Montage of our solar system

The rings of Saturn will all but disappear for a few moments on May 22 in a rare astronomical magic act that will allow astronomers to look for new moons and other features that are normally obscured by the glare of the dazzling rings.

All the world's major telescopes, including the orbiting Hubble Space Telescope, will focus on Saturn during the 24-minute event.

The phenomenon is known to astronomers as a Saturn ring plane crossing. This year and next, the rings will be seen edge-on from the Earth's perspective on three occasions -- May 22 and August 10, 1995, and February 11, 1996 -- and from the Sun's perspective once. This event only happens about every 15 years. Ring plane crossings provide astronomers with unique views of the Saturnian system. With the rings temporarily invisible as viewed from Earth, faint objects near the planet are easier to see. Thirteen of Saturn's 18 known moons have been discovered during past ring plane crossings.

The faint, outermost E-ring is also easier to detect when viewed edge-on due to the greater amount of material in the line-of-sight. Thus, observations made over the course of the ring plane crossing also can be used to gather new information on the thickness of the rings and to search for new rings.

The event is of special interest to scientists and engineers at NASA's Jet Propulsion Laboratory in fine-tuning the flight path of the Cassini spacecraft, scheduled for launch on a mission to Saturn in 1997, jointly conducted by NASA, the European Space Agency and Italian space agency. Any new data on the location and density of material in the rings will help the Cassini team plan the most advantageous and safest course for the spacecraft to take when it flies through the rings upon arrival at Saturn in 2004.

"We're going in awfully close with Cassini," said mission scientist Dr. Linda Horn of JPL, "so the more we know about the boundaries of the rings, the more confident we'll be." Plans call for the spacecraft to fly through a 25,000-kilometer (about 15,500-mile) gap between the F- and G-rings, then closely over the broad C-ring. Later, the spacecraft will make several passes through the E-ring.

Astronomers hope to refine measurements of Saturn's small, inner moons during the ring plane crossing. Better estimates of the moon sizes will be useful in targeting Cassini's observations of those satellite, Horn said.

In 1610, Galileo Galilei, without knowing it, became the first person to observe Saturn's rings with his small telescope. Astonished by what appeared to be "handles" or large moons on either side of the planet, he wrote that Saturn appeared to be "triple-bodied. This is to say to my very great amazement, Saturn was seen to me to be not a single star, but three together, which almost touch each other."

Galileo was even more amazed when the appendages he'd observed a couple of years earlier had now disappeared. "I do not know what to say in a case so surprising, so unlooked for and so novel," he wrote in 1612. Without realizing it, he'd witnessed a Saturn ring plane crossing.

It was not until 1655 that the nature of Saturn's rings began to be understood. In that year, Dutch astronomer Christiaan Huygens proposed that Saturn was surrounded by a solid ring. Improved observations over the next 340 years, most recently by the Voyager 1 and 2 spacecraft flybys of 1980-81 and views from the Hubble Space Telescope, have led to scientists' current understanding of Saturn's rings.

The rings are now known to be numerous, dynamically complex and made up of countless particles of ice ranging in size from boulders to snowflakes, with some rock mixed in. They are thought to be the remains of comets, meteoroids and possibly small moons that have been captured and torn apart by Saturn's gravity. From telescopes on Earth, Saturn and its rings present one of the most beautiful sights in the night sky.

The rings are a prime target for the science instruments aboard the Cassini spacecraft, whose mission is to study the Saturnian system while orbiting the planet for four years. Cassini will also carry the European Space Agency's Huygens Probe to be dropped into the atmosphere of Saturn's large moon Titan. As it parachutes downward, the Huygens Probe will return information about Titan's atmosphere and surface. In some ways, scientists believe Titan resembles Earth as it existed in a primordial stage before life developed.

The Cassini Project is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science in Washington, D.C.

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