Quadruple Saturn Moon Transit
Saturn is famous for the intriguing rings that encircle it. As Saturn orbits the Sun, though, our view of its rings changes. Roughly every 15 years (halfway through Saturn’s almost-30-year orbit), Saturn’s rings appear edge-on, sometimes seeming to disappear altogether. Because many of Saturn’s moons orbit the planet in the same plane as the rings, they appear to cross in front of the planet during this time.
On February 24, 2009, when Saturn’s rings were nearly edge-on, Hubble tracked four of Saturn’s moons as they passed across the face of the giant ringed planet. In this image, captured with Hubble’s Wide Field Planetary Camera 2, Saturn’s large, orangish moon Titan casts a deep, round shadow near the upper edge of Saturn’s disk. The smaller moon Mimas and its shadow appear as white and black dots to the lower left of Titan, just above the long, thin shadow projected onto Saturn by the planet’s rings. Bright Dione and the fainter Enceladus hover above the rings on the far left.
These rare views of Saturn, when the glare of the bright rings is diminished, give astronomers a chance to find and study fainter rings and moons around Saturn. In fact, astronomers discovered Saturn’s hazy E ring and 13 of the planet’s moons at times when the rings were edge-on between 1655 and 1980.
When Saturn’s rings were turned edge-on in 1995, two teams of astronomers, one led by Amanda Bosh of Lowell Observatory and the other by Philip Nicholson of Cornell University, used Hubble to uncover bright, temporary arcs and clumps within Saturn’s narrow F ring that disappeared within weeks. Their Hubble observations also revealed that the orbit of Prometheus, a moon near the F ring, had changed since it was discovered in NASA’s Voyager 1 spacecraft images in 1980, suggesting that Prometheus interacts with the F ring or another nearby moon as they orbit Saturn.
At times when Saturn’s rings appear inclined (not edge-on), astronomers can use Hubble’s Space Telescope Imaging Spectrograph to study the ultraviolet aurorae glowing around Saturn’s south pole. These observations have revealed that Saturn’s aurorae change from day to day in response to fluctuations in the stream of solar particles cascading through Saturn’s magnetic field.
Distance: 6th planet from the Sun; average distance from the Sun is 1.4 billion km (886 million miles) or 9.5 astronomical units (AU)
Image Filters: F439W (B), F555W (V), F675W (R)