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Views of Moons

NASA’S CASSINI SPACECRAFT, in orbit around the planet Saturn, has been sending back some wonderful views of its moons. In particular, it has captured images where one moon seems to float in front of the other. Here we present a selection of recent images.

Cassini image of Titan and Tethys

Can you tell which of these moons in the foreground? It's Titan, the large one (diameter 5,150 kilometres; bigger than our Moon) with the orange atmosphere, with smaller, shiny, icy Tethys in the background. Titan was 2.3 million kilometres from Titan, and 3.4 million from Tethys when it took this image. Saturn's rings can be seen edge-on in the distance.

Cassini image of Rhea and Titan

This black-and-white image shows the moon Rhea (1,528 km diameter) in front of Titan. Cassini was 2 million kilometres from Titan and 1.3 million kilometres from Rhea when it took this image.

Cassini image of Titan and Dione

This view shows Titan again, this time with the much smaller moon Dione (1,123 km diameter) peering around from behind, with Saturn and its rings (edge-on) in the background. Cassini was 2.3 million kilometres from Titan and 3.2 million kilometres from Dione when it took the image. The haze that surrounds Titan can clearly be seen. Titan has a mostly nitrogen atmosphere that extends far from the surface. The surface pressure is about 1.5 times that on Earth.

Cassini image of Titan

In this view, Titan appears to float in front of Saturn and its rings. Titan is not only the second-largest moon in the Solar System; it's also about 300 kilometres wider than the planet Mercury!

More information: Cassini mission

Story by Jonathan Nally. Images courtesy 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 smooth operator

Saturn's moon Telesto

The smooth surface of Saturn's moon Telesto is shown to good effect in this image captured during the Cassini spacecraft's August 27, 2009, fly-by.

Most small, rocky moons are covered in craters, having suffered millions of years of bombardment by meteoroids that happened to come along and find themselves in the wrong place at the wrong time.

Those small moons do not have volcanic, tectonic or other processes to wipe out the traces of those impacts, the craters. (By comparison, on a large world like Earth, there are lots of erosion processes—eg. wind, rain, volcanism—that have obliterated almost all traces of impact craters.)

But in the small moon stakes, Saturn’s Telesto is an exception.

Very small, only 24 kilometres long at its widest point, this moon doesn’t appear to have much in the way of craters. Rather, it seems to be covered by a layer of fine, dust-sized particles of (probably) ice, giving it a smooth appearance.

NASA’s Cassini spacecraft—which has been reconnoitering the Saturnian system since 2004—has taken several images of this mysterious moon. None of the images are very large, as Telesto is tiny and the images were taken from quite large distances.

Cassini’s closest approach to Telesto occurred in October, 2005, during which it managed to capture an image from a distance of approximately 14,500 kilometres (9,000 miles) showing detail down to 86 metres (283 feet) per pixel.

False colour view of Saturn's moon Telesto

A false colour view of Telesto reveals surface variations.

Cassini’s images reveal Telesto’s smooth nature, and show a handful of broad features such as large craters, depressions and mounds.

A special false-colour image shows subtle variations in surface texture across the face of the moon. No one’s really sure what causes the variations, but it’s probably due to small differences in the chemical composition of the ice particles, or maybe their sizes.

Another of Saturn’s moons, Pandora, has a similar dusty surface (see our earlier story on Pandora).

Telesto (pronounced teh-LESS-toh) is also a “Trojan” moon, meaning that it has an orbit that keeps it at the same distance from another body in the same orbit. In this case, Telesto always orbits 60 degrees behind the larger moon Tethys. Another moon, Calypso, orbits 60 degrees in front of Tethys. Telesto is the “leading Trojan,” Calypso is the “trailing Trojan.”

Telesto facts:

  • Discovered: in 1980 in ground-based observations by Brad Smith, Harold Reitsema, Stephen Larson and John Fountain
  • Distance from Saturn: 294,660 km (about 183,090 miles)
  • Equatorial diameter: 30 x 25 x 15 km (19 x 15.5 x 9 miles)
  • Mass: 8 x 10^17 kg

Story by Jonathan Nally, editor SpaceInfo.com.au

Images courtesy 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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