Douglas Esson, age 12, of Salt Lake City, Utah, for his question:
Why don't Saturn's rings come crashing down?
Astronomers began pondering this problem soon after the famous rings of Saturn were discovered. Obviously, the golden circles somehow are suspended out in space, for their lowest level is several thousand miles above the planet's surface. The outer rim of the dazzling ring system has a diameter of some 170,000 miles. Until modern times, no one knew why the planet's gravity did not pull them crashing down to its surface.
In the 1650s, a Dutch astronomer had an idea on this subject. He suggested that the big beauteous rings must, simply must, be made of tiny swarming particles. For if they were sizeable chunks of material, surely the magnificent circles would collapse and crash down to Saturn's surface.
Strange to say, most astronomers agreed with this idea until quite recently. This was indeed strange because, in the meantime, they had figured out how even a ring system of giant boulders could remain above a planet's surface. This possibility involves a couple of cosmic traffic laws which all heavenly bodies must obey.
One is the force of gravity, which mighty Saturn exerts on its rings and outward to the farthest of its ten satellite moons. If this were all, the rings and even the satellites would be pulled down to the planet's surface. But it so happens that there is another cosmic law working to frustrate the pull of gravity.
The satellites of Saturn orbit around their parent planet, just as our moon is in constant orbit around the earth. The rings of Saturn also are in constant orbit around their parent planet. This whirling orbital motion counteracts Saturn's gravity. Instead of falling straight down, their orbital speed causes them to fall in a curved path which takes them around and around.
The strength of Saturn's gravity, as in all planets, diminishes with distance. Hence, its pull is strongest on the lower level of the rings, nearest the surface. Here the ring material should move faster to counteract the stronger downward pull. And this is just what happens. The orbital speed of the inner ring is fastest and the orbital speeds decrease to the slowest ring at the outside edge of the system.
These same rules would keep the rings from crashing down, whether they are made of dust particles or giant boulders. And in 1973, astronomers discovered that Saturn's rings are indeed made of sizeable chunks, perhaps somewhat like rough boulders a yard wide.
This new information was gathered by NASA's radar astronomers, using the 210 foot antenna in the Mojave Desert, California. The radar echoes traveled one and a half billion miles there and back in two and quarter hours. And they revealed that the fabulous rings are swarms of sizeable chunks, all orbiting fast enough to prevent Saturn's gravity from pulling them down to the surface.