of planets from the Sun.

Kirkwood gaps: Areas separating distinct asteroid belts that lie between the orbits of Mars and Jupiter.

Planetesimals: Ancient chunks of matter that are leftover "building blocks" from the formation of the solar system.

Asteroid belt

Most asteroids lie in a main belt between the orbits of Mars and Jupiter. This belt is divided into smaller belts, separated by distances known as Kirkwood gaps (named for their discoverer, nineteenth-century American astronomer Daniel Kirkwood). Kirkwood gaps are spaces in which the gravitational attraction of Jupiter prevents any object from maintaining an orbit.

Not all asteroids reside in the main belt. Trojan asteroids are located in two clusters, one on either side of Jupiter. This arrangement is a consequence of the gravitational attraction of Jupiter and the Sun. Another class of asteroids crosses the orbits of several planets.

Impacts with Earth

One group of asteroids, called Apollo objects, cross Earth's orbit. These bodies may come relatively close to Earth, and some have even collided with the planet. In 1908, an asteroid about one-tenth of a mile in diameter came through the atmosphere and exploded above central Siberia. The blast produced a mushroom cloud, wiped out a herd of reindeer, scorched and uprooted trees for miles around, and shattered windows 600 miles away. Asteroids ten times the size of this Siberian asteroid are estimated to hit Earth every few hundred thousand years.

Some 65 million years ago an asteroid struck Earth off the northern tip of the Yucatan Peninsula of Mexico. The asteroid, known as Chicxulub (pronounced cheek-soo-LOOB), left a crater under the Gulf of Mexico with an estimated diameter of 110 miles (177 kilometers). Some scientists believe the resulting inferno from the impact incinerated hundreds of thousands of species of plant and animals, and caused the extinction of the dinosaurs.

About 250 million years ago, an asteroid 4 to 8 miles (6.4 to 12.8 kilometers) across slammed into Earth with the force of more than a million earthquakes, setting off what scientists theorize was the most catastrophic of mass extinctions. The impact carved out a crater some 75 miles (121 kilometers) wide and caused huge volcanic eruptions that buried much of the planet in lava. It blasted millions of tons of rock and dust into the sky, blotting out the Sun's rays. It also brought about changes in sea levels and climatic shifts. These changes killed 90 percent of the marine species and 70 percent of the backboned land animals that lived during that geologic period. Scientists discovered evidence of this devastating extinction by digging deep into Earth's core. Trapped with soccer-ballshaped carbon molecules called buckminsterfullerenes or buckyballs, scientists found a mix of helium and argon gases similar to that found in certain stars but unlike anything that could form naturally on Earth.

Scientists now believe a flurry of asteroids barraged Earth and the Moon some 4 billion years ago, roughly the same time life was forming on the planet. In a study released at the end of 2000, scientists reported that a rain of asteroids lasting from 20 million to 200 million years melted rocks, blasted out craters, and reshaped the surface of both the Moon and Earth. On the Moon, the bombardment produced the great basins that are clearly visible from Earth. The asteroids that slammed into Earth, ten times larger than the Chicxulub asteroid, blasted craters rim to rim the size of present-day continents, vaporized the oceans, and filled the atmosphere with life-choking fog. Some scientists theorize that the impacts may have affected the evolution of life, forcing it to begin anew, or may have brought minerals, water, or even the building blocks of life to the planet.

Orbiting, then landing on, an asteroid

In April 2000, after traveling some 2 billion miles (3.2 billion kilometers) since it left Earth on February 17, 1996, the unmanned spacecraft NEAR (Near Earth Asteroid Rendezvous) Shoemaker began a circular orbit around the asteroid Eros. It was the first time a spacecraft had orbited an asteroid. During its one-year mission around Eros, the 1,100-pound (500-kilogram) spacecraft settled into an orbit that at one point was as close 3 miles (5 kilometers) above the potato-shaped asteroid. Eros, named after the Greek god of physical love, is one of the larger asteroids in the solar system, measuring about 21 miles (34 kilometers) long and 8 miles (13 kilometers) thick. It is called a near-Earth asteroid because its orbit crosses that of Earth and poses a potential collision danger. Scientists estimate it is 4.5 billion years old, almost unchanged since the beginning of the solar system.

Loaded with six instruments, NEAR Shoemaker took measurements to determine the mass, density, chemical composition, and other geological characteristics of the asteroid. It also beamed back to Earth some 160,000 images of Eros. On February 12, 2001, NEAR Shoemaker used the last of its fuel in an attempt to land on the surface of Eros. The craft had not been designed with landing gear, and mission scientists had given it a 1 percent chance of survival. Bumping into the asteroid at a mere 4 miles (6.4 kilometers) per hour, however, the hardy spacecraft survived, becoming the first spacecraft to land on an asteroid. On its way down to the surface, NEAR Shoemaker continued to transmit pictures back to Earth. Once on the surface, it continued to collect invaluable data about the oddly shaped asteroid. Even though scientists will probably study the data for years, they did learn early on that the asteroid does not tumble wildly through space. Instead, it rotates around one axis, much like a planet.

An end to its successful five-year, $225 million mission came on February 28, 2001, when scientists put NEAR Shoemaker into a planned hibernation. Scientists did not believe the spacecraft would survive the frigid darkness of winter on Eros, when temperatures would plummet to 238°F (150°C). There is a very slight chance it might come alive again sometime in 2002 when full sunlight returns to that part of the asteroid.

[See also Meteors and meteorites ]

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asteroid, planetoid, or minor planet, small body orbiting the sun. More than 300,000 asteroids have been identified and cataloged; more than a million are believed to exist in the main belt between Mars and Jupiter, with many more in the Kuiper belt beyond Neptune. Most asteroids are irregularly shaped, unlike the spherically shaped major planets. The largest asteroids, Quaoar (diameter c.800 mi/1,300 km) and Ixion and Varuna (both c.750 mi/1,200 km), reside in the Kuiper belt (see comet) and are about half the size of Pluto. The largest main-belt asteroid, Ceres, has a diameter of c.630 mi (1,000 km) and is considered a dwarf planet; the three next largest in size are Pallas, Vesta, and Juno. Only Vesta can be seen with the naked eye. Many known asteroids are no larger than a few kilometers; in 1991, an asteroid only 33 ft (10 m) in diameter was found. Many asteroids are so small that their sizes cannot be measured directly; in many cases, their sizes have been estimated from their brightness and distances. The average orbital distance of the main-belt asteroids from the sun is about 2.9 astronomical units (AU).

The orbits of most main-belt asteroids lie partially between the orbits of Mars and Jupiter. These asteroids, also known as cisjovian asteroids, are divided into subgroups named for the main asteroid in the grouping: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles, and Hildas. The near-earth asteroids, which closely approach the earth, are classed as Atens (with orbits between the earth and the sun), Apollos (with orbits similar to that of the earth), and Amors (with orbits between the earth and Mars). Asteroids that share a planet's orbit are known as Trojan asteroids. First found in Jupiter's orbit, Trojan asteroids have also been found in the orbits of Mars, Neptune, and earth. Centaurs are asteroids with orbits in the outer solar system.

Asteroids are also classified by composition and albedo, most being one of three types. The majority (C-type) are similar to carbon-chrondite meteorites with approximately the same composition as the sun (excluding hydrogen) and are relatively dark. Those with a composition of nickel iron mixed with silicates of iron and magnesium (S-type) are relatively bright. The M-type are composed of nickel iron and are bright. Some of the Trojan asteroids appear to be captured comets, composed of ice and dirt, rather than rocky asteroids.

Toward the end of the 18th cent. astronomers were searching for a planet whose orbit should, according to Bode's law, have an average distance from the sun of 2.8 AU On Jan. 1, 1801, G. Piazzi discovered Ceres while studying the sky in the constellation Taurus; Ceres was later found to have an orbit very near that predicted by Bode's law. Ceres and the asteroids Juno, Pallas, and Vesta, which were discovered soon (1802–7) after Ceres, were initially regarded as planets by many astronomers, a view that was not overturned until additional asteroids were identified in the 1840s and 50s. By 1890 more than 300 asteroids had been discovered by visual means. In 1891, Max Wolf introduced the method of identifying an asteroid by the record of its path on an exposed photographic plate; it appears as a short line in a time exposure, rather than as the sharp point of a star. Brucia was the first asteroid discovered by this method. A more modern approach uses two photographs taken less than an hour apart and examined through a stereomicroscope that allows the asteroid to appear suspended above the background of stars. Still more modern techniques were employed in the discoveries of Ixion, found in 2001 using virtual telescope techniques, and of Quaoar, found in 2002 using photographs taken with the Hubble Space Telescope.

Roughly 1,500 asteroids 330 ft (100 m) or larger in diameter have been identified that regularly intersect the orbit of the earth at a distance of 0.5 AU or less (half the distance to the sun). Over geologic time, similarly sized asteroids in similar orbits have struck the earth, and it is estimated that some 4,700 of these asteroids exist. NASA has estimated that a so-called potentially hazardous asteroid (or comet) might cause a global disaster once in several hundred thousand years, and a local disaster once every 10,000 years. Hermes, discovered in 1937 and subsequently lost until 2003 when it was identified as a pair of asteroids, comes within 378,000 mi (608,000 km), and Eros comes within 14 million mi (22 million km). More recently, a small asteroid provisionally designated 2002 MN, 150–360 ft (45–110 m) in diameter, passed within 75,000 mi (121,000 km) of the earth—about a third of the distance to the moon—in 2002. In 2009 an apparent asteroid impact was observed on Jupiter; estimated to be 0.3 mi (.5 km) wide, the object would have caused widespread devastation had it struck the earth.

Even smaller asteroids, between 100 ft and 330 ft (30 m and 100 m) in diameter, are capable of causing significant damage should they strike near a population center, and astronomers have observed about several hundred small asteroids, most measuring less than 55 yd (50 m) across, in near-earth orbits that are spread thinly between the earth and Mars. Many of these small asteroids have orbits that intersect the earth's. In 2013 one such asteroid, 2012 DA14, 150 ft (45 m) in diameter, passed by the earth only 17,200 mi (27,700 km) away. A stony asteroid of roughly similar size may have been responsible the widepsread destruction that occurred in 1908 in the isolated Tunguska Basin. A study of data collected by the Comprehensive Nuclear Test Ban Treaty Organization from 2000 to 2013, in which 26 asteroid explosions, largely in the atmosphere and over the oceans, were recorded, has led researchers associated with the B612 Foundation to conclude that an asteroid capable of destroying a city strikes the earth about once every 100 years.

Asteroids have been implicated in several mass extinctions of large numbers of animal and plant species in the past. From evidence found in sediments, Luis Walter Alvarez and others hypothesized that the great mass extinction of the dinosaurs and other species 65 million years ago, at the end of the Cretaceous period, was caused by the atmospheric and climatic effects of an asteroid impact; a possible crater exists in the Yucatán region of Mexico. In 1992, scientists reported that the appearance of patterns of shattered quartz crystals imbedded in Triassic shale and other fossil evidence suggest that another major mass extinction, about 200 million years ago, was caused by three closely spaced asteroid impacts.

The origin of asteroids is unclear; one theory claims that they were formed from material that could not condense into a single planet because of perturbation effects involving Jupiter. Some asteroids are actually nuclei of comets that are no longer active.

The space probe Galileo, which passed near and photographed Gaspra (1991) and Ida (1993), provided the first close images of an asteroid. The pictures revealed that Ida has a natural satellite, Dactyl. Ida, in the main asteroid belt between Mars and Jupiter, is about 35 mi (56 km) long and 15 mi (24 km) in diameter. Its tiny moon is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida. Since then several other asteroids have been found to have companions, leading astronomers to believe that it may not be uncommon. The probe NEAR-Shoemaker examined Mathilde (1997) on its way to rendezvous (1999) and orbit (2000) Eros. After providing the most information ever obtained about an asteroid (measurements of size, shape, mass, and gravitational field; elemental and mineral composition of the surface; topographic mapping; and measurement of the magnetic field and its interaction with the solar wind), NEAR-Shoemaker made an originally unplanned landing on Eros in 2001, returning close-up images as it descended and data about surface composition. In 1999 the probe Deep Space 1 accomplished the then closest-ever flyby of an asteroid, coming within 16 mi (26 km) of the surface of Braille; spectroscopic data suggests that Braille broke off from Vesta millions of years ago. Other probes have since studied asteroids, including Rosetta,Hayabusa, which returned with samples of the asteroid Itokawa in 2010, and Dawn, which studied Vesta in 2011–12 and then orbited Ceres beginning in 2015.

See T. Gehrels and M. S. Matthews, ed., Hazards Due to Comets and Asteroids (1995); J. S. Lewis, Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment (1997); C. T. Russell, ed., The Near Earth Asteroid Rendezvous Mission (1998); N. F. Michelson, The Asteroid Ephemeris 1900–2050 (1999).

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Asteroids are small bodies in spacethe numerous leftover planetesimals from which the planets were made nearly 4.6 billion years ago. Most are in the "main belt," which is a doughnut-like volume of space between Mars and Jupiter (about 2.1 to 3.2 astronomical units [AU] from the Sun; one AU is equal to the mean distance between Earth and the Sun). The Trojans are two groups of asteroids around 60 degrees ahead of (and behind) Jupiter in its orbit (5.2 AU from the Sun). Asteroids range in location from within Earth's orbit to the outer solar system, where the distinction between asteroids and comets blurs.

Some asteroids orbit at a solar distance where their year is matched to Jupiter's year. For example, the Hilda asteroids circle the Sun three times for every two revolutions of Jupiter. Other Jupiter-asteroid relationships are unstable, so asteroids are missing from those locations. For example, gaps occur in the main belt where asteroids orbit the Sun twice and three times each Jovian year. These gaps are called Kirkwood gaps. Any asteroids originally formed in such locations have been kicked out of the asteroid belt by Jupiter's strong gravitational forces, so no asteroids remain there.

Many asteroids are members of groups with very similar orbital shapes, tilts, and solar distances. These so-called families were formed when asteroids smashed into each other at interasteroidal velocities of 5 kilometers per second (3 miles per second). Fragments from such explosive disruptions became separate asteroids.

Asteroid Sizes, Shapes, and Compositions

Ceres, the first asteroid to be discovered (on January 1, 1801), remains the largest asteroid found to date; it is about 1,000 kilometers (620 miles) in diameter. Dozens of asteroids range from 200 to 300 kilometers (124 to 186 miles) in diameter, thousands are the size of a small city, and hundreds of billions are house-sized. Indeed, asteroids grade into the rocks that occasionally burn through our atmosphere as fireballs and the even smaller grains of sand that produce meteors ("shooting stars") in a clear, dark sky. Collected remnants are called meteorites. All are debris from the cratering and catastrophic disruptions of inter-asteroidal collisions.

Asteroids are small and distant, so even in telescopes they are only faint points of light gradually moving against the backdrop of the stars. Astronomers use telescopes to measure asteroid motions, brightnesses, and the spectral colors of sunlight reflected from their surfaces. Asteroid brightnesses change every few hours as they spin, first brightening when they are broadside to us and fading when end-on. From these data, astronomers infer that most asteroids have irregular, nonspherical shapes and spin every few minutes (for some very small asteroids) to less often than once a month.

Different minerals reflect sunlight (at ultraviolet , visible, and infrared wavelengths ) in different ways. So the spectra of asteroids enable astronomers to infer what they are made of. Many are made of primitive materials, such as rocky minerals and flecks of metal, from which it is believed the planets were made. Such is the case with the ordinary chondrites, the most common meteorites in museums. Most asteroids are exceedingly dark in color, and are apparently rich in carbon and other black compounds, including the uncommon carbonaceous meteorites . Such fragile, C-type materials are abundant in space but often disintegrate when passing through Earth's atmosphere. C-type asteroids may even contain water ice deep below their surfaces.

While most asteroids survived fairly unchanged from the earliest epochs of solar system history, others were heated and melted. The metal flecks sank to form iron cores (like nickel-iron meteorites), while lighter rocks floated upwards and flowed out across their surfaces, like lavas do on Earth. Vesta, one of the largest asteroids, appears to be covered with lava; certain lava-like meteorites probably came from Vesta. Metallic asteroids are rare but are readily recognized by Earth-based radar observations because metal reflects radar pulses well.

New techniques in astronomy, such as radar delay-Doppler mapping and adaptive optics (which unblurs the twinkling of visible light induced by Earth's atmosphere), have revealed a variety of asteroid shapes and configurations. One asteroid, named Antiope, is a double body: Two separate bodies, each 80 kilometers (50 miles) across and separated by 160 kilometers (100 miles), orbit about each other every sixteen hours. Other asteroids have satellites (e.g., moonlets) and still others have very odd shapes (e.g., dumbbells).

Spacecraft Studies of Asteroids

The best (though most expensive) way to study an asteroid, of course, is to send a spacecraft. Three main-belt asteroidsGaspra, Ida, and Mathildewere visited in the 1990s by spacecraft en route to other targets. But even during the few minutes available for close-up observations during such high-speed encounters, scientists obtained images a hundred times sharper than the best possible images from Earth.

The most thorough study of an asteroid was of Eros by the Near Earth Asteroid Rendezvous spacecraft (which was renamed NEAR Shoemaker, after American astronomer Eugene Shoemaker, who first thought of the enterprise). Eros is a 34-kilometer-long (21-mile-long), Earth-approaching asteroid. NEAR Shoemaker orbited Eros until February 12, 2001, when it was landed on the asteroid's surface. Its instruments were designed specifically for asteroid studies. It revealed Eros to be an oddly shaped, heavily cratered object, with ridges and grooves, and covered by a million boulders, each larger than a house. Eros is made of minerals much like the ordinary chondrite meteorites .

Near Earth Asteroids

A few asteroids escape from the main belt through Kirkwood gaps and move around the Sun on elongated orbits that can cross the orbits of Mars and Earth. If an asteroid comes within 0.3 AU of Earth, it is called a near Earth asteroid (NEA). More than half of the estimated 1,000 NEAs larger than 1 kilometer (0.6 mile) in diameter have been discovered. Orbits of NEAs are not stable, and within a few million years they collide with the Sun, crash into a planet, or are ejected from the solar system.

The Threat of Impacts.

If a 2-kilometer (1.2-mile) NEA struck Earth, it would explode as 100,000 megatons of TNT, more than the world's nuclear weapons arsenal. It would contaminate the stratosphere with so much Sun-darkening dust that humans would lose an entire growing season worldwide, resulting in mass starvation and threatening civilization as we know it. Such a collision happens about once every million years, so there is one chance in 10,000 of one occurring during the twenty-first century. A 10-or 15-kilometer (6-or 9-mile) asteroid, like the one that caused the extinction of the dinosaurs 65 million years ago, hits every 50 or 100 million years with a force of 100 million megatons.

Though the chances of dying by asteroid impact are similar to the chances of dying in an air crash, society has done little to address the impact hazard. Modest telescopic searches for threatening objects are underway in several countries. Given months to a few years warning, ground zero could be evacuated and food could be saved to endure an impact winter . If given many years, or decades, of warning, high-tech space missions could be launched in an attempt to study and then divert the oncoming body.

see also Asteroid Mining (volume 4); Impacts (volume 4); Close Encounters (volume 2); Galilei, Galileo (volume 2); Meteorites (volume 2); Planetesimals (volume 2); Shoemaker, Eugene (volume 2); Small Bodies (volume 2).

Clark R. Chapman


Chapman, Clark R. "Asteroids." In The New Solar System, 4th ed., ed. J. Kelly Beatty, Carolyn Collins Petersen, and Andrew Chaikin. New York and Cambridge, UK: Sky Publishing Corp. and Cambridge University Press, 1999.

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

Veverka, Joseph, Mark Robinson, and Pete Thomas. "NEAR at Eros: Imaging and Spectral Results." Science 289 (2000):2088-2097.

Yeomans, Donald K. "Small Bodies of the Solar System." Nature 404 (2000):829-832.

Internet Resources

Arnett, Bill. "Asteroids." <>.

Near-Earth Object Program. NASA Jet Propulsion Laboratory, California Institute of Technology. <>.

Astrobiology See Astrobiology (Volume 4).

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Asteroids are rocky material left over from the formation of the solar system that orbit the Sun , but are too small to be

viewed as planets. Most asteroids are composed of stone, iron , nickel, or a combination of the three ingredients, and resemble terrestrial rocks in appearance. Asteroids can range in size from pebble-sized rocks up to almost 1,000 km in diameter. Asteroids whose orbits will eventually cause them to collide with Earth are known as meteoroids . When the heat and friction of entering Earth's atmosphere at high velocity causes the meteoroid to burn brightly in its path across the sky, it is known as a meteor. Particles or chunks of the meteor that survive the atmospheric entry and fall to Earth are meteorites. Asteroids are classified according to their composition, size, or location. Although Near-Earth Asteroids (NEAs) have been observed in Earth's orbit, the vast majority of asteroids, including the largest asteroid Ceres, are located in the Main Asteroid Belt between Mars and Jupiter.

The astronomer Johannes Kepler (15711630) was the first to postulate the existence of a hidden planet between Mars and Jupiter, a theory long considered by future astronomers, and in the region now known to contain the solar system's Main Asteroid Belt. In 1766, Johannes Titius (17291796), a professor of mathematics and physics in Germany, developed a formula for calculating planetary distances that also suggested a planet belonged between Mars and Jupiter. When the planet Uranus was discovered in 1781, it fit into the formula, causing many scientists to be even more certain that the hidden planet existed. One astronomer, Franz Xaver, proposed the formation of a society of astronomers that would be responsible for looking in assigned areas of the sky for the mystery planet.

Father Giuseppe Piazzi (17461826), was involved in such a search at this time. During the night of New Year's Eve, 1800, he saw a small star in Taurus. Because he couldn't find it listed in star catalogues, he observed it over several nights. Piazzi discovered that the body moved relative to the fixed stars, so it had to be an object that belonged to the solar system. Discovering the largest asteroid in the solar system, Piazzi gave this object the name of Ceres, the patron goddess of Sicily. Piazzi was unable, however, to calculate Ceres's orbit from so few observations. A German mathematician, Carl Friedrich Gauss, became intrigued with the problem and invented a new method for orbit calculations. Using his technique, the small object was rediscovered in the winter of 180102. That same winter, another German, Heinrich Olbers (17581840), found a second planetoid: Pallas.

This second discovery sparked a debate: were these two objects remnants of some planet's catastrophe, or did they always exist in their present form? It is now known that all the asteroids together would produce an object much smaller than our moon , so it is unlikely they were ever in one piece. Scientists generally agree that asteroids are leftovers from the formation of the solar system out of the solar nebula.

In 1804 and 1807, two more asteroids were found. The third was called Juno, and the fourth was dubbed Vesta. These were the only planetoids found until the mid-1800s, when telescopic equipment and techniques improved. From 1854 until 1870, five new asteroids were discovered every year. The all-time champion asteroid hunter in the days before photography was Johann Palisa (18481932) who found 53 by 1900, and added many more before his death.

In 1891, the German astronomer Maximilian Wolf (18631932) began using photographic techniques to search for asteroids. He had his telescope set up to follow the apparent motion of the stars, so that any other object like an asteroid would produce a short line in a photographic image rather than a dot like the stars. There had been about 300 asteroids found up until his time, but the use of photography opened the floodgates. Wolf alone discovered 228 asteroids. Astronomers now estimate that roughly 100,000 asteroids exist that are bright enough to appear on photographs taken from Earth.

Asteroids are not uniformly distributed in space . The huge planet Jupiter has captured some planetoids, called Trojan asteroids, which are found in two clusters ahead and behind the giant planet. They gather at these two points because of the gravitational forces of the Sun and Jupiter. In addition to these, there are other asteroids that have odd orbits that bring them into the inner regions of the solar system. A few have come close to the earth: in 1937, Hermes swept within 600,000 miles of the earth (only twice the distance from the Earth to the Moon); in 1989, another asteroid came within 500,000 miles of our planet. There is evidence that occasionally an asteroid, or a piece of one, has collided with the earth; one of the best-preserved impact craters can be seen in Arizona.

Because they are remnants of the beginnings of our solar system, asteroids can provide astronomers with valuable information about the conditions under which the solar system was formed.

See also Barringer meteor crater; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Comets; Hubble Space Telescope; Meteoroids and meteorites; Solar system

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asteroid A small rocky or metallic body orbiting the Sun in the asteroid belt between the orbits of Mars and Jupiter (from about 2.2 to 3.2 AU (see ASTRONOMICAL UNIT), with one family (Trojans) at 5 AU occupying two of the jovian Lagrangian Points). Two families (Apollo and Aten, as well as some Amor objects) have orbits that cross Earth's orbit. The largest asteroid is 1 Ceres (diameter 987 ± 150 km). About 30 exceed 200 km in diameter and over 3000 have been identified. Meteorites are probable samples of the asteroid belt. See APOLLO (1862); CASTALIA (4769); CERES (1); CHIRON (2060); EROS (433); GASPRA (951); GEOGRAPHOS (1620); ICARUS (1566); IDA (243); JUNO (3); MCAULIFFE (3352); MATHILDE (253); MIMISTROBELL (3840); NEREUS (4660); PALLAS (2); SHIPKA (2530); TOUTATIS (4179); and VESTA (4).

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asteroid Small body in an independent orbit around the Sun. The majority move between the orbits of Mars and Jupiter, in the main asteroid belt. The largest asteroid (and the first to be discovered) was Ceres, with a diameter of 913km (567mi). There are thought to be a million asteroids with a diameter greater than 1km (0.6mi); below this, they decrease in size to dust particles. Some very small objects find their way to Earth as meteorites. Today, nearly 6000 asteroids have been catalogued and have had their orbits calculated. This figure is increasing by several hundred a year. At least 10,000 more have been observed, but not often enough for an orbit to be calculated. Some of the larger asteroids are spherical, but most are irregularly shaped, and a wide variety of compositional types have been identified. Asteroids almost certainly originate from the time of the formation of the Solar System and are not remnants of a large planet that disintegrated, as was once thought.;

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Asteroid ★★ 1997

Re-edited version of the NBC TV miniseries emphasizes the special effects and action, which should help this routine disaster flick. Astronomer Lily McKee (Sciorra) discovers that several giant asteroids are on a collision course with Kansas City. She contacts FEMA and gets hotshot director Jack Wallach (Biehn) anxious to help out (and not just with the rock problem). Naturally, the citizens freak and one asteroid hits but there's an even bigger one on the way. 120m/C VHS, DVD . Michael Biehn, Annabella Sciorra, Don Franklin, Anne-Marie Johnson, Anthony Zerbe, Carlos Gomez, Jensen (Jennifer) Daggett, Michael Weatherly, Frank McRae, Denis Arndt; D: Bradford May; W: Robbyn Burger, Scott Sturgeon; C: David Hennings, Thomas Del Ruth; M: Shirley Walker. TV

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asteroidavoid, Boyd, Coed, droid, Floyd, Freud, Lloyd, overjoyed, self-employed, unalloyed, underemployed, unemployed, void •geoid • amoeboid (US ameboid) •globoid • cuboid • gadoid • typhoid •fungoid • discoid • tabloid • colloid •celluloid • mongoloid • alkaloid •coralloid • crystalloid • prismoid •arachnoid • sphenoid • hominoid •crinoid, echinoid •solenoid • humanoid • paranoid •hypoid • anthropoid • gabbroid •android • steroid • thyroid • hydroid •spheroid • meteoroid • Murgatroyd •Polaroid •haemorrhoid (US hemorrhoid) •asteroid • schizoid • factoid • mastoid •deltoid • planetoid • ovoid • trapezoid •rhizoid

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as·ter·oid / ˈastəˌroid/ • n. a small rocky body orbiting the sun. Large numbers of these, ranging in size from nearly 600 miles (1,000 km) across to dust particles, are found (as the asteroid belt) esp. between the orbits of Mars and Jupiter. DERIVATIVES: as·ter·oi·dal / ˌastəˈroidl/ adj.

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asteroid XIX. — Gr. asteroeidḗs star-like, f. astḗr STAR; see -OID.