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Planetary News: Cassini-Huygens (2005)Cassini Completes Initial Reconnaissance of Saturn's Icy MoonsBy Emily LakdawallaDecember 7, 2005 Cassini has just wrapped up a season of daring close approaches to Saturn's icy satellites: Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion. The photos from these flybys have revealed amazing detail in the structures of craters, grooves, and chasms crossing the frigid surfaces of these little worlds.
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Highest-resolution global Mimas mosaic Because the Mimas flyby was a relatively distant one, this image represents the highest-resolution view available of Mimas' surface. Credit: NASA / JPL / Space Science Institute |
Enceladus
Flyby date: July 14, 2005
Flyby altitude: 172 kilometers (107 miles)
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Highest-resolution global Enceladus mosaic Credit: NASA / JPL / Space Science Institute |
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Closest-ever view of Enceladus Credit: NASA / JPL / Space Science Institute |
Tethys
Flyby date: September 24, 2005
Flyby altitude: 1,500 kilometers (932 miles)
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Highest-resolution global mosaic of Tethys Credit: NASA / JPL / Space Science Institute |
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Closest-ever view of Tethys Credit: NASA / JPL / Space Science Institute |
Dione
Flyby date: October 11, 2005
Flyby altitude: 500 kilometers (311 miles)
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Highest-resolution global mosaic of Dione Credit: NASA / JPL / Space Science Institute |
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Close-up view of Dione Credit: NASA / JPL / Space Science Institute |
Rhea
Flyby date: November 26, 2005
Flyby altitude: 500 kilometers (311 miles)
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Highest-resolution global mosaic of Rhea Credit: NASA / JPL / Space Science Institute |
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Closest-ever view of Rhea Credit: NASA / JPL / Space Science Institute |
Hyperion
Flyby date: September 26, 2005
Flyby altitude: 514 kilometers (319 miles)
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Highest-resolution global mosaic of Hyperion Credit: NASA / JPL / Space Science Institute |
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Closest-ever view of Hyperion Credit: NASA / JPL / Space Science Institute |
Iapetus
Flyby date: December 31, 2004
Flyby altitude: 123,402 kilometers (76,678 miles)
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Saturn's Moon Iapetus This flyby was a very distant one. Fortunately, a close Iaptus flyby is planned for September 10, 2007. Credit: NASA / JPL / Space Science Institute |
On to the Magnetotail
Cassini is now shifting priorities from the icy satellites to Saturn's magnetotail. "For the time period we just ended, we specifically targeted Cassini for icy satellites," says David Seal, a Cassini mission planner. "Any time you do that, it takes delta V" -- in other words, Cassini must spend precious quantities of its limited amount of remaining fuel in order to achieve the close flyby geometry. At the same time, Cassini's orbit also had to lie very nearly in the same plane as the rings and moons' orbits, because that geometry increased the chances that Cassini's path would be close enough to the path of a moon to allow the planning of a close encounter.
Cassini is moving on to the exploration of the magnetosheath and magnetotail. Saturn's magnetic field is blown out behind the planet by the solar wind -- charged particles streaming off the Sun. To explore this magnetotail region, Cassini must change the geometry of its orbit, which presently lies in the plane of Saturn's rings and offers views mostly of the side, not the back, of the Saturn system. "The orbit geometry we are going for gives us passage through the magnetotail region and the current sheet," Seal explained. "MAPS [the magnetometer and plasma science instrument suite] has a requirement for us to be farther away than 40 Saturn radii" at the anti-Sun longitude. A distance of 40 Saturn radii -- roughly 2.5 million kilometers (1.5 million miles) -- is far enough from Saturn that Cassini can't achieve that distance just by enlarging its orbit. The distance and position requirement mean that Cassini's whole orbit must be rotated so that the orbit's apoapsis lies behind Saturn.
Saturn's magnetosphere
This diagram shows the complex structure of Saturn's magnetosphere. Throughout the four-year mission, Cassini will fly through all regions of it, mapping its strength and orientation, and how Titan interacts with it. Credit: Imperial College Cassini Magnetometer group |
So Cassini will expend precious fuel and seven gravity-assist flybys of Titan to rotate its elliptical orbit clockwise around Saturn until its apoapsis lies behind Saturn. But, Seal explained, there is one more twist required to set up the orbit. "The Sun isn't in the plane of Saturn's rings right now," while Cassini's orbit currently is. "The orbit has to have some inclination to it for the apoapsis to get into the magnetotail." An eighth gravity-assist flyby of Titan will tip Cassini's orbit inclination by 15 degrees, finally setting up the proper geometry for the magnetotail studies in August 2006.
This long journey to Saturn's magnetotail means that over the next year, Cassini will be spending more and more time on the night side of Saturn. Nighttime is not the ideal time to capture images in visible light wavelengths. But instruments that do not depend on sunlight are not bothered by the shift in geometry. In particular, Cassini's in-situ instruments must fly to as many different geometries as possible within the Saturn system in order to develop a three-dimensional map of the magnetic fields, plasma, and neutral particles in which the Saturn system is embedded. At the same time, while the season of close flybys of Saturn's icy satellites is over, encounters with Titan will continue. Cassini's next Titan flyby will occur in two weeks, on December 26, 2005.
Cassini's tour: Icy Satellites and Magnetotail, September 7, 2005, to July 22, 2006
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 green ovals show Cassini's looping path, which eventually takes the orbit apoapsis around to Saturn's night side, opposite the Sun. Credit: NASA / JPL |
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