Today (August 17, 2015) NASA’s Cassini spacecraft will zip past Saturn’s moon Dione, the final close flyby of this icy satellite during the spacecraft’s long mission.
Cassini’s closest approach, within 295 miles (474 kilometers) of Dione’s surface, will occur at 11:33 a.m. PDT (2:33 p.m. EDT). Mission controllers expect fresh images to begin arriving on Earth within a couple of days following the encounter.
Cassini scientists have a bevy of investigations planned for Dione. Gravity-science data from the flyby will improve scientists’ knowledge of the moon’s internal structure and allow comparisons to Saturn’s other moons. Cassini has performed this sort of gravity science investigation with only a handful of Saturn’s 62 known moons.
During the flyby, Cassini’s cameras and spectrometers will get a high-resolution peek at Dione’s north pole at a resolution of only a few feet (or meters). In addition, Cassini’s Composite Infrared Spectrometer instrument will map areas on the icy moon that have unusual thermal anomalies — those regions are especially good at trapping heat. Meanwhile, the mission’s Cosmic Dust Analyzer continues its search for dust particles emitted from Dione.
This flyby will be the fifth targeted encounter with Dione of Cassini’s tour at Saturn. Targeted encounters require maneuvers to precisely steer the spacecraft toward a desired path above a moon. The spacecraft executed a 12-second burn using its thrusters on Aug. 9, which fine-tuned the trajectory to enable the upcoming encounter.
Cassini’s closest-ever flyby of Dione was in December 2011, at a distance of 60 miles (100 kilometers). Those previous close Cassini flybys yielded high-resolution views of the bright, wispy terrain on Dione first seen during the Voyager mission. Cassini’s sharp views revealed the bright features to be a system of braided canyons with bright walls. Scientists also have been eager to find out if Dione has geologic activity, like Saturn’s geyser-spouting moon Enceladus, but at a much lower level.
Bonnie Buratti is a Cassini science team member at NASA’s Jet Propulsion Laboratory in Pasadena, California. She said:
Dione has been an enigma, giving hints of active geologic processes, including a transient atmosphere and evidence of ice volcanoes. But we’ve never found the smoking gun. The fifth flyby of Dione will be our last chance.
Cassini has been orbiting Saturn since 2004. After a series of close moon flybys in late 2015, the spacecraft will depart Saturn’s equatorial plane – where moon flybys occur most frequently – to begin a year-long setup of the mission’s daring final year. For its grand finale, Cassini will repeatedly dive through the space between Saturn and its rings.
Scott Edgington is Cassini mission deputy project scientist at JPL. He said”
This will be our last chance to see Dione up close for many years to come. Cassini has provided insights into this icy moon’s mysteries, along with a rich data set and a host of new questions for scientists to ponder.
Bottom line: NASA’s Cassin spacecraft makes its final close flyby of Saturn’s moon Dione on Monday, August 17, 2015.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.