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Space Topics: Cassini-Huygens

Cassini’s Tour of the Saturn System

Click to enlarge > The view from Iapetus From a distance of roughly three million kilometers (two million miles), Cassini contemplated almost the entire system of Saturn, its rings, and its moons. Credit: NASA / JPL / SSI

To visualize Cassini's tour, be sure to check out Eyes on the Solar System at the JPL website!

Update October 25, 2010: The table has been totally replaced with a more recent version of the mission's trajectory information (dated December 7, 2009), which shifted encounter times by small amounts (fewer than five minutes in most cases). As a result of these timing shifts, flyby distances also changed; changes are more significant to nontargeted flyby distances than targeted ones. Also, a choice has been made to include nontargeted flybys of the "rocks" (the moons smaller than Hyperion); however, Cassini often does not take data on these smaller moons even when the opportunities are there.

During more than 13 years within the Saturn system, NASA's Cassini spacecraft will orbit the planet more than 250 times. However, Cassini does not keep to one placid circular path around Saturn. In order to view Saturn, its rings, its moons, and its magnetosphere from every possible angle, Cassini will use its rockets and a total of 127 gravity-assist flybys of Saturn's largest moon, Titan, to tweak the orbit's size, period, speed, inclination, and distance from Saturn.

Each of Saturn's moons orbits the planet at a different speed and distance, so in order to place Cassini at the right place at the right time to observe a moon, Cassini's navigators have to perform rocket firings to fine-tune the spacecraft's orbit. Any moon encounter that is set up with such a rocket firing is called a targeted flyby. Targeted flybys usually have close-approach distances within 2,000 kilometers (1,200 miles) and can be closer than 25 kilometers (15 miles) to the icy moons, but targeted flybys of Titan are (with only one exception) no closer than 950 kilometers (590 miles) because of its extended atmosphere. In addition to these targeted flybys, Cassini also takes advantage of lucky alignments, when Cassini's path happens to take it relatively close to a moon, to perform nontargeted flybys. Nontargeted flybys are typically much more distant than targeted flybys.

The mission can be split into phases based upon the inclination of Cassini's orbit. Low-inclination orbits, near the ring plane, permit Cassini to have close encounters with icy moons. High-inclination orbits result in fewer moon flybys (except for Titan, which Cassini always returns to eventually), but allow views onto the rings and atmosphere and permit the in-situ instruments to explore all parts of the magnetic field and energetic particle environment.

Click to enlarge > Cassini's four-year tour of the Saturn system The left panel looks down upon the Saturn system from above Saturn's north pole, perpendicular to the rings. The Sun shines toward saturn from the right (the positive X direction). The right panel looks across at the Saturn system in a view parallel to the rings, again with the Sun toward the right. The units along the X, Y, and Z axes are "Rs" or "Saturn Radii," that is, half the width of Saturn, or 60,330 kilometers. The dotted white circles in the left view show the orbits of Iapetus (bigger circle) and Titan (smaller circle). The colored ovals show the circuitous path of Cassini through the Saturn system. Different colors represent different phases of the mission. The small yellow circle at the very center shows the position of Saturn's rings. Credit: NASA / JPL

Approach

late 2002 to June 2004

Cassini captured its first image of Saturn on October 21, 2002. During the approach, navigators used the cameras to refine the orbits of the moons and scientists performed tests with their instruments from afar to determine what instrument settings would be appropriate for each planet, ring, and moon target.

Orbit Insertion and Huygens Mission

June 2004 to February 15, 2005

On its way in to Saturn orbit, Cassini flew by the outermost large moon, Phoebe, and then completed a daring pass through and over the ring system during its orbit insertion on June 30. The next three orbits, numbered A through C, included the first three flybys of Titan as well as good views of Mimas, Enceladus, Rhea, Tethys, Dione, and Iapetus. The phase culminated in the successful Huygens descent to the surface of Titan on January 14.

Click to enlarge > The solid white curve shows the circuitous path of Cassini through the Saturn system. Cassini approached Saturn from below and ahead of the planet (on its dawn side). Credit: NASA / JPL

Ring Occultation Sequences

February 15 to September 7, 2005

During this phase, including orbits 3 to 14, Cassini probed the fine structure of the rings. High-resolution images, movies, and radio and stellar occultations of the rings and ring-moons showed complex ringlets and wave structures. There were four Titan flybys, and three close passes by Enceladus that yielded the discovery of active vents at its south pole. Pandora, Epimetheus, Mimas, Tethys, Dione, Rhea, and Hyperion received more distant scrutiny.

Click to enlarge > The orange curves show Cassini's path. Cassini began in an equatorial orbit, but dipped to an inclined orbit, which it then repeated seven times. Credit: NASA / JPL

Icy Satellites and Magnetotail

September 7, 2005, to July 22, 2006

Orbits 14 through 26 lie in the plane of the rings and moons. Cassini got the best views during the mission of many of the icy satellites, including Calypso, Tethys, Telesto, Dione, Rhea, and Hyperion, and more distant views of Janus, Mimas, Enceladus, and Helene. There were a total of nine Titan flybys. The edge-on view also permitted excellent insight into the dynamics of Saturn's atmosphere.

Click to enlarge > The green ovals show Saturn's looping path. Over this period Cassini remained in an equatorial orbit, but the orbit was rotated around eventually to reach behind Saturn. Credit: NASA / JPL

180-Degree Transfer; Rings and MAPS

July 22, 2006, to June 29, 2007

This phase comprises orbits 26 through 47. The orbits will become smaller and faster and tilt upward, permitting extensive surveying of Saturn's magnetosphere and atmosphere, and giving the first-ever polar views of Saturn and its rings. There will be 17 Titan flybys.

Click to enlarge > The solid blue curves show the path of Cassini. During this period the orbit begins in Saturn's ring plane and becomes smaller and more inclined with time until Cassini meets Titan on the opposite side of Saturn (the "180 degree transfer") and the orbit is returned to an equatorial one. Credit: NASA / JPL

Iapetus Flyby and Icy Satellites

June 30 to September 14, 2007

This brief mission phase, orbits 47 through 49, included one long orbit that brought Cassini out to Iapetus. There were two flybys of Titan, one of Rhea, and the best view of Helene throughout the primary mission.

Click to enlarge > The solid yellow curve shows Cassini's path. A special effort was required to kick Cassini out to an encounter with distant Iapetus. Credit: NASA / JPL

Rings and MAPS

September 14, 2007 to July 1, 2008

During this last phase of Cassini's primary mission, comprising orbits 49 through 78, the orbit's inclination was once again increased at the same time that it iswasshrunk smaller and smaller in order to permit observations of the ring system in greater and greater detail. There were be nine Titan flybys, as well as the best views throughout the primary mission of Epimetheus and Enceladus. The primary mission ended with Cassini on course for another Titan gravity assist, which inagurated the extended mission.

Click to enlarge > The solid red curves shows the circuitous path of Cassini through the Saturn system. Cassini's final orbits will become smaller and smaller and will reach a nearly polar inclination. Credit: NASA / JPL

Extended Mission

July 1, 2008 - October 11, 2010

A plan for the extended mission was adopted in early February, 2007. The initial plans for the extended mission covered two years (through July 1, 2010), as described in the Outer Planets Assessment Group presentation by Linda Spilker, but later documents place the end date in October 2010. Measuring through October 2010, the extended mission, also known as the Equinox Mission because Saturn passed through its equinox on August 11, 2009, included 27 Titan flybys. Enceladus was revisited seven times, two of them with 25-kilometer altitudes. There were also reasonably close flybys of Mimas (6 of them), Tethys (8), Dione (3, one of them from 500 kilometers altitude), Rhea (5, one of them from 100 kilometers), as well as many flybys of small moons, including a 1,500-kilometer-altitude flyby of Helene. But the main event was the equinox. As the Sun passed through the plane of Saturn's rings, moons threw shadows onto Saturn and the rings, and vertical structures in the rings threw shadows of their own, illuminating the subtle structures within them. The extended mission also added significantly to the RADAR imaging coverage of Titan (22% was covered in the prime mission, another 8% in the extended mission).

The extended mission can be split into four main phases based on the inclination of the orbits and the science goals for each phase: Inclined (July 1, 2008 to April 4, 2009), Equinox (April 4 to October 12, 2009), Equatorial (October 12, 2009 to May 20, 2010), and Northern Titan (May 20 to October 11, 2010). These splits were made here on the basis of Linda Spiker's presentation and are not formal terms used by the mission -- they serve just to break up the table into more manageable pieces.

XM: Inclined Phase

July 1, 2008 to April 4, 2009

Cassini ended its Prime Mission on a highly inclined orbit. Titan encounters occurred on the outbound leg of Cassini's orbit. Toward the end of this first phase of the extended mission, Cassini positioned itself for a "180 degree transfer," in which it circularized its orbit, met Titan on one side of Saturn and then again just half a Titan orbit later, and then lengthened its elliptical orbit again, switching its apoapsis to the other side of Saturn.

XM: Equinox

April 4 to October 12, 2009

With the "pi transfer" complete, Cassini orbited with apoapses high above Saturn's northern latitudes, which saw the Sun slowly creeping higher into the sky. On August 11, 2009, the Sun rose on the northern face of the rings for the first time in 14 years. Cassini kept constant watch on the rings, taking advantage of the low-angle sunlight to observe topography in the rings from the long shadows they threw. With a year lasting 30 Earth years, the equinox was a sedate affair, and Cassini was able to observe equinox-related phenomena for months on either side of August 11.

XM: Equatorial

October 12, 2009 to May 20, 2010

With orbital inclinations approaching zero, Cassini focused on Saturn's icy moons, with four close passes by Enceladus (in November 2009 and April and May 2010), and one each of Rhea (March 2010) and Dione (April 2010).