NASA Cassini: Saturn's moon Rhea, Epimetheus transiting

This image was taken by NASA Cassini’s narrow-angle camera on 24 March 2010, and processed by amateur astronomer Gordan Ugarković. 

A monochrome version was previously released by NASA as PIA12638: Big and Small Before Rings.

Saturn has a great many more moons than our planet, a whopping 62.

A single moon, Titan, accounts for an overwhelming 96% of all the material orbit the planet, with a group of six other smaller moons dominating the rest.

The other 55 small satellites whizzing around Saturn make up the tiny remainder along with the gas giant’s famous rings. One of the subjects of this Cassini image, Rhea, belongs to that group of six.

Set against a backdrop showing Saturn and its intricate system of icy rings, Rhea dominates the scene and dwarfs its tiny companion, one of the 55 small satellites known as Epimetheus.

Although they appear to be close to one another, this is a trick of perspective – this view was obtained when Cassini was some 1.2 million km from Rhea, and 1.6 million km from Epimetheus, meaning the moons themselves had a hefty separation of 400 000 km.

However, even if they were nearer to each other, Rhea would still loom large over Epimetheus: at 1528 km across and just under half the size of our own Moon, Rhea is well over 10 times the size of Epimetheus, which is a modest 113 km across.

As is traditional for the earliest discovered moons of Saturn, both are named after figures from Greek mythology: the Titan Rhea (“mother of the gods”) and Prometheus’ brother Epimetheus (“after thinker” or “hindsight”).

Cassini Set to Do Retinal Scan of Saturnian Eyeball Mimas

NASA's Cassini spacecraft made its closest examination yet of Mimas, an eyeball-shaped moon of Saturn that has also been likened to the Death Star of "Star Wars."

The spacecraft returned the highest-resolution images yet of this battered satellite.

Mimas bears the mark of a violent, giant impact from the past - the 140-kilometer-wide (88-mile-wide) Herschel Crater - and scientists hope the encounter will help them explain why the moon was not blown to smithereens when the impact happened. They will also be trying to count smaller dings inside the basin of Herschel Crater so they can better estimate its age.

In addition, Cassini's composite infrared spectrometer was used to determine the thermal signature of the moon, and other instruments made measurements to learn more about the surface composition.

The Mimas flyby involved a significant amount of skill because the spacecraft passed through a dusty region to get there. Mission managers have planned for the Cassini spacecraft to lead with its high-gain antenna to provide a barrier of protection.

At closest approach, the spacecraft came within 9,500 kilometers (5,900 miles) of the moon. Cassini began taking images and measurements shortly after closest approach.

Mimas is an inner moon of Saturn that averages 396 kilometers (246 miles) in diameter. The diameter of Herschel Crater is about one-third that of the entire moon. The walls of the crater are about 5 kilometers (3 miles) high, and parts of the floor are approximately 10 kilometers (6 miles) deep.