Written by Jia-Rui C. Cook
NASA’s Jet Propulsion Laboratory
Pasadena, CA – It’s been nearly two years since NASA’s Cassini spacecraft has had views like these of Saturn’s glorious rings. These views are possible again because Cassini has changed the angle at which it orbits Saturn and regularly passes above and below Saturn’s equatorial plane.
Steeply inclined orbits around the Saturn system also allow scientists to get better views of the poles and atmosphere of Saturn and its moons.Cassini’s recent return of ring images has started to pay off. A group of scientists has restarted the team’s studies of propeller-shaped gaps. These gaps are cleared out by objects that are smaller than known moons but larger than typical ring particles.
Cassini scientists haven’t seen propellers in two years. Matt Tiscareno, a Cassini imaging team associate at Cornell University, Ithaca, NY, and colleagues have been following these objects for several years. Because some of the propellers are exactly where models predicted they would be, scientists believe they are seeing some old friends again.
Scientists are eagerly waiting for the other data that will come from this change in perspective. What’s the secret to getting Cassini to orbit at such high angles?
Cassini’s lead navigator, Duane Roth, explains:
Cassini Has a Special View of Saturn These Days – How Did It Get There?
For the past 18 months, NASA’s Cassini spacecraft has been orbiting Saturn in practically the same plane as the one that slices through the planet’s equator. Beginning with the Titan flyby on May 22nd, navigators started to tilt Cassini’s orbit in order to obtain a different view of the Saturnian system.
The measure of the spacecraft orbit’s tilt relative to Saturn’s equator is referred to as its inclination. The recent Titan flyby raised Cassini’s inclination to nearly 16 degrees. Seven more Titan flybys will ultimately raise Cassini’s inclination to nearly 62 degrees by April 2013. On Earth, an orbit with a 62-degree inclination would pass as far north as Alaska and, at its southernmost point, skirt the latitude containing the tip of the Antarctic Peninsula.
You may wonder why this change has been planned and how this feat is achieved. The “why” is to allow scientists to observe Saturn and the rings from different geometries in order to obtain a more comprehensive three-dimensional understanding of the Saturnian system.
For instance, because Saturn’s rings lie within Saturn’s equatorial plane, they appear as a thin line when viewed by Cassini in a near-zero-degree orbit inclination. From higher inclinations, however, Cassini can view the broad expanse of the rings, making out details within individual ringlets and helping to unlock the secrets of ring origin and formation. Some of those images have already started to come in.
The “how” is by using the gravity of Titan — Saturn’s largest moon by far — to change the spacecraft’s trajectory. Like the rings and Cassini’s previous orbit, Titan revolves around Saturn within a plane very close to Saturn’s equatorial plane.
As Cassini flies past Titan, Titan’s gravity bends the spacecraft’s path by pulling it towards the moon’s center — similar to a ball bearing rolling on a smooth horizontal surface past a magnet. Near Titan, the motion is confined to a plane containing the spacecraft’s path and Titan’s center of mass. If this “local” plane coincides with Cassini’s orbital plane about Saturn, the trajectory’s inclination will remain unchanged.
However, if this plane differs from Cassini’s orbital plane about Saturn, then the bending from Titan’s gravity will have a component out of Cassini’s orbital plane with Saturn, and this will change the tilt of the spacecraft’s orbit. Repeated Titan flybys will raise Cassini’s orbit inclination to nearly 62 degrees by April of next year and then lower it back to the Saturn equatorial plane in March 2015.