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Gallery – Dione and friends

Saturn's moon Dione

Saturn's moon Dione, seen along with half of Saturn and the planets rings.

SATURN’S MOON DIONE coasts along in its orbit appearing in front of its parent planet in this Cassini spacecraft view.

The wispy terrain on the trailing hemisphere of Dione (1,123 kilometres wide) can be seen on the left of the moon here.

Dione (pronounced dy-OH-nee) is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.

Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.

Telesto and Epimetheus

The moon Telesto can be seen above the rings on the left, and Epimetheus is just on the bottom edge of the rings.

The tiny moon Telesto (25 kilometres wide) is visible as a white speck above and to the left of the rings in this view. Epimetheus (113 kilometres) appears just below the rings near the centre of the image. This view looks toward the northern, sunlit side of the rings from just above the ringplane.

The image was taken with the Cassini spacecraft narrow-angle camera on July 18, 2011. The view was acquired at a distance of approximately 2.2 million kilometres from Dione.

Adapted from information issued by NASA / JPL-Caltech / Space Science Institute.

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One image, five moons

Five of Saturn's moons in one image

Five of Saturn's moons appear in this single image taken by NASA's Cassini spacecraft.

NASA’s CASSINI SPACECRAFT snapped this image showing part of Saturn’s rings edge-on, and with five of the giant planet’s moons in the same frame.

The moon Rhea (1,528 kilometres in diameter) dominates the image, and is in the foreground of the tableau. Below it and appearing to sit on the rings, is Dione (1,123km wide). Dione is actually far in the background.

Just to the right of Dione is what looks like a small bump in the rings. This is actually Prometheus (86km wide), a “shepherd moon” that orbits Saturn along the inner edge of the F ring.

The tiny dot off to the right of the rings is Epimetheus (113km wide), and the larger moon right on the edge of the image is Tethys (1,062km wide). Epimetheus is very interesting, as it shares almost exactly the same orbit as another moon, Janus. In fact, their orbits are different by a factor of only 50 kilometres. And every now and then they come close together and swap positions!

Cassini was about 61,000 kilometres from Rhea when it took this image on January 11, 2011. Detail can be seen on Rhea down to about 2km per pixel.

Just so that you know what each of the moons looks like close up, here are images of them, also taken by Cassini.

Saturn's moon Rhea.

Saturn's moon Rhea.

Saturn's moon Dione.

Saturn's moon Dione.

Saturn's moon Prometheus.

Saturn's moon Prometheus.

Saturn's moon Epimetheus.

Saturn's moon Epimetheus.

Saturn's moon Tethys.

Saturn's moon Tethys.

Adapted from information issued by NASA/JPL/Space Science Institute.

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Saturn’s Siamese twin moons

Saturn's moon Epimetheus

Saturn's moon Epimetheus, as seen by the Cassini spacecraft narrow-angle camera from a distance of approximately 37,400 kilometres. A large crater dominates one half of the 116km-wide moon.

  • Epimetheus and Janus circle Saturn at almost the same distance
  • Every four years they swap orbits with each other
  • Both moons are probably loose piles of rocky, icy rubble

Epimetheus (pronounced ep-ee-MEE-thee-us) and the neighbouring moon Janus have been referred to as the Siamese twins of Saturn because they circle Saturn in nearly the same orbit. This co-orbital condition (also called a 1:1 resonance) confused astronomers, who at first could not believe that two moons could share nearly identical orbits without colliding.

The two moons lie amongst Saturn’s rings at a distance from Saturn of roughly 151,500 kilometres (94,100 miles). One moon orbits 50 kilometres (31 miles) higher (farther away from the planet) and consequently moves slightly slower than the other. The slight velocity difference means the inner satellite catches up to the other in approximately four Earth years.

When this happens, the gravitational interaction between the two pulls the inner moon it to a higher orbit. At the same time, the catching-up inner moon drags the leading outer moon backward so that it drops into a lower orbit. The result is that the two exchange places, and the nearest they approach is within 15,000 kilometres (6,200 miles).

At their most recent trade in early 2010, Epimetheus’ orbital radius dropped by approximately 80 kilometres (50 miles) while Janus’ orbit increased by only approximately 20 kilometres (12.4 miles). Janus’ orbit changed only a quarter of that of Epimetheus because Janus is four times more massive than Epimetheus.

Both of the moons are “phase locked” with Saturn, which means that one side always faces toward the planet. And being so close to Saturn, they orbit around it in less than 17 hours.

Saturn's moon Epimetheus

The Cassini spacecraft snapped this image of Epimetheus from a distance of about 107,000 kilometres.

Rubble piles

Epimetheus and Janus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturnian system because both moons have ancient cratered surfaces, many with soft edges because of dust. They also have some grooves (similar to grooves on the Martian moon Phobos) suggesting some glancing blows from other bodies.

They are both thought to be composed of largely of water ice, but their density of less than 0.7 is much less than that of water. Thus, they are probably “rubble piles”—each a collection of numerous pieces held together loosely by gravity.

Each moon has dark, smoother areas, along with brighter areas of terrain. One interpretation of this is that the darker material evidently moves down slopes, leaving shinier material such as water ice on the walls of fractures.

Their temperature is approximately -195 degrees Celsius (-319 degrees Fahrenheit). Their reflectivity (or albedo) of 0.7 to 0.8 in the visual range again suggests a composition largely of water ice.

Epimetheus has several craters larger than 30 kilometres, including Hilaeira and Pollux.

Saturn's moon Janus

Janus, seen here, swaps orbits with Epimetheus every four Earth years.

Discovery

French astronomer Audouin Dollfus spotted a moon of Saturn on December 15, 1966, for which he proposed the name “Janus.” On December 18 of the same year, Richard Walker made a similar observation, now credited as the discovery of Epimetheus.

At the time, astronomers believed that there was only one moon, unofficially known as “Janus,” in the given orbit.

Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Observations by the Voyager I spacecraft confirmed this in 1980, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.

The Cassini spacecraft has made several close approaches and provided detailed images of the moon since it achieved orbit around Saturn in 2004.

Nineteenth-century English astronomer John Herschel suggested that the moons of Saturn be associated with mythical brothers and sisters of Kronus, known to the Romans as Saturn. (The International Astronomical Union now controls the official naming of astronomical bodies.)

The name Epimetheus comes from the Greek god (or titan) Epimetheus (or hindsight) who was the brother of Prometheus (foresight). Together, they represented humanity. The craters on Epimetheus include Hilaeira (who was a priestess of Artemis and Athena) and Pollux (who was a warrior in The Illiad and who carried off Hilaeira).

Astronomers also refer to Epimetheus as Saturn XI and as S/1980 S3, and they refer to Janus as Saturn X and as S/1980 S1.

Epimetheus data:

  • Discovered: 1966 by R. Walker
  • Distance from Saturn: 151,422 km
  • Period of orbit around Saturn: 16.7 hours
  • Diameter: 138 x 110 x 110 km
  • Mass: 5.3 x 10^17 kg

Adapted from information issued by NASA / JPL / Space Science Institute.

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Moons with a view

NASA’s Cassini spacecraft has been orbiting Saturn since July 2004. The ringed planet has more than 60 moons, and Cassini has taken numerous images of them.

Sometimes, when the angles are just right, Cassini’s camera can fit more than one moon into its field of view—with one moon in the background and one in the foreground.

Many of the moons orbit near or within the planet’s famous rings, so the rings often appear in the images too.

Here’s a selection of recent shots showing some of Saturn’s natural satellites, large and small.

Rhea, Prometheus and Saturn's rings

In this view, the moon Rhea (1,530km wide) is on the far side of the rings. Much smaller Prometheus (86km wide) is on the nearside, orbiting between the main portion of the rings and the thin outer F ring. Camera distance to Rhea: approx. 1.6 million km. Camera distance to Prometheus: approx. 1 million km.

Dione and Titan

The cratered and cracked moon Dione (1,120km wide) seems to hang suspended in place in front of Titan (5,150km wide) in the background. Camera distance to Dione: approx 1.8 million km. Camera distance to Titan: approx. 2.7 million km.

Tethys and Dione

Dione, in the foreground of this image, appears darker than the moon Tethys (1,070km wide). Tethys appears brighter because it has a higher albedo than Dione, meaning Tethys reflects more sunlight. Camera distance to Dione: approx. 1.2 million km. Camera distance to Tethys: 1.8 million km.

Epimetheus and Janus

Saturn's moon Epimetheus (86km wide) moves in front of the larger moon Janus (179km wide) as seen by the Cassini spacecraft. Camera distance to Epimetheus: approx. 2.1 million km. Camera distance to Janus: 2.2 million km.

Janus and Prometheus

In this image, Janus is on the far side of Saturn's rings. Prometheus is on the nearside, orbiting in the gap between the main rings and the outer, thin F ring. Camera distance to Janus: approx. 1.1 million km. Camera distance to Prometheus: 1 million km.

Images courtesy of NASA / JPL / Space Science Institute.

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Saturnian moons line up

Image showing the moons Rhea and Epimetheus with Saturn and its rings in the background.

Image showing the moons Rhea and Epimetheus with Saturn and its rings in the background.

This amazing black and white image shows two of Saturn’s moons, Rhea and Epimetheus, against a backdrop of the planet and its rings.

Saturn has more than 60 known moons, each a different size and orbiting at different distances from the planet. They orbit at different speeds, and often overtake each other, leading to views like this when the Cassini spacecraft’s camera is pointed in the right direction.

Although they look close, the two moons are actually far apart. The view was obtained at a distance of approximately 1.2 million kilometres (746,000 miles) from Rhea, while Epimetheus is 400,000 kilometres further away at 1.6 million kilometres (994,000 miles).

The image gives a good indication of the scale of things in the Saturnian system. At 1,528 kilometres (949 miles) diameter, Rhea is by no means Saturn’s largest moon, yet it is more than one-tenth the width of Earth. Compare that with the huge bulk of Saturn in the background.

Epimetheus is tiny, only 113 kilometres (70 miles) wide.

At Cassini’s huge distance when it took this image, detail as small as 7 kilometres (4 miles) per pixel can be seen on Rhea, and 10 kilometres (6 miles) per pixel on Epimetheus.

Adapted from information issued by NASA / JPL / Space Science Institute.