![]() ![]() Most of Saturn’s rings lie within what’s known as the Roche limit-the distance a satellite can orbit a large object without the planet’s tidal force overpowering the object’s own gravity and tearing it apart. But there seemed to be little serious interest in the question of how long they would last. Others thought they were formed by colliding moons, asteroids, comets or even the remainders of dwarf planets, perhaps as recently as ten million years ago. Some believed they appeared when the planet first pulled itself together about 4.5 billion years ago. A recent study showed that parts of the B-ring-the brightest ring of all-are only three to ten feet thick.Īstronomers have long wondered about the origins of Saturn’s rings. The average thickness of the main rings is believed to be no more than 30 feet. But they’re so thin that during Saturn’s equinoxes, when the light from the Sun hits the rings straight on, they all but disappear when viewed from the Earth. The rings have such a wide breadth their outermost circumference is greater than the distance from Earth to the Moon. The particles in the inner rings move faster than those in the outer rings, because they are fighting against a stronger gravitational pull. The ice particles fall into a ring shape because each one follows a similar orbital path. They stay in orbit around Saturn for the same reason the Moon stays in orbit around the Earth: Their speed is fast enough to just barely counteract the gravitational pull of the planet, keeping them at a distance. (This summer, NASA announced a new mission, dubbed Dragonfly, to Titan, Saturn's largest moon.) The spacecraft confirmed that the rings are made up mostly of water ice-chunks ranging in size from submicroscopic particles to boulders dozens of feet wide. Not the men but the $4 billion NASA Cassini-Huygens mission that was launched in 1997 and orbited Saturn and its moons until 2017. It took Cassini and Huygens again to make the first direct measurements of the rings. Giovanni Cassini famously spotted a gap in what looked like a single giant ring around Saturn he also discovered four of the planet's moons. This article is a selection from the September 2019 issue of Smithsonian magazine Buy Subscribe to Smithsonian magazine now for just $12 Some of the rings have full-on moons roaming within them. Astronomers now know that this “ring” is actually made up of eight main rings and thousands of other ringlets and divisions. What appeared to be one ring turned out to be even more complex. Then, in 1659, the Dutch astronomer Christiaan Huygens first made the suggestion that Saturn was surrounded by “a thin, flat ring, nowhere touching, and inclined to the ecliptic.” The Italian-French astronomer Giovanni Cassini went a step further in 1675 when he noticed a puzzling slim, dark gap almost in the middle of the ring. The best minds of the 17th century came up with all sorts of theories: Saturn was ellipsoidal, or surrounded by vapors, or actually a spheroid with two dark patches, or had a corona that rotated with the planet. ![]() “What is to be said concerning so strange a metamorphosis?” he wondered. “The fact is that the planet Saturn is not one alone,” he wrote to a counselor of the Grand Duke of Tuscany, “but is composed of three.” Two years later, though, with the rings tilted directly toward the Sun edgewise and basically invisible from Earth, Galileo was astonished to find that the two mysterious companions were gone. His primitive telescope gave him only a slightly better view of the heavens than did the naked eye, and in 1610 he thought he saw two undiscovered bodies flanking Saturn, one on each side. It was Galileo Galilei who first spotted something there. ![]() And not even Tycho Brahe, the Danish nobleman and alchemist, who attempted to calculate Saturn’s diameter (he was way off). Not Nicolaus Copernicus, who showed that the Earth was just another planet orbiting the Sun. Not Ptolemy or the Greco-Romans, who nonetheless discerned that Saturn was farther from Earth than Mercury or Venus. Not the astronomers of ancient India, Egypt, Babylon or the Islamic world. But for most of history, human beings couldn’t see the rings. If someone asked you to draw a planet other than ours, you would likely draw Saturn, and that is because of its rings. ![]()
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