Asteroids, also called minor planets, are rocky objects in orbit around the sun. Most asteroids orbit the sun between Mars and Jupiter, moving in the same direction as the planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. Some asteroids, called Apollo Asteroids, cross the orbit of Earth. It has been estimated that there are around 1000 Earth-crossing asteroids with a diameter of a kilometer or more.
Most comets are believed to be composed of rocky material and water ice. A few have highly elliptical orbits that bring them very close to the sun and swing them deeply into space, often beyond the orbit of Pluto.
The most widely accepted theory of the origin of comets is that there is a huge
cloud of comets called the Oort Cloud (after the Dutch Astronomer Jan H. Oort
who proposed the theory), of perhaps 10
comets orbiting the sun at a distance
of about 50,000 AU (just under a light year). These comets are near the
boundary between the gravitational forces of the sun and the gravitational
forces of other stars with which the sun comes into interstellar proximity
every several thousand years. According to the theory, these stellar passings
perturb the orbits of the comets within the Oort cloud. As a result, some
comets may be captured by the visiting star, some may be lost to interstellar
space, and some may begin to "fall" toward the sun. Actually, the comet is
still in orbit around the sun as it "falls." However, the orbit has been
modified from a relatively circular orbit to an extremely elliptical one.
These are the comets we observe.
A comet entering our planetary system may come under the gravitational influence of the planets as it comes within about 30 AU, especially Jupiter which is at about 5 AU, and its path may be perturbed again. A comet may be accelerated onto a hyperbolic (open) curve, which will cause it to leave the solar system. Unlike the planets that have orbits in nearly the same plane, comet orbits are oriented randomly in space. Comets have been known to break up on closest approach to the sun. Discovered early in 1993, comet Shoemaker-Levy 9 had broken up apparently because of its close passage to Jupiter. It had been captured into orbit about Jupiter and would collide with the planet in July of 1994. The spectacular collision was widely observed.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the sun. The dense, inner coma often appears pointlike, but the actual necleus is rarely seen from Earth because it is too small and dim. The coma and the nucleus together constitute the head of the comet.
As many comets approach the sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the sun. When far from the sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the sun, fluorescence usually becomes more intense than reflected light.
As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.
The sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the sun.
Each time a comet visits the sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.
Meteoroids are small, often microscopic, solid particles that are in orbit around the sun. We see meteoroids as bright meteors when they enter Earth's atmosphere at high speed as they burn up from frictional heat. Any part of a meteor that reaches the ground is called a meteorite. As volatiles boil off from comets they carry small solid particles with them. Particles released from comets in this way becomes a source for meteoroids, causing meteor showers as the Earth passes through them. Meteoroids also come from the asteroid belt.
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