The solar system. The scale of the solar system is defined in relative terms (astronomical units) and in absolute terms (using radar techniques). Computations of the size of the solar system are based on Kepler's third law. The nine planets and many of the minor bodies revolve about the sun in roughly circular orbits, moving in the same direction and nearly the same orbital plane.
The sun is the central and principal object of the solar system. The sun contains 99.86 percent of the mass of the solar system and, because of its great mass, the sun dominates the dynamics of the solar system. The sun also steadily radiates great amounts of electromagnetic energy and intermittently emits streams of high-energy subatomic particles.
The nine major planets had a common origin but different chemical, physical, and geological histories. The matter that formed the solar system can be categorized as either rocky, icy, or gaseous. On this basis, the inner group of solid planets, known as the terrestrial planets, are composed of rocky materials; the outer group, known as the Jovian planets, are primarily icy and gaseous bodies.
Satellites revolve about the planets. All but four of the 49 known satellites belong to the Jovian planets. Saturn, Jupiter, and Uranus also have ring systems composed of icy or ice-coated particles.
Minor members of the solar system are asteroids, meteoroids, comets, and the so-called interplanetary medium of dust and gas particles.
Earth-based study of planets. The physical and chemical properties of planets are studied by means of photometry (measuring the amount of radiant energy) and spectroscopy (determining the particular wavelengths composing white light). Radiated energy may be emHted as part of normal thermal or selective nonthermal processes. Studies of thermal radiation give information about surface and atmospheric temperatures, densities, and chemical composition. Studies of nonthermal radiation can provide information about other physical processes. The surfaces of Mercury, Venus, and Mars have also been mapped by radar techniques.
The planets. The planets divide by physical and chemical properties and location into two groups: the terrestrial planets and the Jovian planets with Pluto fitting neither group. The terrestrial planets-Mercury, Venus, earth, Mars, and to some extent the moon - are small rocky bodies with large mean densities, slow rotation, thin or no atmospheres, and few if any satellites. They are the ones closest to the sun. By contrast the Jovian planets-Jupiter, Saturn, Uranus, and Neptune-are large planets composed of light elements with small mean densities, rapid rotation, thick atmospheres, and many satellites. They are widely separated from each other and are located far from the sun.
Exploration of the solar system. The vast space exploration program of the United States and other countries has been motivated by the search for knowledge, a vision of exploration as a good in its own right, the expectation of practical benefits, and interest in maintaining national prestige. The strategy used so far for planetary exploration can be divided into the four phases of reconnaissance, exploration, intensive study, and exploitation or utilization.
