Flybys are a major element of Cassini's tour. The spacecraft's looping, elliptical path around Saturn is carefully designed to enable occasional visits to the many moons in the system. All flybys provide an opportunity to learn more about Saturn's icy satellites, and encounters with giant Titan are actually used to navigate the spacecraft, changing its orbit or setting up future flybys.
Many of the most exciting encounters are "targeted" flybys, for which Cassini's flight path is steered so the spacecraft will pass by a specific moon at a predetermined distance, referred to as "closest approach." Cassini's targeted flybys have yielded incredible close-up views and many groundbreaking science results. Visits to Dione and Hyperion, for example, as well as the daring Oct. 2008 dives through the Enceladus plume, have provided some of the great highlights of the mission.
Some flybys are more distant, "non-targeted" encounters. Non-targeted flybys occur when a moon happens to be close enough to Cassini's path that it can be easily observed. Typical distances are large, anywhere from several thousand to a million kilometers. Non-targeted flybys often yield exciting science results, including spectacular images of moons like Telesto.
In addition to occasional burns from Cassini's main rocket engine, the navigation team uses flybys of Saturn's largest moon, Titan, to direct the spacecraft's path. These flybys also provide excellent opportunities for scientists to study the smog-enshrouded moon and learn more about processes affecting its surface.
During a flyby Cassini approaches the target moon at great speed. The spacecraft keeps its instruments precisely pointed toward a target using either its reaction wheels or thrusters, which spin the spacecraft in order to track the moon as it passes by. Thrusters are also used to keep Cassini from tumbling when it experiences drag while passing through Titan's upper atmosphere during close flybys and during the deepest plunges through the icy plume of Enceladus.
Flybys are a critical part of the Cassini mission, for both science and navigation purposes. From more than a billion kilometers (nearly a billion miles) away on Earth, the Cassini team is able to send the spacecraft hurtling past Saturn's moons at closest approach distances as small as 16 miles (25 kilometers)!