ASTEROID

also called Minor Planet, or Planetoid, any of a host of small rocky astronomical objects found primarily between the orbits of Mars and Jupiter. By the 1990s, more than 7,000 asteroids had been observed at two or more oppositions, and 5,000 of them had been assigned numbers, which is done as soon as accurate orbital elements have been determined. Asteroids are smaller than any of the nine major planets of the solar system. There are very few large asteroids; about 30 have a diameter of more than 200 km (124 miles). Ceres, the largest known minor planet, has a diameter of roughly 935 km (581 miles), and Pallas, the second in size, measures only 535 km (332 miles) across. Approximately 250 asteroids have a diameter of at least 100 km (62 miles). It is estimated that millions of asteroids of boulder size exist in the solar system. These smaller objects are probably formed when larger asteroids collide. A few of them strike the Earth's surface in the form of meteorites (see meteorite). The largest asteroids are massive enough for their gravity to have molded them into spheres during formation. This hypothesis is supported by the lack of regular variation in their brightness that would result if these objects were asymmetric in shape. Their rotation would manifest regularly a varying amount of reflecting surface area. Smaller asteroids may have a wide range of shapes. Icarus, for example, is nearly spherical, with a diameter of only 2 km (1.2 miles). Eros, however, is more of a slab, having dimensions of approximately 10 15 30 km (6 9 18.6 miles). In 199394 the spacecraft Galileo, which was passing through the asteroid belt on its way to Jupiter, produced images showing an asteroid that has its own tiny moon. Ida, a potato-shaped asteroid with a length of about 56 km, is orbited at a distance of roughly 100 km by a rock that is about 1.5 km in diameter and is the smallest known natural satellite in the solar system. Asteroids whose orbits cross that of the Earth on a nearly continuous basis are called Apollo asteroids. About 91 of these asteroids have definitely been identified. Some astronomers would like to mount a full-scale search for such asteroids, partly out of a fear that they may collide with the Earth. Knowing of their existence and calculating their orbits would afford a possibility of altering their motion away from the Earth. Collisions with larger asteroids are rare, but smaller ones are more numerous. It is estimated that a few asteroids with a diameter of 1 km (0.6 mile) may collide with the Earth within a period of 1,000,000 years. If an asteroid of this size were to collide with the Earth, it would produce an explosion with as much force as several hydrogen bombs. The crater created by its impact would be about 13 km (8 miles) across. A short-term disturbance in the world's climate could result, and a collision in the ocean could be catastrophic. Some investigators believe that the extinction of the dinosaurs and many other land and marine animals at the end of the Cretaceous Period (about 65 million years ago) was triggered by the impact just north of the Yucatan Peninsula of an asteroid or meteorite measuring some 10 km (6 miles) in diameter. Since the 1970s astronomers have applied sophisticated analytical techniques to the reflecting spectra of asteroids to complement the analysis of the composition of meteorites in the laboratory. Such efforts have resulted in a more extensive understanding of the composition of asteroids. Asteroids as well as meteorites appear to be composed of varying proportions of stony and metallic (principally iron) materials. Numerous objects of both types contain a considerable amount of carbon, which makes them dark, with low albedos (i.e., reflective powers). Such objects are referred to as carbonaceous chondrites and are considered to have been the first materials to coalesce out of the primordial nebula from which the solar system is believed to have originated. They have avoided any subsequent alteration (for example, melting induced by heat from radioactivity within young asteroids or structural metamorphism induced by meteoric impact). No planet could have formed in the region between Mars and Jupiter because of the gravitational influence of the latter, which is the largest planet in the solar system and which resembles a star more than a planet. Its influence would have stirred up the preplanetary material in the asteroid belt during the formation of the solar system, causing the material to crash and break up rather than coalesce to build up to a planet-sized object. Calculations show that if all of the asteroids were to fuse into one object, they would form an object comparable in size to only one of the larger satellites of the solar system, such as the Moon or a Galilean satellite of Jupiter. There are gaps in the orbital distances of the asteroids from the Sun because of Jupiter's gravitational influence. These interruptions, known as Kirkwood gaps, are a simple fraction (i.e., 1/3 and 1/2) of the orbital period of Jupiter. This resonance phenomenon has the planet passing by any asteroid in the Kirkwood gaps every two or three asteroid years, depending on which gap. The repeated tugging induces an asteroid into larger, longer orbits closer to Jupiter. Eventually, however, an asteroid's resonance with Jupiter disappears as its orbit increases. The discovery of the first asteroid (Ceres in 1801) came as no surprise because of Bode's law (q.v.), a scheme that yields a sequence of numbers analogous to the spacing of the planets. (The sequence breaks down for the later-discovered outer planets.) One of these numbers corresponded to a distance from the Sun between the orbits of Mars and Jupiter where there was no planet. Ceres, then, appeared to prove Bode's law, when it was observed to orbit at the very distance indicated by the sequence. The subsequent discoveries of Pallas in 1802, Juno in 1804, and Vesta in 1807, however, confused matters. Astronomers concluded that these objects resulted from the fragmentation of a major planet. This idea is no longer considered viable for three reasons. First, as discussed earlier, all the asteroids together do not constitute an object the size of a major planetary body. Second, Jupiter's gravitational influence would have prevented a sizable planet from forming. Third, astronomers have found that asteroids at different distances from the Sun differ in composition and density. With increasing distance away from the centre of the solar system, there is a progression from stony-metallic to watery, carbonaceous-stony material and an attendant decrease in density. This condition could not have resulted from the breakup or explosion of a planet because an occurrence of that type would have necessarily produced a random distribution of density and composition. By contrast, such a condition seems consistent with the process that, according to present theory, led to the formation of the solar system and corresponds to the trend observed in the major planets. Asteroid Ida and its satellite. Seen at a distance of about 10,870 km (6,755 miles), Ida shows the also called minor planet, or planetoid, any of a host of small rocky bodies, about 1,000 km or less in diameter, that orbit the Sun primarily between the orbits of Mars and Jupiter. It is because of their small size and large numbers relative to the nine major planets that asteroids are also called minor planets. The two designations are frequently used interchangeably, though dynamicists, astronomers who study individual objects with dynamically interesting orbits or groups of objects with similar orbital characteristics, generally use the term minor planet, whereas those who study the physical properties of such objects usually refer to them as asteroids. Additional reading Summary articles can be found in Richard P. Binzel, M. Antonietta Barucci, and Marcello Fulchignoni, The Origins of the Asteroids, Scientific American, 265(4):8894 (October 1991). Review and research papers are collected in Richard P. Binzel, Tom Gehrels, and Mildred Shapley Matthews (eds.), Asteroids II (1989); and C.-I. Lagerkvist et al. (eds.), Asteroids, Comets, Meteors III (1990). A.H. Delsemme (ed.), Comets, Asteroids, Meteorites: Interrelations, Evolution, and Origins (1977), an extensive collection of colloquium papers, provides a scholarly overview of these minor solar system bodies. Armand H. Delsemme Edward F. Tedesco