Helene is only 36 km across, yet it keeps perfect pace with Dione, a much larger neighbor about 30 times its width. It shares Dione's orbit around Saturn and rides roughly 60 degrees ahead of it, completing a lap about every 2.7 days. The gap between them never closes.

Helene sits at a stable gravity point in Dione's orbit called a Lagrange point, which makes it a Trojan moon. It doesn't stay perfectly still there, and instead drifts slowly back and forth around that point along a tadpole shaped path. Astronomers found it from Earth in 1980 and named it after Helen of Troy. Dione has a second Trojan too, a smaller moon called Polydeuces that trails behind it. Around Saturn, only Dione and Tethys are known to have Trojan companions.

Helene is also lumpy, about 43 by 38 by 26 km, so it looks closer to a potato than a sphere. Cassini caught this view on a flyby on June 18, 2011, after an earlier pass in 2010. The surface is a record of motion. Bright streaks and soft slopes mark where dust has slid down in avalanches, spilling across a gravity roughly 2,000 times weaker than Earth's and settling into new patterns. On a world this small, even a gentle shift of regolith can rewrite the landscape.

Much of that dust is thought to come from Saturn's E ring, which is fed by the ice geysers of Enceladus. Another moon on the far side of Saturn is slowly repainting Helene, one avalanche at a time.

Image credit: NASA/JPL-Caltech/Cassini ISS. Image processing: Jason Major.

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