Rhea is just appearing from behind Titan. The image was taken by the Cassini spacecraft a million kilometers away from Titan. Rhea was about twice that distance from Titan. [Image credit: NASA/JPL.Space Science Institute]

Saturn is famous for its rings, but it also has 145 known moons, more than any other planet.
Saturn's biggest moon Titan is larger than the planet Mercury, but a bit smaller than the Solar System's biggest moon, Jupiter's Ganymede.

Although Rhea is Saturn's second biggest moon, it's still small.
Click to see how Rhea compares to the Moon and to Earth. Three Rhea-sized objects would fit on the USA with room to spare.

Saturn and its moons are around 9.5 AU from the Sun.
An AU is an Astronomical Unit, the distance from the Earth to the Sun. Saturn is nearly ten times farther from the Sun than Earth is.

Jean-Dominique Cassini, director of the Paris Observatory, discovered Rhea in 1672, but astronomers knew very little about the moon for over three hundred years.
Cassini discovered Rhea plus three more of Saturn's moons. Over the centuries, telescopes got bigger and better. But even in the big telescopes of the 20th century the moons were just dots. The first close-up pictures came from the Voyager 1 space probe, but the best ones are from the Cassini spacecraft.

A day that's four and a half Earth days long
Rhea is farther away from Saturn than the Moon is from Earth, so we might expect it to orbit more slowly than the Moon does. Yet the Moon takes over 27 days to orbit, and Rhea takes only four and a half days, so its months are very short. Saturn is more massive than Earth, and its gravity pulls more strongly on its moons. If Rhea didn't move fast, it would fall out of orbit and into Saturn.

A dirty snowball
Some of the icy moons of the giant planets have rocky centers, but not Rhea. It's made mostly of ice, with about a quarter of it rock. Instead of a rocky core, the rock and ice are all mixed together.

An oxygen-rich atmosphere
The oxygen sounds exciting, but it's far too thin to breathe. It's an exosphere, not a proper atmosphere. Earth has both an exosphere and an atmosphere. The exosphere is where the atmosphere is so thin that it's merging with outer space.

Extremely cold.
It's easy to guess that Rhea is cold if it's ten times farther from the Sun than Earth is. And unlike Earth, Rhea doesn't have an atmosphere to act like a blanket. Even in direct sunlight, the temperature is -174° C (-281° F). Sometimes people say that something is “as hard as a rock.” This is usually just an expression, but on Rhea it's so cold that ice really is as hard as a rock.

For two centuries Saturn's moons were nameless.
When Danish astronomer Christiaan Huygens discovered Titan, he just called it Saturni Luna. That was Latin for "Saturn's Moon". When more moons were discovered, astronomers used Roman numerals to number them. Titan was Saturn I and Rhea was Saturn II. By the mid nineteenth century there were several moons. English astronomer John Herschel was finding it tiresome to keep them straight, so he gave them names for his own use.

Cronus and the Titans
Saturn is the Roman name for the Greek god Cronus. He got to be king of the Titans after overthrowing his father Uranus the sky god. Herschel called the moons after Titans, and Rhea was the wife of Cronus. Herschel's names are now official, though the moons still have their Roman numerals too.