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Quadruple Saturn Moon Transit Snapped by Hubble

On February 24, 2009, four moons of Saturn lined up in front of their parent planet to be photographed by the Hubble Space Telescope. The large orange moon Titan, and the small white icy moons Mimas, Dione, and Enceladus paraded across the face of the giant ringed planet. Due to the sun angle, they were each preceded by their own shadow.

These rare moon transits only happen when the tilt of Saturn’s ring plane is nearly “edge on” as seen from the Earth. Saturn’s rings will be perfectly edge on to our line of sight on August 10, 2009, and September 4, 2009. Unfortunately, Saturn will be too close to the sun to be seen by viewers on Earth at that time. This “ring plane crossing” occurs every 14-15 years. In 1995-96 Hubble witnessed the ring plane crossing event, as well as many moon transits, and even helped discover several new moons of Saturn.

Early 2009 was a favorable time for viewers with small telescopes to watch moon and shadow transits crossing the face of Saturn. Titan, Saturn’s largest moon, crossed Saturn on four separate occasions: January 24, February 9, February 24, and March 12, although not all events were visible from all locations on Earth.

Under exceptional viewing conditions, ground observers may even notice subtle features on the surface of Saturn. Scientists studying the atmospheres of the giant planets have noted that the banded structure in Saturn's atmosphere is similar to Jupiter's.

These pictures were taken with Hubble’s Wide Field Planetary Camera 2 on February 24, 2009, when Saturn was at a distance of roughly 775 million miles (1.25 billion kilometers) from Earth. Hubble can see details as small as 190 miles (300 km) across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet.

The moons, from left to right are: Enceladus and its shadow, Dione and its shadow, Titan and Mimas (their moons are already off the edge of the disk).

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: M.H. Wong (STScI/UC Berkeley) and C. Go (Philippines)