Bill Gladwell, age 14, of Missoula, Mt. for his question:
HOW MANY SATELLITES ORBIT SATURN?
Saturn is the sixth planet in order of distance from the sun and the second largest in the solar system. Saturn is famous for its rings and its satellites. More than 20 satellites have been discovered orbiting Saturn. Their diameters range from 12 to 3,200 miles.
The satellites consist mostly of the lighter, icy substances that prevailed in the outer parts of the gas and dust nebula from which the solar system was formed and where radiation from the distant sun could not evaporate the frozen gases.
The five larger inner satellites Mimas, Enceladus, Tethys, Dione and Rhea are roughly spherical in shape and composed mostly of water ice. Rocky material may constitute up to 40 percent of Dione's mass.
The surfaces of the five are heavily cratered by meteorite impacts. Enceladus has a smoother surface than the others, the least cratered area on its surface being less than a few hundred million years old.
Astronomers suspect that Enceladus supplies particles to the E ring, which neighbors Enceladus' orbit. Mimas, far from being smooth, displays an impact crater the diameter of which is one third of the diameter of the satellite itself.
Tethys also bears a large crater and a valley 63 miles in width that stretches more than 1,200 miles across the surface.
Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces. Some scientists conjecture these were caused either by ice ejected from craters by meteorites, or by fresh ice that has migrated from the interior.
The famous rings of Saturn are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G and E rings. These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet.
The visible rings of Saturn stretch out to a distance of 84,650 miles from Saturn's center, but in many regions they may be only 16.4 feet thick. They are thought to consist of aggregates of rock, frozen gases and water ice ranging in size from less than 0.0002 inches in diameter to about 33 feet in diameter from dust to boulder size. An instrument aboard Voyager 2 in August, 1981, counted more than 100,000 ringlets in the Saturnian system.
The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television imaged five new faint rings within Cassini's division.
The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that exhibit rippling density variations. The gravitational interaction between rings and satellites, which cause these density waves is still not completely understood.
The B ring appears bright when viewed from the side illuminated by the sun, but dark on the other side because it is dense enough to block most of the sunlight.