So far, 62 moons have been discovered orbiting around Saturn, though only 53 have officially been named. Discovered in 1684, Dione is a small moon 1,123 kilometers (698 miles) in diameter, and it orbits Saturn every 2.7 days at a distance of 377,400 kilometers (234,000 miles) – about the same distance that our moon orbits around Earth.
“Dione has been an enigma, giving hints of active geologic processes, including a transient atmosphere and evidence of ice volcanoes,” Bonnie Buratti of NASA's Jet Propulsion Laboratory said in a statement last week. “But we've never found the smoking gun. The fifth flyby of Dione will be our last chance.” Dione isn’t as active as the geyser-spouting Enceladus, though we have seen that its fractured surface is covered by bright linear chasms that contrast dramatically with its more typical round impact craters (pictured above).
This flyby’s closest approach – within 474 kilometers (295 miles) of Dione's surface – occurred at 2:33 p.m. Eastern Time on Monday, August 17. This was the fifth targeted encounter with Dione, and in order to fine-tune its trajectory, Cassini had executed a 12-second burn using its thrusters on August 9. A lot of upcoming analyses are planned for Dione. Cassini’s cameras and spectrometers have taken a high-resolution look at the moon’s north pole, which hasn’t been observed closely. Gravity-science data will shed light on the moon’s internal structure and the rigidity of the outer shell. And a variety of other instruments are mapping areas that trap heat and searching for dust particles that are emitted. Some raw images have already been received, and you can browse them here.
A view of Saturn's moon Dione captured by NASA's Cassini spacecraft during a close flyby on June 16, 2015. The diagonal line near the upper left is the rings of Saturn in the distance. NASA/JPL-Caltech/Space Science Institute.
Cassini’s closest-ever flyby of Dione took place during December 2011 at a distance of 100 kilometers (60 miles). That and other previous close flybys have yielded sharp views of Dione’s terrain, which was first observed during the Voyager mission. The thin wispy lines captured in (poor resolution) Voyager images from decades ago turned out to be a system of braided canyons with bright walls.
This flyby is our last chance to see Dione up close for many years. Cassini has been orbiting Saturn since 2004, and after a series of close moon flybys scheduled for the remainder of this year, the spacecraft will leave Saturn's equatorial plane to start setting up for the mission's final year: In 2017, Cassini will repeatedly dive through the space between Saturn and its rings.
Next up on the Saturn Tour, a Titan flyby is scheduled for September 28.