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Cassini's Tour de Saturn

by Bruce Moomaw
Cameron Park - May 25, 2001
In my last article on the Cassini Saturn probe, I described the first of the four "phases" of its four-year orbital tour of Saturn and its rings and moons, during which it will use a constant sequence of gravity-assist flybys of Titan - 44 flybys during its 74 orbits around Saturn during those four years - to keep radically modifying its orbit to study different parts of the Saturn system.

Each phase represents a period in which Cassini will carry out orbital maneuvers of a different general type to make a particular kind of scientific observation.

In Phase I - running from Cassini's arrival at Saturn on July 1, 2004, through August 2005 - Cassini will make a total of six Titan flybys, in order to shorten its initial very elongated orbit around Saturn, drop off the Huygens probe to parachute into Titan's atmosphere and radio back its results to Cassini as the latter flies past the moon, and then make a series of orbits around Saturn allowing it to pass its radio beam to Earth through cross-sections of Saturn's rings seven times (the so-called "ring occultations"), and make two close flybys of Saturn's small but intriguingly geologically active moon Enceladus.

In this last part of my series on Cassini and Huygens, I'll briefly describe the remaining three phases of the tour.

Cassini will initiate Phase II of the tour in September 2005.

During Phase I, the apoapsis of its elongated orbit is on the "dawn" side of Saturn, ahead of the planet on its orbital path around the Sun.

But scientists very much want Cassini to make several elongated orbits pointing directly away from the Sun on Saturn's night side, so that they can use its magnetospheric instruments to make a detailed study of the structure of the "magnetotail" - the great, fluctuating wake, millions of miles long, which Saturn's magnetic field plows in the solar wind streaming outward past the planet, in which some currents of charged particles flow out away from the planet and others double back and return.

(Galileo made a similar survey of Jupiter's magnetotail once during its two-year primary tour, and again during its extended mission.)

To do this, Cassini will make a series of nine more Titan flybys from September 7, 2005 through July 2, 2006, using them to gradually twist its orbit's long axis about a quarter of the way round the planet - from 14 degrees on the daylight side of the planet's dawn "terminator" (the border between its day and nightsides) to only 16 degrees from its midnight point directly away from the Sun - and also somewhat elongating its orbits until it is finally flying down the magnetotail to a distance of fully 4.2 million km from Saturn.

(The first four orbits during its Tour Phase III, in fact, will then move as close as seven degrees to the midnight point - although they'll also be less elongated.) During this migration period, its orbits will alternatively lengthen and shorten, alternating between 23-day and 39-day periods around Saturn.

The Phase II Titan flybys will be somewhat more distant than those during Phase I - none of them will come closer than 1470 km to Titan's surface.

During this period, Cassini will also make "targeted" (that is, deliberately aimed-for and close) flybys of three more of Saturn's smaller icy moons. First comes Hyperion on Sept. 26 - the strange little moon, shaped rather like a fat hamburger patty, which orbits a relatively close distance beyond Titan and has an orbital period exactly equal to 4/3 of Titan's 16-day period.

Titan's repeated gravitational tuggings at these regularly timed "resonant" intervals seem to have had a very strange effect on Hyperion's rotation, thanks to its irregular shape.

Instead of keeping one fact perpetually toward its home planet as most moons do, Hyperion seems to rotate "chaotically" - with not just the speed but even the direction of its rotation changing constantly in a way so complex as to be completely unpredictable; sometimes it twists its rotational direction around up to 90 degrees, and at other times it even reverses its spin!

The fact that Hyperion is so non-spherical despite the fact that it is (on the average) 290 km wide suggests that after its formation it was hit by something big that partially shattered it, in which case much of its material may have crashed into Titan (or even splattered one side of Saturn's distant moon Iapetus, of which I'll say more later).

Cassini will make a 1000-km flyby of Hyperion for closeup photos and composition studies.

Only 15 days later, it will make a 500-km flyby of Dione - a bigger icy moon, 1120 km in diameter, which is an orthodox spherical shape.

Dione, although it doesn't show strong evidence of being currently geologically active like Enceladus, does possess many light-colored streaks, and other large areas where craters were erased from its surface - all of which suggest that during its early days it too had a great deal of geological activity in which eruptions of melted water-ammonia mixture erupted from its interior.

(By the way, because this mixture would have contained far more ammonia than your traditional household cleanser and would also have been extremely cold on the surface, it would actually have been thick and gooey, like Earth's rock lava.) Dione also has some effect on the way in which Saturn's radiation belts emit radio waves, which suggests that a very small amount of gas may still be venting from some surface features.

15 days after that, Cassini will make a 500-km flyby by Rhea, Saturn's second-biggest moon (1530 km in diameter).

Rhea also shows many light-colored streaks from ancient icy "cryovolcanic" eruptions, but - paradoxically - seems to have been less geologically active than Dione. Perhaps its biggest puzzle is an overabundance of small craters on its surface, suggesting that it may have been pelted by debris from the event that shattered Hyperion.

  • Part One - Two - Three - Four - Five

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