The 170,000-mile-wide ring system around Saturn will appear to disappear Aug. 11 as the planet's equator will be directly in line with the photons of light streaming in from the sun. On earth, the phenomenon is known as an equinox and it occurs every year about March 21 (spring equinox) and Sept. 22 (autumnal equinox).
But the Earth is not surrounded by rings consisting of 35 trillion-trillion tons of ice, dust and rock fragments.
"Saturn has been performing the 'ring plane crossing' illusion about every 15 years since the rings formed, perhaps as long as 4.5 billion years ago, so by now it is pretty good at it," said Spilker. "The magician's tools required to perform this trick are pure sunlight, a planet that wobbles, and a main ring system that may be almost 200-thousand miles wide, but only 30 feet thick," Linda Spilker, deputy project scientist for the Cassini Saturn mission at NASA's JPL (Jet Propulsion Laboratory) in Pasadena, Calif., said in a statement.
Spilker said the Saturn equinox occurs twice about every 15 years.
"Whenever equinox occurs on Saturn, sunlight will hit Saturn's thin rings, the ring plane, edge-on," said Spilker. "The light reflecting off this extremely narrow band is so small that for all intents and purposes the rings simply vanish."
Observing it all will be the Cassini telescope spacecraft, which has been watching Saturn for five years. The spacecraft's instruments have discovered new rings, moons, as well as changed the way astronomers look at Saturn's ring system. The Aug. 11 Saturn equinox will be Cassini's first view of the disappearing rings.
Cassini's thermal instrument is tasked with measuring the temperature of both sides of the rings as the sun sets to look at how the rings cool as they go through this seasonal change. The spacecraft's cameras are looking for topographic features in the rings, like tiny moons and possible ring warps, which are only visible at equinox, while the near-infrared and ultraviolet instruments will be on the hunt for signs of seasonal change on the planet.
"The great thing is we are not sure what we will find," said Spilker. "Like any great magician, Saturn never fails to impress."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Cassini orbiter was designed, developed and assembled at JPL. JPL manages the mission for the Science Mission Directorate at NASA Headquarters in Washington.