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)

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

The Cassini spacecraft orbiting Saturn has returned first images of many of the planet’s moons. This first orbit will be Cassini’s longest, as Saturn and its moons are used to shrink the orbit down to something more science activity-friendly. In 89 days the spacecraft will make a close flyby of Titan.

On its way out, Cassini took the closest images of Titan ever captured. While visual light images depicted the same smog-covered moon, other frequencies of light were used to take images of the surface. Mysteries abound. The predicted lakes or seas of methane were not seen. Instead, the images revealed a diverse surface possibly modified by different geological processes as well as bright methane clouds hovering near the south pole. When Cassini comes around for another pass it will be much closer, allowing much clearer images.

The images of the other moons reveal little more than Voyager images from the 1980s. Future orbits of Cassini will target specific moons, giving each its own moment in the spotlight.