Minor Objects: Asteroids, Comets, and More

Four basic categories of smaller materials exist in the solar system: meteoroids; asteroids (or minor planets); comets; and dust and gas. These categories are differentiated on the basis of chemistry, orbital characteristics, and their origins.

Meteoroids

Meteoroids are basically the smaller bodies in between the planets, defined as any rocky‐metallic objects less than 100 meters, or alternatively 1 kilometer, in size. It is these objects that generally fall to Earth. While heated to incandescence by atmospheric friction during their passage through the atmosphere, they are termed meteors. A fragment that survives to hit the ground is known as a meteorite.

Astronomers distinguish two types of meteors: the sporadic, whose orbital paths intersect that of Earth in random directions; and shower meteors, which are the remains of old comets that have left lots of small particles and dust in a common orbit. The material of sporadic meteors originates from the breakup of larger asteroids and old comets and the scattering of the debris away from the original orbits. When the orbit of shower meteors intersects that of Earth, numerous meteors may be viewed coming in from the same point, or radiant, in the sky. The association of meteors with comets is well known with the Leonids (observable around November 16 with a radiant in the constellation of Leo), representing the debris of Comet 1866I, and the Perseids (about August 11), which is the debris of Comet 1862III.

A typical meteor is only 0.25 grams and enters the atmosphere with a velocity of 30 km/s and a kinetic energy of approximately a 200,000 watt‐second, allowing frictional heating to produce an incandescence equivalent to a 20,000 watt light bulb burning for 10 seconds. Daily, 10,000,000 meteors enter the atmosphere, equivalent to about 20 tons of material. The smaller and more fragile material that doesn't survive passage through the atmosphere is primarily from comets. Larger meteors, which are more solid, less fragile, and of asteroidal origin, also hit Earth about 25 times a year (the largest recovered meteorite is about 50 tons). Every 100 million years, an object 10 kilometers in diameter can be expected to strike Earth producing an impact that resembles the event that explains the demise of the dinosaurs at the end of the Cretaceous period. Evidence of some 200 large meteor craters remain preserved (but mostly hidden by erosion) on Earth's surface. One of the most recent and best known meteor crators that is preserved, the Barringer Meteor Crater in northern Arizona, is 25,000 years old, 4,200 feet in diameter, and has a depth of 600 feet. It represents an impact due to a 50,000‐ton object.

Chemically, meteorites are classified into three types: irons, composed of 90 percent iron and 10 percent nickel), (representing about 5 percent of meteor falls), stony‐irons, of mixed composition (1 percent of meteor falls), and stones (95 percent of meteor falls). The latter are composed of various types of silicates but are not quite chemically identical to Earth rocks. The majority of these stones are chondrites, containing chondrules, microscopic spherules of elements that appear to have condensed out of a gas. About 5 percent are carbonaceous chondites, high in carbon and volatile elements, and are believed to be the most primitive and unaltered materials found in the solar system. These meteorite classes provide evidence for the existence of chemically differentiated planetesimals (compare with the differentiation of the terrestrial planets), which have since broken up. Age‐dating of meteorites yields the basic data for the age of the solar system, 4.6 billion years.

Asteroids

Asteroids, the largest non‐planetary or non‐lunar objects in the solar system, are those objects larger than 100 meters, or 1 kilometer, in diameter. The largest asteroid is Ceres, with a diameter of 1,000 km, followed by Pallas (600 km), Vesta (540 km), and Juno (250 km). The number of asteroids in the solar system increases rapidly the smaller they are, with ten asteroids larger than 160 km, 300 larger than 40 km, and some 100,000 asteroids larger than 1 kilometer.

The vast majority of asteroids (94 percent) are found between Mars and Jupiter in the asteroid belt, with orbital periods about the Sun of 3.3 to 6 years and orbital radii of 2.2 to 3.3 AU about the Sun. Within the asteroid belt, the asteroid distribution is not uniform. Few objects are found with orbital periods an integral fraction (1/2, 1/3, 2/5, and so on) of the orbital period of Jupiter. These gaps in the radial distributions of asteroids are called Kirkwood's Gaps, and are the result of accumulated gravitational perturbations by massive Jupiter, which altered the orbits to larger or smaller orbits. Cumulatively, the asteroids amount to a total mass of only 1/1,600 that of Earth and are apparently just debris left over from the formation of the solar system. Reflected sunlight from these objects shows that most of them represent three main types (compare with meteorites): those of predominantly metallic composition (highly reflective M‐type asteroids, about 10 percent), those of stony composition with some metals (reddish S‐type, 15 percent, and more common in the inner asteroid belt), and those of stony composition with high carbon content (dark C‐type, 75 percent, more abundant in the outer asteroid belt). Asteroids with different proportions of silicates and metals come from the breakup of larger asteroidal bodies that once were (partially) molten, allowing chemical differentiation at time of formation.

Elsewhere in the solar system exist other groups of asteroids. The Trojan asteroids are locked into a stable gravitational configuration with Jupiter, orbiting the Sun at positions 60 degrees ahead or behind in its orbit. (These positions are known as the Lagrange L4 and L5 points, after the French mathematician who showed that given two bodies in orbit about each other, there are two other positions where a smaller third body may be gravitationally trapped). The Apollo asteroids (also called Earth‐crossing asteroids or near‐Earth objects) have orbits in the inner part of the solar system. These asteroids number a few dozen and are mostly about 1 kilometer in diameter. One of these small bodies likely will hit Earth every million years or so. In the outer solar system, we find the asteroid Chiron in the outer part of the solar system, whose 51‐year orbit is probably not stable. Its diameter is between 160 and 640 kilometers, but its origin and composition are unknown. It may or may not be unique.

Comets

The structure of a typical comet includes gas and dust tails, a coma, and a nucleus (see Figure 1). The diffuse gas or plasma tail always points directly away from the Sun because of interaction with the solar wind. These tails are the largest structures in the solar system, up to 1 AU (150 million kilometers) in length. The tails are formed by sublimation of ice from the solid nucleus of the comet and look bluish due to re‐emission of absorbed sunlight (fluorescence). Tail gases include compounds such as OH, CN, C −2, H, C −3, CO +, NH −2, CH, and so on, for example, (ionized) fragments of ice molecules CO −2, H −2O, NH −3, and CH −4. A dust tail, appearing yellowish because of reflected sunlight, can sometimes be seen as a distinct feature pointing in a direction intermediate between the cometary path and the direction away from the Sun. The coma is the diffuse region around the nucleus of the comet, a region of relatively dense gas. Interior to the coma is the nucleus, a mass of mostly water ice with rocky particles (Whipple's dirty iceberg). Observation of the nucleus of Halley's Comet by spacecraft showed it to have an extremely dark surface, probably much like the dirty crust left on a snowpile melting in a parking lot. Typical cometary masses are about a billion tons with a size a few kilometers in diameter (Halley's Comet, for example, was measured to be an elongated object 15 kilometers long by 8 kilometers in diameter). Jets caused by gas boiling out of the nucleus sometimes can be observed, often forming an anti‐tail. Jets can be a significant influence in changing a cometary orbit.




Figure 1

Schematic diagram of a comet.


Astronomers recognize two major groups of comets: long period comets, with orbital periods of a few hundred to a million years or more; and the short period comets, with periods of 3 to 200 years. The former comets have orbits that are extremely elongated and move into the inner solar system at all angles. The latter have smaller elliptical orbits with predominantly direct orbits in the plane of the ecliptic. In the inner solar system, the short period comets may have their orbits altered, specifically by the gravitation of Jupiter. There are about 45 bodies in Jupiter's family of comets with periods of five to ten years. Their orbits are not stable because of continued perturbations by Jupiter. In 1992, a dramatic perturbation between Comet Shoemaker‐Levy and Jupiter occurred, with the comet breaking into some 20 fragments whose new orbit about Jupiter caused them to enter that planet's atmosphere some two years later.

Because comets are composed of ice that is slowly lost through solar heating, comet lifetimes are short compared to the age of the solar system. If a comet's perihelion is less than 1 AU, a typical lifetime will be about 100 orbital periods. The solid rocky material once held together by the ice spreads out along the cometary orbit. When Earth intersects this orbit, meteor showers occur. The finite lifetime of comets shows that a source of comets that continually supplies new ones must exist. One source is the Oort Cloud, a vast distribution of billions of comets occupying a region 100,000 AU in diameter. Occasionally, a comet is perturbed by a passing star, thus sending it into the inner part of the solar system as a long period comet. The total mass of the Oort Cloud is much less than that of the Sun. A second reservoir of comets, the source of the majority of short period comets, is a flattened disk in the plane of the solar system, but exterior to the orbit of Neptune. About two dozen objects with diameters of 50 to 500 kilometers have been detected in orbits out to 50 AU; but likely there are thousands more of these larger ones and millions of smaller ones in this Kuiper Belt.

Dust and gas

Dust and gas are the smallest constituents of the solar system. The presence of dust is revealed by its reflection of sunlight to produce the zodiacal light, a brightening of the sky in the direction of the plane of the ecliptic, which is best observed before sunrise or after sunset; and the gegenschein (or opposite light), again a brightening of the sky, but seen in the direction nearly opposite the position of the Sun. This brightening is caused by backscattered sunlight. Mapping of the sky by satellites using infra‐red radiation has also detected thermal emission from bands of dust around the ecliptic, at the distance of the asteroid belt. The number of these dust belts agrees with the collision rate for major asteroids and the time for dust produced in such collisions to disperse.

Gas in the solar system is the result of the solar wind, a constant outflow of charged particles from the outer atmosphere of the Sun, which moves past Earth with a velocity of 400 km/s. This outflow is variable with a higher flux when the Sun is active. Exceptional flows of particles can cause disturbances in the magnetosphere of Earth, which can disturb long distance radio communication, affect satellites, and generate current anomalies in electrical power grids on the planet.