RING SYSTEMS
One of the most exciting developments in planetary research in recent years has been the discovery of ring systems for Uranus and jupiter. (Saturn's rings had been discovered with the introduction of the telescope into astronomy.) Rings are actually individual, small solid bodies in orbit about a planet in its equatorial plane. They are thus very small satellites of the planet. Uranus's and jupiter's rings do not contain so many tiny satellites as do Saturn's; so they are much fainter than the ring system about Satu rn and have managed to escape detection until recently. Although no ring system has yet been found for Neptune, it is possible that it also has a faint set of rings similar to those around the other three Jovian planets.
MINOR MEMBERS
The asteroids, or minor planets, are rocky bodies whose diameter vary from a few between 150 and 1000 kilometers down to thousands less tha a kilomter across. Most asteroids are found between Mars and Jupiter, traveling around the sun in the same direction as the planets. However, many of them orbit the sun in the vicinity of the earth's orbit, with some in fairly elliptic orbits. In recent years the term "asteroid" has been expanded to include small objects, which are presumably not comets, located in the outer portion of the solar system. It is unlikely that their physical makeup is like that of those in the inner part of the solar system.
The meteoroids range in size from irregular solid bodies, called meteorites when they strike the ground, to tiny particles, called meteors if they merely flash through the atmosphere. As we go down the scale in size, the number of meteoroids increases very rapidly. They are composed of rocky material and are apparently related to asteroids and comets. All the meteoroids are satellites of the sun and are apparently moving in a wide variety of orbits, as best as we can determine.
Unlike the planetary bodies, most comets move around the sun in highly eccentric orbits with very long periods of revolution and at all angles of inclination. Some comets with short periods are regular visitors to the vicinity of the earth. The small masses of comets mean that it is possible for the larger planets, jupiter in particular, to alter their orbits. Astronomers believe a comet to be a "dirty iceball" that is a conglomerate of icy materials mixed with rocky matter, while most of the asteroids and meteoroids are composed of a rocky material. The cometary composition is apparently characteristic of many bodies in the outer solar system. We shall have more to say in later chapters about the relationships of asteroids, meteoroids, comets, satellites, and planets to each other.
The interplanetary medium is primarily gas particles-mostly protons and electrons-that are ejected from the sun's atmosphere at several hundred kilometers per second. These subatomic particles form the solar wind. Some dust is there too, most of it being cometary debris. Despite huge numbers of gas and dust particles, interplanetary space has fewer bits of matter and is a better vacuum than can be made in a terrestrial laboratory.
