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Cassini's Tour de Saturn

by Bruce Moomaw
Cameron Park - May 25, 2001
Phase III of Cassini's orbital tour begins on July 22, 2006.

While the apoapsis of its elongated orbit was over the middle of Saturn's nightside during Phase II, the goal in Phase III is to swing it around fully 150 degrees so that the apoapsis is over Saturn's dayside, allowing really long-term sweeping photographic observations of Saturn's cloud patterns.

This was the scientific goal of the Galileo spacecraft at Jupiter that was most seriously hurt by the failure of its high-gain antenna to deploy. Since Galileo's weight limitations forced it to carry only one camera - a narrow-angle, telephoto one - it was actually supposed to devote most of its photography to providing maps of Jupiter's cloud patterns all over its dayside by stitching together full-disk mosaic view of the planet out of multitudes of narrow-angle photos.

This naturally had to be abandoned in the revised mission, in which the data return rate was only a tiny trickle of what had been originally planned and Galileo's images had to be rationed and devoted to more important scientific goals.

Cassini should do vastly better at Saturn.

Not only will it be capable of communicating at a full 100,000 bits per second, but it carries two cameras - including a wide-angle one - so that it can easily provide both sweeping views of Saturn's overall cloud patterns and zoom in on detailed studies of individual features.

Galileo carried only a short-wavelength near-IR spectrometer, excellent for mapping the solid surface materials of Jupiter's moons, but somewhat limited in the amount of data it could provide on Jupiter's atmosphere - although it did still provide much useful data, including water-vapor maps which confirmed that Galileo's entry probe (by sheer chance) had parachuted down into a highly atypical "warm spot" on Jupiter where a temporary strong updraft of warm air from its interior had erased most of the local clouds and drastically reduced the local humidity.

The Voyagers, by contrast, during their flybys carried only a longer-wavelength IR spectrometer which optimized measurements of atmospheric gases and air temperatures at a wide variety of greater depths in the atmospheres of the giant planets during their brief flybys.

Cassini carries both kinds of IR spectrometers, and they're considerably improved models.

During its earlier orbits of Saturn, Cassini will of course already have done a great deal of observation of Saturn's weather patterns.

But during Phase I - when it will have been looking at Saturn from the side - it will only be able to photograph cloud patterns on about half of Saturn's dayside, and thus will be able to monitor changes in individual cloud features over time for only one-quarter of a Saturnian day (about 2 2/3 hours) before the planet's rotation swings them around out of view.

During Phase II, Cassini will spend most of its time looking at Saturn's nightside, which still allows a great deal of useful IR mapping of Saturn's nightside weather but rules out photographic observations (except for lightning and auroras).

So, despite its improved observational abilities, scientists want to provide Cassini with one period during which it can monitor cloud features all over Saturn's disk and observe changes in any individual cloud feature over a period of closer to half a Saturnian day.

Thus the need to swing its apoapsis around 150 degrees, so that Cassini is in an elongated orbit that extends over Saturn's dayside.

There are two possible ways to do this.

The obvious one is to simply continue doing what Cassini did to swing its apoapsis around to Saturn's nightside during Phase II (and Galileo did throughout its orbital tour of Jupiter): make repeated flybys of Titan to gradually twist the long axis of Cassini's orbit around Saturn's equator like the hand on a clock - this time twisting it fully 180 degrees around the planet from the nightside to the dayside.

But there is an alternative technique, more complex and less obvious, which Cassini's planners finally decided would serve it better in this particular orbital tour: the "180-degree transfer".

In this technique, Cassini will make repeated flybys of Titan's north polar region to do something that neither it nor Galileo did before: sharply tilt the inclination of its orbit.

During the first two tour phases, Cassini will never be in an orbit tilted more than 22 degrees to Saturn's equator (just as Galileo, during all its lengthy mission around Jupiter, has always been in orbits close to Jupiter's equatorial plane to maximize the study of Jupiter's moons, and so has not been able to get a really good look at Jupiter's polar regions).

Indeed, during Phase II, Cassini's orbit will be completely in the equatorial plane, giving it no good views of the rings since they will be edge-on to it.

But during the first nine Titan flybys of Phase III, Cassini will gradually tilt its orbital inclination up to an angle of fully 60 degrees to Saturn's equator, giving it both an excellent view both of Saturn's polar regions and a splendid sweeping view of the rings' structure.

The first two of these flybys will also trim down its apoapsis so that Cassini, like Titan itself, orbits the planet every 16 days - but its periapsis will still be considerably closer to Saturn than Titan, and its apoapsis will still be a good deal farther away than Titan.

Then the other seven flybys will then gradually lower its apoapsis and raise its periapsis so that its orbit bears a closer and closer resemblance to Titan's circular orbit around Saturn, until finally (after the "Titan-24" flyby on Jan. 29, 2007) it will be in a circular orbit around Saturn exactly like Titan's - except that it's tilted 59 degrees to Saturn's equator and thus to Titan's orbit.

Then - after completing one and a half such orbits around Saturn - it will make its 24th flyby of Titan on the other side of the planet (the "180-degree transfer" of the title), beginning a new series of eight Titan flybys at 16-day intervals on that side of the planet which will gradually re-reduce the inclination of its orbit and re-elongate it, in a mirror-image reverse of the previous sequence of flybys - with Cassini's apoapsis, this time, on Saturn's dayside.

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