Asteroids

Asteroids are small bodies in space—the 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 asteroids—Gaspra, Ida, and Mathilde—were 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

Bibliography

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." <http://www.seds.org/nineplanets/nineplanets/asteroids.html>.

Near-Earth Object Program. NASA Jet Propulsion Laboratory, California Institute of Technology. <http://neo.jpl.nasa.gov/>.

Astrobiology See Astrobiology (Volume 4).

Asteroids

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 (1571–1630) 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 (1729–1796), 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 (1746–1826), 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 1801–02. That same winter, another German, Heinrich Olbers (1758–1840), 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 (1848–1932) who found 53 by 1900, and added many more before his death.

In 1891, the German astronomer Maximilian Wolf (1863–1932) 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

asteroid Any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the asteroid belt) between the orbits of Mars and Jupiter; also known as a minor planet. They range in size from almost 1000 km for Ceres (the first asteroid discovered, in 1801) down to less than 10 m for the smallest so far detected. The total mass of all asteroids is 4 × 10²¹ kg, about one-twentieth the mass of the Moon.

When an asteroid is discovered it is given a temporary designation, consisting of the year of discovery followed by two letters; the first indicates the half-month during which the asteroid was discovered, and the second the order of discovery within that half-month. Only when an accurate orbit has been determined is it assigned a permanent number, and the discoverer then has the right to name it. Increasing numbers are being discovered in dedicated searches such as the Catalina Sky Survey, Lincoln Near-Earth Asteroid Research, Lowell Observatory Near-Earth Object Search, Near Earth Asteroid Tracking, Pan-STARRS, and Spacewatch. There are thought to be between 1–2 million larger than 1 km, mostly in the main belt, although only around 10% of these are currently known.

The orbits of most asteroids have higher eccentricities and inclinations than those of the major planets. Within the main asteroid belt, orbital eccentricities average about 0.15, and inclinations about 10°; occasionally they exceed 0.5 and 30°, respectively, more typical of the orbits of short-period comets. Indeed, some objects classified as asteroids may be defunct cometary nuclei. Rotation periods of asteroids range from a few hours to several weeks, but are typically 6–24 hours. The larger asteroids are roughly spherical, but those smaller than 150 km are commonly elongated or irregular. Radar studies of a few asteroids have revealed that some may be dumbbell-shaped or possibly double; these include Castalia and Toutatis. A few asteroids have small moons, the first of which was photographed in orbit around Ida by the Galileo space probe.

Some main-belt asteroids form groups with similar orbital characteristics (semimajor axis, orbital eccentricity, and inclination), for example the Cybele, Hilda, Hungaria, and Phocaea groups. Where the group seems to have originated from the break-up of a single parent body, it is called a Hirayama family. A small percentage of asteroids orbit outside the main asteroid belt. Members of the Amor group cross the orbit of Mars, while Apollo and Aten group asteroids cross that of Earth; these three groups are collectively termed near-Earth asteroids. Farther out, the Trojan asteroids orbit at Jupiter's distance, while beyond Neptune is the Kuiper Belt.

Asteroids are divided into various classes according to their reflectance spectra, which reveal differences in composition. The proportion of different asteroid classes changes markedly with increasing distance from the Sun. S-class (silicaceous) asteroids predominate in the inner main belt (at less than 2.4 AU). C-class (carbonaceous) asteroids are more prevalent in the middle and outer regions of the belt, with a peak near 3 AU. The dark asteroids near the outer edge of the main belt have a reddish tinge, and may be richer in organic components; these are the P-class asteroids. Still farther out, many of the Trojan asteroids are even redder; they are termed D-class asteroids. There is an apparent concentration of M-class (metallic) asteroids in the middle of the belt, at 2.5–3.0 AU.

Asteroids are thought to have formed through the accretion of metre-sized bodies, but were prevented from aggregating into a planet by the gravitational effect of Jupiter, which had already formed. In addition, some planetesimals left over from the formation of Jupiter may have been scattered into the asteroid belt. The largest asteroids were heated by the decay of radioactive isotopes within them. They melted and became differentiated, acquiring a metallic core, overlain by a mantle and crust. Subsequent collisions led to fragmentation, and almost every asteroid is probably a fragment of a once-larger body. In addition, most meteorites are believed to be pieces of asteroids.

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.

http://lpl.arizona.edu/nineplanets/nineplanets/asteroids.html; http://jpl.nasa.gov

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).

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.

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

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