Spectacular false color Voyager 2 image of Saturn's rings taken on August 23, 1981 from a range of 3 million km. Taken with three separate filters - ultraviolet,blue and green - this images shows that the C Ring, shown in blue, and the B Ring, shown in yellow, have possible traces of elements different from each other.

Written by Ron Baalke

1. What exactly is a Saturn ring plane crossing? How often does a Saturn ring plane crossing occur? Why is it so infrequent?

It takes Saturn 29.5 years to complete one revolution around the Sun. As it circles the Sun, the angle of the Saturn's rings relative to the Sun varies by 27.3 degrees. Twice during the 29.5 years, the rings are edge-on to the Sun. Since, as seen from Saturn, the Earth appears not more then 6 degrees from the Sun, it too crosses the ring plane at around the same time. Since Saturn's rings are so thin, when they are edge-on to the Earth, they appear to disappear when viewed with a small telescope. Some additional characteristics of a ring plane crossing:

• There may be 1 or 3 Earth crossings (always an odd number) during any half-orbit of Saturn (15 years).
• If there is only one Earth crossing then Saturn and the Earth will be on almost opposite sides of the Sun, making observations of Saturn difficult.
• If there are three crossings, the middle one is near opposition (when Saturn and the Earth are closest to each other) and the other two are near quadratures.
• The chance of three intersections is about 53% and the chance of one intersection is about 47%.
• There is an occasional case where the Earth hangs in the plane without passing through it.

2. Why is the Saturn ring plane crossing important?

Any new information obtained during the 1995-1996 ring plane crossing may prove to be invaluable for the upcoming Cassini mission to Saturn. For example, Cassini will actually pass through the outer rings of Saturn during its orbit insertion - passing between the F and G rings at a distance of 2.67 Saturn radii from the planet. Observations of the G ring can help assess the relative risk to Cassini before it arrives at Saturn.

When the rings of Saturn are nearly edge-on to Earth, the glare from the rings is reduced considerably, and faint objects near Saturn are easier to see. Months before and after the ring plane crossings, unique observations of Saturn, its rings and moons can be made from Earth which are available at no other time. For example:

• This is the best time to discover a new moon around Saturn. Thirteen of Saturn's moons have been discovered around the time of a ring plane crossing.
During the Saturn ring plane crossing on May 22, 1995, the Hubble Space Telescope discovered four new probable moons orbiting Saturn.
• A large number of eclipses and occultations of the satellites by Saturn will occur, and accurate timings of these events can be obtained during a ring plane crossing. This in turn is used to improve the ephemerides of the satellites.
• The cloudtops of Saturn are more easily observed without the glare from the rings.
• Direct observations of the thickness of the main rings can be performed.
• The tenuous E and G rings may be observed from Earth, which are normally obscured from the glare from Saturn's rings.
• The small inner moons of Saturn, Prometheus, Pandora, Janus and Epimetheus can be observed with better precision.
• Observations of the dark side of the rings and of the C ring are possible during a ring plane crossing event.

3. After 1996, when is the next ring plane crossing?

The Saturn ring plane crossings only occur about every 15 years. The next ring plane crossing will be the hard-to-observe single crossing which will occur on September 4, 2009. The next ring plane crossing after that is also a single crossing on March 23, 2025. In fact, the next triple ring plane crossing won't happen again until 2038!

4. Do any of the other planets have rings?

Yes, Jupiter, Uranus and Neptune all have rings. Of course, none of the rings around the other planets are as spectacular as Saturn's, but it appears to be a common phenomena for large planets.

5. How did the rings form around Saturn?

There are currently three theories on how the rings formed around Saturn:

1. The rings are remnants of material of the early solar system that never formed into satellites because they were within the Roche limit.
2. A medium size moon of Saturn strayed inside the Roche limit and was pulled apart by tidal forces, forming the rings.
3. The ring material is the remains of a satellite shattered by meteor impacts.

6. What is the Roche limit?

The Roche limit was first described fy Edouard Roche in 1848. The Roche limit is the closest distance an object can come to another object without being pulled apart by tidal forces. If a planet and a moon have identical densities, then the Roche limit is 2.446 times the radius of the planet. A large moon orbiting inside the Roche limit will be destroyed. The Earth's Roche limit is 18,470 km (11,470 miles). If our Moon ever ventured within this Roche limit, it would be pulled apart by tidal forces and the Earth would have rings. The four gaseous outer planets do have their rings systems inside of their respective Roche limit. The Roche limits for the gaseous planets are:

```   Jupiter - 175,000 km (108,000 miles)
Saturn  - 147,000 km ( 92,000 miles)
Uranus  -  62,000 km ( 39,000 miles)
Neptune -  59,000 km ( 37,000 miles)
```

On July 7, 1992, Comet Shoemaker-Levy 9 broke apart in 21 pieces due to tidal forces when it made a close approach of Jupiter which was within the Roche limit. On the subsequent pass, each of the pieces of the comet impacted Jupiter.

7. What are Saturn's rings composed of? What are the size of the ring particles? How thick are the rings?

Near-infrared observations from Earth have shown that the surface of the ring particles is predominately water ice. Some impurities has been detected as well, so some small amount of silicate material may be mixed in with some of the ring particles.

The size of the most of the ring particles ranges from around 1 centimeter to 5 meters. It is likely a few kilometer-sized object exist as well. One of the moons, Pan, is inside the Encke Division and is 20 km (12 miles) in diameter. Additional moonlets may yet to be discovered in some of the other gaps in the rings.

The rings of Saturn are very thin, and results from Voyager 2 indicates that they are are no thicker than 200 meters.

8. What is the cause of the "spokes" seen on the rings?

#### Saturn's Ring Spokes

The spokes are strange dark radial features (as long as 20,000 km!) that move about in curious patterns on the B ring. Since the spokes have been observed on both sides of the ring plane, they are thought to be microscopic grains that have become charged and are levitating away the ring plane. Another possibility is a meteor punched through Saturn's rings, lifting dust particles away from the ring plane. When first observed by Voyager, the spoke movements seemed to defy gravity and had the scientists very perplexed. Since the spokes rotate at the same rate as Saturn's magnetic field, it is apparent that the electromagnetic forces are also at work.

9. What are shepherding moons?

Shepherding moons are satellites that orbit along side a ring. Due to gravitational effects from the shepherding moon, the edges of the rings are kept sharp and distinct. If the shepherding moon was not present, then the ring material would have a tendency to spread out. If two satellites are orbiting on both sides of the ring, then ring will be constrained on both sides into a narrow band.

#### Voyager 2 Image of Prometheus and Pandora Shepherding Saturn's Narrow F Ring

The concept of shepherding moons was first proposed by Peter Goldreich and Scott Tremaine in 1979 to explain why the Uranus' rings were so narrow. Voyager 1 discovered the first pair of shepherding moons, Prometheus and Pandora, in 1981, shepherding the narrow F ring. Voyager 2 later found shepherding moons at Uranus in 1986.

During the Saturn ring plane crossing on May 22, 1995, the Hubble Space Telescope discovered a third shepherding moon orbiting near the F-Ring.

10. Are we sending any spacecraft to Saturn?

Yes, the Cassini mission was launched in 1997. It will arrive at Saturn in 2004, and deploy a probe to land on Saturn's largest moon, Titan.

11. Who was the first to see Saturn's rings?

Galileo Galilei became the first to observe Saturn's rings in 1610 with his 20-power telescope. He thought the rings were "handles" or large moons on either side of the planet.