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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 the "Cassini at Saturn Interactive Explorer" at the JPL website!

Update February 10, 2010: The table has been overhauled with complete information on the extended-extended mission, which begins on July 1, 2010.

During its 6-Earth-year mission within the Saturn system, NASA's Cassini spacecraft will orbit the planet more than 134 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 70 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 can be no closer than 950 kilometers (590 miles) because of its extended atmosphere. There are 70 targeted flybys of Titan, eight of Enceladus, and fewer of the other moons. 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 primary mission is split into seven main phases. Within each phase, Cassini's orbit is designed for specific science goals. They are:

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 (COMPLETED)

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 (COMPLETED)

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 (COMPLETED)

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 (CURRENT PHASE)

August 31, 2007 to July 1, 2008

During this last phase of Cassini's primary mission, comprising orbits 49 through 78, the orbit's inclination will once again be increased at the same time that it is shrunk smaller and smaller in order to permit observations of the ring system in greater and greater detail. There will be nine Titan flybys, as well as the best views throughout the mission of Epimetheus and Enceladus. The primary mission ends with Cassini on course for another Titan gravity assist, which will inagurate 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 - June 30, 2010

A plan for the extended mission was adopted in early February, 2007. The plan will include 21 Titan flybys. Enceladus will be revisited seven times, two of them with 25-kilometer altitudes. There will also be 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.

Extended-extended Mission

July 1, 2010 - September 18, 2017

A plan for the extended mission was adopted in early February, 2010. The plan inlcudes more than seven years of continued science at Saturn, allowing Cassini to observe the season progress from spring to full summer in the northern hemisphere; all together, the mission will have lasted half a Saturnian year.

The extended-extended mission (described here by John Spencer) will include 56 gravity-assist flybys of Titan and 12 close encounters with Enceladus. In addition, there will be three very close flybys of Dione, two of Tethys, and one each of Methone, Tlesto, Rhea, Epimetheus, and Aegaeon. But the best is saved for last. On November 29, 2016, the spacecraft's periapsis will drop to just 10,000 kilometers beyond the F ring. There will be 20 "F ring" orbits. Then, with a final close Titan flyby on April 22, 2019, the periapsis will be shifted into the narrow zone between the innermost edge of the D ring and Saturn itself; the orbit will pass within 3,800 kilometers of Saturn's cloud tops. The last 23 orbits are called the "proximal" orbits. A final nudge from Titan on September 11, 2017 will push Cassini into an orbit that intersects Saturn. Like Galileo at Jupiter, Cassini will vaporize from frictional heating as it creates a last fireball in Saturn's atmosphere. This last act at Saturn will unfold on September 15, 2017.