Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)      

Every about 15 years, Earth crosses the plane of Saturn’s magnificent rings, and the rings turn edge-on, as viewed from Earth. Therefore, the extremely thin rings almost disappear! This rare cosmic alignment, termed ring plane crossing, provides an opportunity to study the structure of the rings and to discover new small moons of Saturn, particularly those that orbit close to, or within the rings.

Also, near a ring plane crossing, the larger moons of Saturn frequently pass in front of their parent giant planet, presenting fascinating views for telescopic observations. As Earth is near a ring plane crossing, on 24 February 2009, the Hubble Space Telescope (HST) pictured four moons of Saturn crossing the flattened disc of the planet. The transiting moons are: Titan (about 5,150 km across), Saturn’s largest moon, Mimas (about 397 km across), a bizarre icy moon, Dione (about 1,126 km across), one of the larger icy moons of Saturn, and Enceladus (about 500 km across), a geologically-active, enigmatic moon.

In this mesmerizing view, Titan is the orange-tinged object. Its shadow is visible as a big dark dot on the north polar region of Saturn. Mimas shines as a small point of light, below Titan and above the rings. It casts a shadow, visible as a small black dot, onto the equatorial region of Saturn. Dione and Enceladus are the star-like objects near the rings, to the left; Dione is the brighter object. The event is known as a quadruple transit. The image also reveals clearly the cloud bands of Saturn. The dark band running across the orb of the planet slightly above the rings is the shadow of the rings cast on the planet.

Saturn's rings will be exactly edge-on to our line of sight on 10 August 2009, and 4 September 2009. But, unfortunately, Saturn will then be too close to the Sun in the sky to be observed from Earth. Between May 1995 and March 1996, there were three ring plane crossings. In that period, HST pictured the edge-on rings, as well as moon transits; the images even helped discover several new Saturnian moons.

In early 2009, there was a favorable opportunity to view moon and shadow transits. These events were observable even in small telescopes. Titan passed in front of Saturn on four occasions: 24 January, 9 February, 24 February, and 12 March, but some of these transits were not visible from all locations on Earth.

The picture was taken with HST's Wide Field Planetary Camera 2, on 24 February 2009, when Saturn was at a distance of approximately 1.25 billion km from Earth. Hubble can detect details as small as 300 km across on Saturn.

Further Reading

Hubble Site
http://hubblesite.org/
Saturn Ring Plane Crossings of 1995-1996
http://www2.jpl.nasa.gov/saturn/saturn.html

Aymen Mohamed Ibrahem
Senior Astronomy Specialist