Sen— The Cassini spacecraft made its last close flyby of Saturn's moon Rhea on March 9.
Rhea is the second largest moon of Saturn, with a diameter of 1,528 kilometers (949 miles). It is tidally locked in phase with Saturn so one side always faces toward the planet. It has a density 1.233 times that of liquid water suggesting that Rhea is three quarters ice and one quarter rock.
The primary purpose of the flyby was to probe the internal structure of the moon to understand whether the moon is homogeneous all the way through or whether it has differentiated into the layers of core, mantle and crust. To do this the Cassini probe flew by Rhea at an altitude of 620 miles (997 kilometres) while the gravitational pull of Rhea was measured against the spacecraft's steady radio link to NASA's Deep Space Network on Earth.
Data from Cassini's cosmic dust analyzer was also collected to try to detect any dusty debris flying off the surface from tiny meteoroid bombardments. This data will help scientists understand the rate at which "foreign" objects are raining into the Saturn system.
During closest approach the imaging team rode along and captured 12 images of Rhea's rough and icy surface. These raw, unprocessed images of the battered icy moon show an ancient, cratered surface bearing the scars of collisions with many space rocks.
The camera was pointing toward Rhea at approximately 3778 kilometers away. Credit: NASA/JPL-Caltech/SSI
Outbound from Rhea, Cassini's cameras captured a set of global images from a distance of about 167,000 miles (269,000 kilometers).
The camera was pointing toward Rhea at approximately 62880 kilometers away. Credit: NASA/JPL-Caltech/SSI
Cassini launched in October 1997, arriving at Saturn in 2004. It is now on its second extension, called the Cassini Solstice Mission, which goes through September 2017. This was Cassini's fourth close encounter with Rhea. The spacecraft will pass the moon again on February 10, 2015, at about 29,000 miles (47,000 kilometers), but this is not a targeted flyby.
Cassini Imaging Team Leader Carolyn Porco posted a message saying "And now begins the end...Take a good, luxurious look at these sights from another world, as they will be the last close-ups you'll ever see of this particular moon. Our mission at Saturn has been ongoing for nearly 9 years, and is slated to continue for another 4. Targeted flybys of Dione in June and August of 2015, and of Enceladus in October and December of 2015, are all that remains on the docket for detailed exploration of Saturn's medium-sized moons. We're nearing the end, so let's enjoy it while we can!"