Jewel of the Solar System
The next stop on our tour brings us to one of the most spectacular sights in the Solar System. The sixth planet from the Sun is a large, bright gas giant surrounded by thousands of delicate, glistening rings. As we get closer to the planet, we can see even more rings. What originally appears a two rings now reveals itself to be a complex system composed of thousands of smaller ringlets. There are rings and there are rings inside of rings. Surrounding all of this stunning beauty is a system of 62 moons, ranging from the tiny to the truly gigantic. Seven of these natural satellites are large enough to warrant further investigation. This is the planet Saturn and its system of rings and moons.
Rings and Moons
Without a doubt, the most striking feature of the Saturnian system is Saturn's amazing rings. This complex ring system is composed mainly of water ice fragments. These fragments range in size from dust specs to chunks the size of a car. Although they span 175,000 miles (282,000 km), they are less than a mile thick. Because of this, the rings are totally invisible when viewed from the side. Saturn's rings were first observed by Galileo when he viewed Saturn through his telescope in 1610. Early observations showed two rings, but later missions to the planet revealed that there are actually thousands of smaller rings. Recent observations have shown that the rings are much more complex than previously thought, with unexplained thick and thin areas and elaborate spiral structures. It has also been discovered that some of the rings are kept in place and shaped by the gravitational effects of small moons known as shepherd moons.
The Saturnian system contains a large number of moons, nearly as many as the planet Jupiter. So far, 62 moons have been discovered orbiting the planet. Seven of these moons are quite large. The largest is named Titan, and is actually larger than the planet Mercury. The rest of these moons are much smaller, and many of them thought to be asteroids that were captured by the strong gravitational forces of Saturn.
Twenty four of Saturn's moons are classified as regular satellites. This means that they orbit the planet in a nearly circular, equatorial orbit in the same direction as its rotation. They are thought to have formed at the same time as the planet as it coalesced from the dust and gas of the primordial nebula. The remaining thirty eight moons are classified as irregular satellites. This means that they orbit much farther away from the planet and their orbits are highly eccentric and inclined to the equatorial plane. Many of the irregular satellites have retrograde orbits, meaning they orbit Saturn in the opposite direction as its rotation. Astronomers believe that most irregular satellites were once wandering asteroids or comets that were captured by the planet's gravitational force.
A few of Saturn's small moons actually orbit inside or very near the planet's ring system. Their gravity has the effect of shaping the rings and keeping them sharp and in line. They are also responsible for creating some of the gaps between the rings. These moons are known as shepherd moons, because of their "herding" effect on the rings.
Exploring the Saturnian System
The Saturnian system has been explored by only four spacecraft. The first was Pioneer 11, which flew past the ringed planet in 1979. It flew close to the planet's cloud layers and sent back a low resolution images of Saturn and a few of its moons. Unfortunately, the images were too low resolution to make out any surface features. However, the spacecraft did discover Saturn's faint F-ring and revealed that the gaps between some of the planet's rings are not completely empty but are filled with fine material.
In November of 1980, the Saturnian system was visited by the Voyager 1 spacecraft. Nine months later, in August of 1982, Voyager 2 entered the Saturnian system. The two Voyager spacecraft were able to send back much higher resolution images of Saturn and its rings. They also gave us our first close-up views of many of Saturn's moons. Five new moons were discovered on these missions, and Saturn's rings were revealed to be composed of thousands of smaller rings. While Voyager 2 would continue on to Uranus, this was the final stop for Voyager 1.
In July of 2004, the Cassini-Huygens spacecraft entered the Saturnian system for the first extended survey. Cassini spent six years studying and photographing Saturn and its many moons. A special landing probe names Huygens was dropped into the atmosphere of Saturn's largest moon, Titan, where it showed us the first images from the surface of an alien moon. The Cassini spacecraft later confirmed the existence of liquid methane lakes on the Titan. Throughout its six year mission in the Saturnian system, Cassini discovered four news moons and revealed the presence of liquid water geysers on Saturn's moon, Enceladus. Thousands of high resolution images were captured of Saturn and its moons, giving us our best views yet of the Saturnian system.
The next mission to study the Saturnian system will likely be the Titan Saturn System Mission or TSSM. This will be a joint venture between NASA and the European Space Agency. TSSM is expected to study Saturn and its moons in depth when it is finally launched. So far, no launch date has been set.