SATURN: LORD OF THE RINGS

The latest spacecraft to visit Saturn has given us our best views yet of the giant ringed planet.

by Terence Dickinson
SkyNews Editor

Imagine a gigantic globe more than 700 times larger than the Earth made entirely of gas and clouds--hydrogen, helium, methane, ammonia--the froth of the universe. This colossal object is light enough to float in water... if a large enough ocean could be found to launch it. Now surround the great world with trillion moons and spin them into a thin disk.

What do we have?

Saturn.

Immediately recognized as a symbol of all things cosmic, Saturn is an icon of the universe. Most people have never seen the planet for real. It looks like a bright star to the eye, yet no other sight in the heavens is more impressive in a telescope. The creamy globe of Saturn with its symmetrical bracelet, white as snow, is unforgettable.

My first view of Saturn was at age 15. It was a mild spring evening in the late 1950s. I was stargazing from the backyard in suburban Toronto using a small telescope my parents had given me the previous Christmas.

As Saturn floated into the telescope’s field of view, I was immediately paralyzed with excitement at seeing the famous world for real--the planet looking like a tiny marble with a perfect ring around it suspended in the blackness. Then... I felt myself falling through space! So overwhelmed was I by the exquisite beauty of the remote orb, I had tipped backwards too far on my observing stool and fell with a thump on my back on the grass. Fortunately the ground was soft, and I just stayed there flat on my back for a time, gazing past the telescope tripod at Saturn, one of the great wonders of the universe.

Since the dawn of the space age, four robot spacecraft have explored Saturn. The first, Pioneer 11, flew past the ringed planet in 1979, taking a few pictures on the way by. Voyager 1 followed a year later, then its twin, Voyager 2, flew past in 1981. Equipped with superior cameras, the Voyagers transmitted back a library of beautiful images that remained our best views of Saturn until the Cassini space probe arrived at the planet in January 2004. Rather than cruising past as its predecessors did, Cassini fired its engines and swept into orbit around the ringed giant. From its orbital perch, Cassini has transmitted a flood of images back to Earth.

Research over the years is pointing more and more toward the idea that Saturn was not born with this adornment. Most likely Saturn's rings are a temporary rather than permanent structure, though by “temporary” I mean in the astronomical sense--that is, many millions of years, but still significantly less than the 4.6-billion-year age of the solar system.

The thinking is that the rings were most likely created when a comet smashed into one of Saturn's moons and some of the debris remained in orbit around Saturn and eventually formed into rings. Another possibility is that the rings are debris from a collision of two of Saturn’s moons.

Saturn is nine times the Earth-Sun distance from us this month. A radio signal takes 75 minutes to reach the Cassini spacecraft as it loops in its elongated orbit around Saturn. (Radio signals travel at the same speed as light: For example, from the Earth to the Moon in just over a second.)

If an object the size of Earth were at the same distance as Saturn, we would see it merely as a dim star, barely visible from a dark location in the country. Instead, Saturn is as bright as the brightest stars in our spring sky this year.

In 1888, American astronomer James Edward Keeler began using the largest telescope in the world at the time, a 36-inch refractor telescope at Lick Observatory near San Jose, California, to study the rings of Saturn. He proved that the rings are not rotating as a single, solid structure as some had thought.

Using a spectroscope attached to the big Lick telescope in 1895, Keeler was able to measure the comparative red shift and blue shift of sunlight reflected off different parts of Saturn's rings. He found that the parts closest to Saturn were moving faster than the parts farther out and correctly concluded that the rings must consist of individual particles revolving around Saturn like swarms of tiny moonlets.

Those ring moonlets range from tiny crystals like those in an ice fog to flying icebergs the size of small mountains. Each particle has its own individual orbit about Saturn, although a gentle jostling occurs as the particles are affected by the gravitational pull of Saturn's major moons, and each other.

The rings are truly enormous in extent. From one edge to the other, they span a distance equivalent to two-thirds of the gulf between the Earth and the Moon. Yet the particles that make up the rings seldom stray more than a few hundred meters from a perfectly flat disc, making the structure about as thick as the height of a 50-story building. A scale model of the rings made of paper the thickness of the page you are now reading would be larger than a football field.

For every house-sized ring boulder there are a million the size of a baseball and trillions the size of a grain of sand. In denser sections of the rings, the baseball-sized particles would be separated by a meter or so, while the house-sized ones would be relatively rare, sometimes kilometers apart. But if the entire ring structure could be melted and refrozen in place as a solid body, it would be a solid disk less than two feet thick.

CELESTIAL HIGHLIGHTS, June 2007

by Todd Carlson
Assistant Editor

On June 5, Jupiter is at opposition, the point at which the Sun, Earth and Jupiter are aligned. The largest planet in our solar system, Jupiter can be found low in the southeast at sunset. By the middle of the night it is due south and easily identified due to its overwhelming brilliance compared to the surrounding stars. Binoculars will reveal up to four of its moons huddled close to the planet.

Venus is at its greatest elongation on June 8, which means it is at its maximum apparent distance from the Sun. Over the next few months Venus will appear to sink farther into the twilight sky on each night.

The Beehive cluster, M44, and Venus form a pretty pairing in the night sky on June 12 and 13. Sitting just one degree above the cluster, binoculars or a telescope at low power will provide the best views. Click here to view the map.

On June 17 the thin crescent Moon will join Venus and the Beehive cluster and create a striking scene. To photograph the event with a digital camera, use a tripod and keep the camera’s zoom lens at wide angle. If the camera’s auto exposure is not quite right (it often isn’t in twilight) use manual settings and bracket exposures between 2 and 10 seconds at ISO 200.

Venus and Saturn appear very close to each other in the western sky shortly after sunset on June 30. This is a very rare occurrence and an event not to be missed if the sky is clear. Both planets will appear in the same telescopic field of view.

From a dark observing location, the Milky Way, our own galaxy, arches over the eastern horizon after 11:00 p.m. on nights without moonlight interference. The plane of our disc-shaped galaxy appears as a faint white-coloured haze that becomes more intense approaching the southern horizon. Scanning the southern Milky Way with binoculars will reveal more stars in this part of the sky than any other since we are looking directly towards the centre of our galaxy.