Dione

What is Dione?

Moons· Diameter 1,123 km. A bright icy moon of Saturn whose trailing hemisphere is scored by wispy streaks — ice cliffs hundreds of meters tall — and which may hide an ocean under its crust. Find it between Tethys and Rhea in the orbit view.

The orbit of Dione at this moment: https://nowsky.org/solar/orbit?fw=moon:saturn:dione&sp=0.008 — replayed live, no login.

Orbit and size

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Parent planet and sibling moons

Frequently asked

How big is Dione?

1,123 km.

How far is Dione from Earth?

377,400 km.

What is notable about Dione?

bright ice-cliff streaks.

Which planet does Dione orbit?

Saturn.

Helene and Polydeuces, the moons that share its orbit

Dione does not have its orbit to itself. Sixty degrees ahead on the same path travels Helene, 35 km across, and sixty degrees behind, Polydeuces, only 3 km. Like the Trojan asteroids of the Sun–Jupiter system, they are trapped at the Lagrange points where Saturn's and Dione's gravity balance; such "Trojan moons" exist nowhere in the solar system but Saturn's (Tethys keeps two as well).

Helene was found from the ground in 1980 and Polydeuces in Cassini images in 2004. Both oscillate loosely around their Lagrange points, drifting away from Dione and back, and Polydeuces swings more than 30 degrees ahead and behind. Following Dione in the orbit view does not draw these small companions, but picturing their places 60 degrees ahead and behind on the same track gives a sense of how crowded the Saturn system is.

The moon in depth

Dione, discovered by Cassini in 1684, is 1,123 km across and orbits 377,400 km from Saturn every 2.74 days. With a density 1.48 times that of water it is one of the rockier mid-sized moons, and it shares its orbit with two tiny Trojan companions, Helene and Polydeuces.

When Voyager first imaged Dione, bright 'wispy' streaks on its trailing side were a mystery. Cassini's close-ups revealed them as ice cliffs hundreds of meters high glinting in the sunlight — fractures left as the crust stretched. Gravity measurements in 2016 hinted at an ocean roughly 100 km beneath the surface, making Dione another candidate water world in Saturn's system after Enceladus.

Track Saturn in the orbit view and you will find Dione orbiting after Mimas, Enceladus and Tethys, just inside Rhea. It also plays a quiet supporting role: its 2:1 resonance with Enceladus keeps the tidal heating alive that powers that moon's ice geysers.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.