Posted on by Jason Major

Watch Two Dark Moons Sneak Into Cassini’s Shots

Raw image of Saturn with two moons acquired by Cassini on March 11, 2014 (NASA/JPL-Caltech/SSI)

On March 11, NASA’s Cassini spacecraft was acquiring some images of Saturn’s back-lit limb when two of its moons decided to make an entrance. Like stage hands in a darkened theatre the moons quickly passed  across the scene, moving between Saturn and the spacecraft and, because of exposure time and spacecraft motion, getting a bit blurred in the process.

In the image above the silhouette of one moon can be seen at bottom right — Mimas, perhaps — while another’s crescent can be made out at upper left… possibly Enceladus. Very cool!

Watch an animation of the moons below:

Two of Saturn's moons drift into the scene on March 11, 2014 (NASA/JPL-Caltech/SSI. Animation by Jason Major.)
Two of Saturn’s moons drift into the scene on March 11, 2014 (NASA/JPL-Caltech/SSI. Animation by Jason Major.)

While I admit I’m not 100% sure which moons these are, based on their apparent shapes, positions, and relative sizes I’d make my guess that these are 318-mile (511-km) -wide Enceladus and the 246-mile (395-km) -wide Mimas.

Possible location of icy spray, if this is Enceladus
Possible location of icy spray, if Enceladus is in fact this moon’s real name

Cassini was 843,762 miles (1,357,903 km) from Saturn when the images were acquired. And, if the larger moon at left is Enceladus, I’m thinking south in these images is up based on the barely-perceptible presence of a lighter area along its top edge that could be icy spray from its southern geysers. (See enlarged detail at right.)

Saturn, of course, is on the right. A small segment of the bright arc of its backlit limb is what’s running diagonally down across the image.

These images have not yet been calibrated or cataloged by NASA or the Cassini team.

See the latest raw images from Cassini on JPL’s mission page here.

*I say “dark moons” but actually Enceladus and Mimas are pretty bright, both being composed of a lot of ice. Enceladus is actually the most reflective world in the Solar System!

Posted on by Jason Major

Cassini Captures a Rarely-Seen Moon

Closest view of Saturn's moon Methone ever captured by Cassini

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While many of us here on Earth were waiting for the Moon to take a bite out of the Sun this past Sunday, Cassini was doing some moon watching of its own, 828.5 million miles away!

The image above is a color-composite raw image of Methone (pronounced meh-tho-nee), a tiny, egg-shaped moon only 2 miles (3 km) across. Discovered by Cassini in 2004, Methone’s orbit lies between Mimas and Enceladus, at a distance of 120,546 miles (194,000 km) from Saturn — that’s about half the distance between Earth and the Moon.

At an altitude of 1,200 miles (1900 km) this was Cassini’s closest pass ever of Methone, a rare visit that occurred after the spacecraft departed the much larger Tethys.

Along with sister moons Pallene and Anthe, Methone is part of a group called the Alkyonides, named after daughters of the god Alkyoneus in Greek mythology. The three moons may be leftovers from a larger swarm of bodies that entered into orbit around Saturn — or they may be pieces that broke off from either Mimas or Enceladus.

Earlier on Sunday, May 20, Cassini paid a relatively close visit to Tethys (pronounced tee-this), a 662-mile (1065-km) -wide moon made almost entirely of ice. One of the most extensively cratered worlds in the Solar System, Tethys’ surface is dominated by craters of all sizes — from the tiniest to the giant 250-mile (400-km) -wide Odysseus crater — as well as gouged by the enormous Ithaca Chasma, a series of deep valleys running nearly form pole to pole.

Saturn's icy moon Tethys with Ithaca Chasma visible, seen by Cassini on May 20, 2012.
Tethys' rugged and heavily-cratered surface near the terminator
Cassini looks down into the 62 mile (100 km) wide Ithaca Chasma

Cassini passed within 34,000 miles (54,000 km) of Tethys on May 20, before heading to Methone and then moving on to its new path toward Titan, a trajectory that will eventually take it up out of Saturn’s equatorial plane into a more inclined orbit in order to better image details of the rings and  Saturn’s poles.

Read more about this flyby on the Cassini mission site here. and see more raw images straight from the spacecraft on the CICLOPS imaging lab site here.

Image credit: NASA / JPL / Space Science Institute. (Color-composite image edited by J. Major.)