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.
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.
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.
NASA / JPL-Caltech
Cassini’s tour: Orbit insertion and Huygens mission, May 14, 2004 to February 15,2005
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).
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.
NASA / JPL-Caltech
Cassini’s tour: Ring occultation sequences, February 15 to September 7, 2005
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.
Fri, Jan 14, 2005 2005-014T17:05
Ascending ring plane crossing r = 18.360 Saturn radii (1107646 km)
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.
NASA / JPL-Caltech
Cassini's tour: Icy Satellites and Magnetotail, September 7, 2005, to July 22, 2006
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.
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.
NASA / JPL-Caltech
Cassini's tour: 180-degree transfer; rings and MAPS; July 22, 2006, to June 30, 2007
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.
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.
NASA / JPL-Caltech
Cassini's tour: Iapetus flyby and icy satellites, June 30 to September 14, 2007
The solid yellow curve shows Cassini's path. A special effort was required to kick Cassini out to an encounter with distant Iapetus.
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 was shrunk 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 inaugurated the extended mission.
NASA / JPL-Caltech
Cassini's tour: Rings and MAPS, August 31, 2007 to end of Nominal Mission, July 1, 2008
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.
Fri, Sep 14, 2007 2007-257T11:38
Apoapsis Rev 50
period = 32.0 days, inclination = 6.1°, r = 59.736 Saturn radii, phase = 42°
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.
Wed, Jul 2, 2008 2008-184T22:44
Ascending ring plane crossing
r = 19.728 Saturn radii (1190164 km)
Thu, Jul 3, 2008 2008-185T21:33
Apoapsis Rev 75
period = 7.0 days, inclination = 74.8°, r = 20.783 Saturn radii, phase = 22°
period = 16.5 days, inclination = 65.2°, r = 19.723 Saturn radii, phase = 29°
Fri, Mar 27, 2009 2009-086T05:10
Ascending ring plane crossing
r = 19.722 Saturn radii (1189837 km)
Sat, Mar 28, 2009 2009-087T05:27
Periapsis Rev 107
R = 19.668 Saturn radii, lat = 21°, phase = 28°
Sat, Apr 4, 2009 2009-094T01:30
Apoapsis Rev 108
period = 16.8 days, inclination = 63.3°, r = 20.921 Saturn radii, phase = 151°
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.
period = 23.8 days, inclination = 12.1°, r = 48.713 Saturn radii, phase = 103°
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).
period = 23.8 days, inclination = 3.0°, r = 48.780 Saturn radii, phase = 101°
October 11, 2010 - September 18, 2017
A plan for the extended-extended mission was adopted in early February, 2010. The plan includes 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, Telesto, 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, 2017, 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.
The extended-extended mission can be split into five main phases phases based on the orbital inclination and the science goals for each phase: Equatorial phase 1, from October 11, 2010 to May 22, 2012; Inclined phase 1, from May 22, 2012, to March 16, 2015; Equatorial Phase 2, from March 16, 2015, to January 16, 2016; Inclined Phase 2, from January 16 to November 29, 2016, and the Proximal Orbits, from November 29, 2016 to Cassini's planned death on September 15, 2017. (These date splits are based on slide 6 from a presentation made by Bob Pappalardo to the Outer Planets Assessment Group in March 2009 and don't represent formal terminology used by the mission; rather, they serve to break this table into more manageable chunks.) The equatorial phases of the mission permit close encounters with icy moons, while the inclined phases are more advantageous for studying Saturn's rings, magnetic field, and plasma environment, and the atmospheres of Saturn and Titan.
XXM: Equatorial Phase 1 (CURRENT PHASE)
October 11, 2010, to May 22, 2012
With orbits within Saturn's ring plane, Cassini will enjoy the opportunity to witness mutual events of moons and plans no fewer than seven targeted flybys of Enceladus (November and December 2010, October and November 2011, March, April, and May 2012), plus one each of Rhea (January 2011) and Dione (December 2011).
This longest phase of the XXM covers nearly three years, with much of that time spent in orbits inclined more than 40 degrees. At first, apoapsis is high over Saturn's southern (winter) latitudes and the unlit side of the rings, but on December 1, 2013, a targeted flyby of Titan shifts the apoapsis to be over the northern (summer) latitudes and the lit side of the rings, where it will remain until the March 16, 2015 flyby. Only one targeted encounter, with Rhea in March 2013, is planned for these three years, but there are 27 Titan flybys.
Low-inclination orbits bring the opportunity for close flybys of the icy moons, including five flybys of Titan, two of Dione (in June and August 2015), and three of Enceladus (in October and December 2015).
This second set of inclined orbits will include 9 flybys of Titan as well as opportunities to probe Titan's poles and Saturn's atmosphere with occultations, watching the weather as the planet and its moons approach summer solstice. The apoapses will be over Saturn's northern (summer) latitudes and the lit face of the rings.
This final, amazing phase of the Cassini mission will consist of 20 "F-ring" orbits and 23 "proximal" orbits. The penultimate Titan flyby on November 29, 2016 will drop Cassini's closest approach to a point just 10,000 kilometers beyond the F ring. Then, with the last close flyby of Titan on April 22, 2017, Cassini's periapsis will jump, in a single leap, from just outside the main ring system into the narrow zone of safety between the inner edge of the innermost D ring and the planet itself, just 3,800 kilometers above the cloud tops. During these proximal orbits, on May 24, 2017, Saturn and its moons will pass through northern summer solstice. A final nudge by a distant Titan on September 11, 2017 will deliver the death blow to Cassini, putting its periapsis below Saturn's clouds. Cassini will not survive that plunge, and the mission will come to an end on September 15.
Courtesy John Spencer
Cassini's end-of-mission "proximal" orbits
A schematic illustration of Cassini's final Saturn orbits in 2016 and 2017, according to the current XXM plan. The view is from directly above Saturn's north pole, with the main ring system shown in gray, and Cassini's path shown in black. The cluster of orbits crossing the lower part of the figure are the "F-ring" orbits which Cassini will follow from November 2016 to April 2017, and the upper cluster of orbits, passing between the rings and the planet, are the "proximal" orbits that will be followed from April 2017 until Saturn impact in September 2017.
End of mission, atmospheric entry into Saturn (1.0 Saturn radii)
Fri, Sep 15, 2017 2017-258T17:05
Descending ring plane crossing
r = 0.988 Saturn radii ( 59589 km)
Fri, Sep 15, 2017 2017-258T17:10
Periapsis Rev 293
R = 0.983 Saturn radii, lat = -7°, phase = 35°
Thanks to Cassini Project Scientist Linda Spilker, Deputy Project Scientist Amanda Hendrix, and Science Planners David Seal, Emily Manor-Chapman, Trina Ray, and Kevin Grazier of NASA's Jet Propulsion Laboratory for providing valuable information for this record of Cassini's tour.
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