Cassini flew by Rhea yesterday in an effort to better understand the heavily cratered world with wispy terrain similar to the ice cliffs and fractures of Dione.

The image above shows the planned image coverage as Cassini passed only 500 kilometers (310 miles) above Rhea on Saturday. When Cassini was still 76,689 kilometers (47,652 miles) away from Rhea it captured the raw image on the right with Saturn’s rings in the background. The image data is still streaming back to earth and should appear on the public Cassini-Huygens mission website in the raw images section sometime today or tomorrow.

From NASA’s mission description document (PDF link):

“November of 2005 includes the final flyby of an amazing string of close icy satellite encounters. As September included the closest-ever encounters with Tethys and Hyperion, October contained the single targeted encounter at Dione . November brings the only targeted encounter at Rhea. Rhea was discovered by Jean-Domnique Cassini (after whom our orbiter is named) in 1672. . In Greek mythology, Rhea is known as the mother of the gods. She is the mother of Demeter, Hades, Hera, Hestia, Poseidon and Zeus.

“The closest approach to Rhea occurs on Saturday, November 26th, at 22:37 spacecraft time (4:49 PM Pacific Time) at an altitude of 500 km (310 miles) above the surface and at a speed of 7.3 kilometers per second (16,330 mph). Rhea has a diameter of 1528 km (949 miles), making it the largest icy satellite. Rhea is spherical in shape. The next-closest encounter with Rhea occurs in 2007 on orbit 49, at a distance of 5000 km.

“This encounter is set up with two maneuvers: an apoapsis maneuver scheduled for November 13, and an approach maneuver, scheduled for November 23. The encounter itself occurs approximately 12 hours prior to periapsis to Saturn. The cleanup maneuver for the flyby occurs just a day after the encounter.

“Occurring on orbit 18, Rhea will be the eighth close encounter with icy satellites, after Phoebe, Enceladus (orbit 3, non-targeted), Enceladus (orbit 4), Enceladus again (orbit 11), Tethys (orbit 15, non-targeted), Hyperion (also orbit 15) and Dione (orbt 16). (It could be argued that the study of Iapetus on orbit C was intense enough even at 120,000 km to merit inclusion in this list, making this encounter the ninth.)”

In one month, Cassini will return to Titan. The spacecraft’s closest approach will be 10,400 kilometers (6,500 miles).

More Information

• Cassini-Huygens Website
  • Raw Images
  • Saturn’s Moons section
  • Rhea Information
  • Mission Description (PDF link)

Ymir, Suttung, Thrym, Mundilfari, Narvi, Tarvos, Siarnaq, Erriapo, Albiorix, Skadi, Paaliaq, Ijiraq, Kiviuq, and 12 more unnamed…

In the outer reaches of the Saturnian system lie at least 26 tiny moons. 25 of these remain faint lights in the sky, 12 of which were announced in May 2005. When Cassini-Huygens entered the Saturnian system in June 2004, it passed by and photographed the other outer moon, Phoebe…

A world previously seen only as a blurry blob taken by the distant Voyager 2 in 1981 resolved in the span of just a few days into a fantastic cratered object with towering cliffs of water ice mostly covered by very dark material. Phoebe became the first object Cassini-Huygens would flyby as it entered the Saturnian system in June 2004.

Did Phoebe form with the inner moons of Saturn during the formation of the system? Probably not. Scientists now believe that Phoebe was somehow captured during an ill-fated incursion from the outer solar system’s Kuiper Belt into the Saturnian system.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Phoebe Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

The excitement of scientists upon Cassini-Huygens entering the Saturnian System was reserved mostly for Titan, Saturn itself, and its rings. That the other moons might be something more than cratered and dead ice bodies was hardly expected.

Enter Iapetus. This strange moon between Phoebe and Titan helped write the exciting new chapter of Saturn moon exploration. Upon the side of Iapetus facing toward its motion around Saturn is a coating of debris “as dark as asphalt,” according to the official NASA Cassini-Huygens page for the moon. The other half of the moon is “bright as snow.”

Perhaps even more surprising was the discovery of a mountain range that neatly straddles the equator of the moon, a ring around Iapetus’ middle that makes little sense at all. Some scientists believe that the formation of this mountain range may provide an explanation for the dark debris, although others believe the debris falls onto the surface from an outside sources.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Iapetus Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

There is a poster-sized image displayed on a board in the public-accessible lobby of the Charles P. Sonnett Space Sciences Building on the University of Arizona campus that correctly portrays Hyperion as one of the reddest objects in our solar system, along side Mars and some of the transneptunion objects. The colors of Hyperion, in both false-color (as above) and in real color, reveal differences in surface composition, a fact that has not yet been fully analyzed by planetary scientists.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Hyperion Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

Titan, the largest of the Saturnian moons, with the thick planet-like atmosphere. The moon with the Earth-like surface, of deeply cut fluid channels, broad sea-like basins, pebbled channel beds, lakes, wind-driven sediments, and occasional craters. The alien moon with water ice as rock carved by periodically flowing methane streams and rivers, with hydrocarbons snowing from its nitrogen and methane atmosphere to collect downstream in basins, with methane clouds and perhaps rain, and with brights spots and warms spots that point to heating caused by unknown mechanisms.

Titan, the latest destination in the search for extraterrestrial life and a key to our own terrestrial history of life. Where the methane at the surface and in the atmosphere must be constantly replenished from an unknown vast reservoir. The moon that has planetary scientists scratching their heads and holding hours long meetings to discuss the latest findings painstakingly gathered by Cassini as it attempts to peer through the atmospheric shroud at various wavelengths.

Titan, a distant neighbor in our solar system that has captured the public imagination unlike any other object since Mars.

Titan, where the best is yet to come…

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Titan Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

Rhea might otherwise be the most boring of the Saturnian moons, what with its ancient craters, airless surface, and lack of any recent activity. But therein lies the mystery. Why are there two distinct regions of craters, suggesting an early resurfacing event, and what are the wispy features on the surface that resemble the more prominent wispy features on Dione? The next Cassini flyby will be of Rhea, and it will occur on November 26, 2005. The distance above the surface will only be 500 kilometers (300 miles) and will likely provide new answers to old questions and new questions with no current answers.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Rhea Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

Somewhere in the raw image above, perhaps the tiny dot in the lower right, may be Polydeuces, a tiny moon discovered by Cassini-Huygens and announced on February 24, 2005 with the following from a mission news report:

“Another discovery was a tiny moon, about 5 kilometers (3 miles) across, recently named Polydeuces. Polydeuces is a companion, or “Trojan” moon of Dione. Trojan moons are found near gravitationally stable points ahead or behind a larger moon. Saturn is the only planet known to have moons with companion Trojan moons.”

There is currently no scheduled flyby of this moon but because of its position near Dione, Polydeuces will likely be captured by Cassini’s cameras at some point during the mission.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Polydeuces Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

The beautiful wispy terrain on Dione has long tantalized planetary scientists looking over low resolution images of the moon. When Cassini flew by Dione in December 16, 2004 it revealed the wisps to be bright ice cliffs created by the fracturing of the moon’s surface, a result completely unexpected by scientists. This would not be the last unexpected surprise.

On October 11, 2005, Cassini traveled even closer over Dione and sent back images of fantastic surface features that demand further scrutiny. Each raw image was more bizarre than the last: oddly shaped craters with dark material in their deepest reaches, fractures scratching the surface, and grooves that seem to ooze across the surface like toothpaste.

The questions, as usual for these Saturnian moons, come fast and furious in the face of such convoluted terrain. How did all of this happen? When did it happen and is the activity ongoing? Is Dione currently a world in turmoil or is it instead the dead remains of a world scrambled very early in its history?

Cassini will continue to seek answers with several more flybys of Dione over the next few years.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Dione Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

Little is known about the tiny moon Helene other than its size (32 kilometers, or 20 miles) and its status as a Trojan moon of Dione (along with Polydeuces). All three moons share an orbit, but Helene lies 60 degrees ahead of Dione and Polydeuces lies 60 degrees behind Dione.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Helene Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

Calypso is a Trojan moon of Tethys orbiting Saturn 60 degrees behind Tethys. Cassini snapped the best image yet of Calypso on September 23, 2005, revealing a colorful and potato-shaped object.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Calypso Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe