Page last updated at 19:48 GMT, Monday, 10 August 2009 20:48 UK

Equinox to reveal Saturn secrets

Shadow of Moonlet
Cassini spotted the "moonlet" in Saturn's B ring

Planetary scientists are keenly observing an equinox on Saturn on 11 August, in a bid to learn more about the gas giant's ring system.

A planet's equinox comes twice a year when the Sun crosses its equator, making day and night the same length.

It takes Saturn nearly 30 Earth years to orbit the Sun, so this is the first equinox since 1994.

Around equinox, irregularities show as long shadows on the otherwise flat surface of the rings.

Objects such as "moonlets" - very small natural satellites, which are often difficult to spot - become easy to detect.

Tiny discovery

Last week, the shadows revealed a new moonlet in Saturn's B ring.

The shadows have been cast at equinox for millennia. But for the first time there is a witness in the shape of Nasa's Cassini spacecraft, which spotted the new object.

This moonlet is about 480km (300 miles) from the outer edge of the B ring.

Nasa says that the size of the shadow means the moonlet protrudes about 200m (660ft) from the ring plane. If it is orbiting in the same plane as the ring material surrounding it, which seems likely, it must be about 400m (1,300ft) across.

At the moment of equinox, the rings turn edge-on to the Sun and reflect almost no sunlight.

"Whenever equinox occurs on Saturn, sunlight will hit Saturn's thin rings, the ring plane, edge-on," said Linda Spilker from Nasa's Jet Propulsion Laboratory in Pasadena.

"The light reflecting off this extremely narrow band is so small that for all intents and purposes the rings simply vanish."

Cassini was launched in October 1997 from Florida's Cape Canaveral Air Force Station. It arrived at Saturn in 2004 to embark on a four-year mission of exploration around the planet and its moons.

The spacecraft it still operating well and has been re-programmed to carry out new tasks. It will now aim to answer some of the questions raised by its earlier observations.

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