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Saturn's Disappearing Rings


Douglas Isbell
Headquarters, Washington, DC               May 18, 1995
(Phone: 202/358-1547)

Mary Beth Murrill
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-6478)

RELEASE: 95-72


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

Many of the world's major telescopes, including NASA's 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 Feb. 11, 1996. 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 also is 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 (JPL), Pasadena, CA, who are fine-tuning the flight path of the Cassini spacecraft. Cassini is scheduled for launch on a mission to Saturn in 1997, jointly conducted by NASA, the European Space Agency and the 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 15,500- mile (25,000-kilometer) 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 moons' sizes will be useful in targeting Cassini's observations of those satellites, according to Horn.

Saturn's rings are 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.

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 also will 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 JPL for NASA's Office of Space Science in Washington, DC.

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