StarDate: March 3, 2010

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The Cassini spacecraft is paying its closest call on one of Saturn's moons today. It'll pass just 1100 miles above the surface of Helene, a chunk of rock that was discovered 30 years ago this month.

Helene is only about 20 miles wide. Its surface is scarred by impact craters. Each impact not only gouges a hole, it blasts out rock and dirt that settle back on the moon's surface. That gives Helene a soft, fluffy look.

Helene's most interesting characteristic is its 2.7-day orbit around Saturn: it shares the orbit with two other moons.

The biggest of the three is Dione, which is about 700 miles in diameter. Its gravity has locked the two other moons in step with it. Helene orbits 60 degrees ahead of Dione -- one-sixth of the way around Saturn. The third moon, Polydeuces, is 60 degrees behind Dione.

All three moons are about a quarter of a million miles from Saturn -- the same distance between Earth and our own moon. That puts them well outside the bulk of Saturn's beautiful rings. But the view from these little worlds would be spectacular. Saturn itself would appear about 40 times wider than a full Moon looks from Earth. And the rings -- seen edge-on -- would form a bright line spanning a quarter of the sky -- a thin slash against butter-colored Saturn and the dark sky around it.

Saturn forms a pretty sight in our own sky tonight. It rises around 9 o'clock, in the western part of Virgo, and looks like a bright golden star.

Script by Damond Benningfield, Copyright 2010

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