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Saturn’s quadruple play

Four Saturnian moons make simultaneous transit snapped by Hubble

March 17, 2009 at 9:51 am

No, Saturn hasn’t suddenly developed a mild case of acne. But on February 24, the Hubble Space Telescope did capture a rare portrait of Saturn as four of its moons simultaneously passed in front of the ringed planet.

QUADRUPLE PASSAGE | In February, the Hubble Space Telescope captured this rare view of four Saturnian moons passing in front of their parent planet. The giant, orange moon Titan appears near Saturn’s north pole while the tiny moon Mimas lies to the far right limb of Saturn, just above the planet’s ring plane. The moon Enceladus lies to the far left, preceded by its black shadow. The bright moon Dione and its shadow follow just to the right. The dark band running across Saturn slightly above the rings is the shadow cast on the planet by the rings. NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

From a distance of 1.25 billion kilometers, Hubble’s Wide Field Planetary Camera 2 recorded the passage of the four moons — giant Titan, Mimas, Dione and Enceladus.

These rare transits happen only when Saturn’s ring plane is nearly edge on as seen from Earth. Saturn’s rings will be perfectly edge on to our line of sight on August 10 and September 4, but the planet will lie too close to the sun to be seen then by terrestrial observers.

During the last ring plane crossing, in 1995 and 1996, Hubble also recorded several Saturnian moon transits and helped find several previously unknown moons.

A very different kind of transit — the passage of extrasolar planets in front of their parent stars — has proven invaluable in discerning the atmosphere and composition of these alien orbs. Some 40 transiting exoplanets are now known.

NASA released the new image of Saturn on March 17.