In addition to the sun and the planets (with their satellites), the solar system contains many smaller objects: asteroids, or minor planets; meteors, some of which collide with the surface of the earth as meteorites; and comets. Asteroids, or more accurately, planetoids, are relatively small planetary bodies.
Various theories have been proposed about the origin and position of asteroids. One theory suggests that a substantial body was smashed by a planetary collision to leave the large number of assorted fragments that remain today. Another theory proposes that the asteroids acquired their present forms through the condensation of solar gas in the early stages of the sun’s formation.
Apart from most asteroids, which lie in the belt between the orbits of Mars and Jupiter, there are other groups of minor planets, each with its own characteristic orbit. The Trojan asteroids, for example, keep to the same orbital path as Jupiter. There are two separate groups—the Achilles group, located about 500 million miles (800 million kilometers) ahead of the planet, and the Patroclus group, at an average of the same distance behind it—both held in these “Lagrangian” positions by Jupiter’s gravitational influence. The Trojans are believed to have been satellites of Jupiter (when it was about 20 times more massive than it presently is) that escaped the planet’s gravitational field to fall into the position they now occupy in Jupiter’s orbit. These asteroids range in average diameter from 2 to 60 miles (3 to 97 kilometers).
Most asteroids do not remain as close to Jupiter as the Trojans, because the perturbations they receive from their proximity to the planet cause their orbits to change. There are specific distances from Jupiter where few asteroids are found, such as 2.1 and 2.5 astronomical units. These relatively barren areas are called Kirkwood gaps and may be the result of an interaction with Jupiter, because these distances correspond to orbital periods that are rational multiples of Jupiter’s orbital period.
Another group of asteroids that do not lie in the main belt is the Apollo group, which passes within the earth’s orbit for a short period as their orbits take them closer to the sun. The Apollo-Amor group of asteroids appear to be remnants of comets whose volatile material has been consumed through repeated interaction with the inner solar system. All that is left of them are their dark, solid nuclei orbiting the sun.
Chiron, the asteroid discovered by Charles Kowal in 1977, may herald a group of remnants from the outer solar system. For much of its orbit, Chiron stays between the orbits of Saturn and Uranus, but for about one-sixth of the time it orbits closer to the sun than Saturn.
Most asteroids appear to be irregular in shape. Variations in the light reflected by several of them indicate that they are elongated bodies that tumble in orbit. The rotation periods of irregular asteroids can be determined by plotting their light curves. One asteroid known to be irregular in shape is 433 Eros, discovered by Carl Witt in 1898. Eros makes close approaches to earth, and at one such approach (in 1931), its elongated shape was observed telescopically by the astronomers Fin-sen and van den Bos.
Another explanation for the variability in light is the suggestion that some asteroids have satellite companions, such as 532 Herculina—a binary asteroid. It was observed that in 1978, Herculina occluded a star, and shortly afterward there was a second dimming of the star, which indicated that Herculina has a companion.
The brightness of an asteroid depends on its size and its albedo, or the fraction of sunlight it reflects. The amount of light reflected in turn depends on the composition of the asteroid. Two basic compositions of asteroids are known: the larger group consists of dark objects and are assumed to be composed of carbonaceous chondrites. The second, smaller group is brighter and reddish in color. This type is rich in silicates.
Ceres, the biggest asteroid, has been calculated to be about 600 miles (970 kilometers) across.
Meteors and meteorites
The breakup of a comet produces a swarm of particles spread out along its orbital path. If one of these particles, or any of the millions of dust grains orbiting the sun, strays too close to the earth, it is pulled down by gravity through the atmosphere and generally burned up, destroyed by friction with the air. The usual result is a brilliant streak of light moving across the night sky—known as a meteor, or more commonly, a shooting star. Meteors are not stars, however, and only their appearance has given rise to their common name.
Meteor particles (meteoroids) orbit the sun in elongated orbits, often in random planes. Earth passes through these orbital paths at certain times of the year, when meteor showers can be seen. Due to the effect of perspective, each meteor in a shower appears to radiate from the same point in the sky, called the radiant, usually named after the constellation in which the point is located. For example, the Leonid shower radiant is in the constellation of Leo. All the meteors in a particular shower travel in parallel paths.
Those meteors that are not members of a meteor shower are known as sporadics and are visible at any time of the year, because they orbit around the sun in the same direction as the planets and near the ecliptic plane.
Most meteoroids disintegrate completely, but some are sufficiently large to partly survive the fall to the earth. These objects are known as meteorites, and certain features on the surface of the earth bear testimony to past meteoric impacts. One of the best-known examples is the large meteorite crater in northeast Arizona, which was created by an impact that occurred many thousands of years ago.
Meteorites are classified into two major types: iron (siderite) and stony (aerolite). Iron meteorites are thought to be remnants of the metallic core of a planet that has broken up. Stony meteorites are made up of various rocks and minerals. The mineral composition of these meteorites suggests that they may once have formed part of the outer layers of a planetary body.
Awesome though comets are, they are fairly unimportant members of the solar system. A typical comet is made up of three main parts: the nucleus, coma, and tail. Most of the comet’s mass is concentrated in the nucleus, which comprises a mixture of the ice and snow of various frozen materials, together with a large proportion of dust. The vaporization of matter from the nucleus results in the appearance of the coma.
Most comets form several tails containing both dust and plasma. They always point away from the sun and can reach lengths of more than 100 million miles (160 million kilometers).
Comets are classified according to their orbital periods. Short-period comets have periods that range from 3 to 25 years. About 80 of these have been observed on repeated occasions.
Intermediate-period comets have periods of between 25 and 200 years. The most notable example is Halley’s Comet. It has a period of 76 to 79 years, and recorded observations of it go back as far as 240 B.C. In this century, it was seen in 1910 and in 1986.
Long-period comets have very elongated orbits that carry them up to 10,000 astronomical units from the sun. Some 450 examples have been observed.
In 1950, Jan Oort put forth the theory that a great number of comets originate in a vast, spherical cloud surrounding the solar system. Comets are released from the cloud and pushed toward the inner regions of the solar system by the gravitational perturbations of passing stars. Astronomers cannot actually observe the cloud, however, and its existence is based only on theory. Another suggestion is that in its orbit, the sun encounters clouds of gas and dust and when going through these clouds, its gravitational pull attracts particles from the cloud that follow it in its orbit. These particles accrete to form the nucleus of a comet.