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Cassini's Epic Tour of the Rings

by Bruce Moomaw
Cameron Park - May 15
The recently-discovered problem with Cassini's ability to record the radio telemetry from Huygens may lead to some reshuffling of the early sequence of events -- the Huygens release may be delayed till the second or even the third flyby, in which case Cassini will instead use its first one or two flybys to try to gauge the exact speed and direction of Titan's high-altitude winds (which will allow it to point its radio dish much more precisely at Huygens as the probe is blown along hundreds of kilometers sideways during the first part of its descent), and maybe for radar mapping as well.

But it's very unlikely that the Huygens release will be delayed past February.

Thereafter, Cassini will continue to make a rapid-fire sequence of Titan flybys precisely calculated to veer itself onto a whole series of orbits carefully designed to study other targets.

In fact, the "Titan-3" flyby will direct Cassini at its first flyby of another of Saturn's moons -- Enceladus, on March 9.

Saturn's other moons, as I said, are too small to significantly bend Cassini's orbit, and so setting up flybys of them is somewhat harder than for Jupiter's moons -- but they're important enough scientifically that Cassini is scheduled to carry out seven deliberately targeted, close flybys of them during its 4-year tour.

(It will also make fully 27 "nontargeted" flybys -- that is, flybys within 100,000 km of a moon, and sometimes within just a few thousand kilometers -- which just happen to occur when the craft is on its way to another destination, but which are nevertheless close enough to allow some very useful scientific observations.

In particular, it will make 7 such flybys of Mimas and 5 of Tethys, neither of which are targeted for any deliberate close flybys during the 4-year primary tour -- as well as 8 of Enceladus, 4 of Dione and 3 of Rhea.)

Enceladus is a little iceball only 500 km across -- but it's nevertheless the second most important target for Cassini among Saturn's moons.

The reason is that, despite its small size, it shows surprising signs of being still geologically active.

It shows clear signs of faults and ancient ice "volcanoes"; the number of craters on its dazzling white surface is small enough that it seems to have been completely resurfaced sometime during the past 5 to 200 million years; and Saturn's "E ring" -- a faint but vast cloud of dust particles that stretches for 480,000 km away from the planet -- seems to be densest around Enceladus' orbit, strongly suggesting that it consists of ice particles expelled from Enceladus by geysers.

(All of Saturn's other moons also show some major signs of icy eruptions and resurfacing during their early days, unlike Jupiter's big moon Callisto. This may be due to the fact that the nebula around Saturn was cold enough that it formed moons containing a large amount of frozen ammonia mixed in with their water ice; and that mixture has a far lower melting point -- minus 97 deg C -- so that much of it may have formed a subsurface melted layer during the moons' warmer early days.

But none of the others shows significant present-day activity.)

Even considering the low melting point of ammonia-water mixture, Enceladus' apparent current activity is a puzzle.

The leading theory is that it's due to the same kind of tidal heating that warms Jupiter's volcanic moon Io -- Enceladus' orbital period is half that of the moon Dione, so the latter's rhythmic "resonant" tuggings have stretched Enceladus' orbit slightly out of the circular, and the resultant changes in the strength of Saturn's tidal pull on Enceladus' surface causes flexing which in turn causes some frictional heat.

But Enceladus' orbit is still close enough to circular that this process should produce only about one percent of the heat needed to melt its internal ice.

The current feeling is that, since Saturn's moons (like Jupiter's and even our own) are very slowly spiralling farther away from their home planet because they're being pulled along by tidal pulling from the rapidly spinning surface of that planet, their orbits may have drifted in and out of more dramatic resonant relationships with each other over geological epochs. For instance, Ganymede's surprising magnetic field is apparently caused by the fact that it still has a molten metal core -- which in turn may have been due to the fact that about a billion years ago it may have undergone a period of having a much more eccentric orbit, and it's still slowly cooling down from that era.

Similarly, Enceladus may still be cooling down from a time when the eccentricity of its orbit, and thus the tidal flexing of its surface by Saturn, were much greater than they are now.

Anyway, Cassini will fly by it at only 500 km range, taking very high-resolution photos, sniffing for any signs of released gases, looking for local warm spots with its long-wavelength "Composite IR Spectrometer", and using its VIMS to analyze the moon's surface ices.

(VIMS can do so much better here than for Titan, since it's not blocked in all but a few spectral wavelengths by an atmosphere -- for instance, it should be able to firmly measure the amount of frozen ammonia on Enceladus.)

Tour Phase I runs through August 22, 2005 -- and during it, Cassini will make three more Titan flybys and a second one of Enceladus (on July 14, at 1000 km).

During its first four Titan flybys, it will never get closer than 1200 km -- but starting with the fifth one in April, it will usually fly by Titan at the minimum permitted altitude of 950 km.

Scientists would of course like to get closer to Titan that this; not only would it allow sharper surface mapping, but Cassini carries an ion and neutral mass spectrometer designed to directly analyze Titan's upper atmospheric gases, and close approaches would also allow better studies of its gravitational field and the way its atmosphere interacts with Saturn's magnetosphere. But -- as I've noted before -- Titan's unique combination of weak grvity and a nevertheless dense atmosphere means that its air towers up above its surface to astonishing heights, and taking Cassini any closer to it might actually lead to dangerously high air friction.

However, scientists will carefully measure Titan's precise air density during Cassini's early flybys, and reserve the option of later lowering some of its flybys to as low as 850 km if it's safe (which would require them to make only very minor adjustments -- "tweaks" -- to the orbits of its overall Tour). Alternatively, if Titan's upper air turns out to be denser than expected, they can easily move its closest flyby range out to as far as 1050 km.

However, Phase I's most important goal has to do not with Saturn's moons, but with its rings.

During its flyby back in 1980, Voyager 1 deliberately flew behind not just Saturn but its rings, as seen from Earth -- and so its radio beam sliced through a cross-section of the rings, so that measurements of the extent to which the particles in the rings scattered the signal allowed calculation of the sizes of the particles in the different rings. The results -- like so much else about the rings -- were puzzling; the biggest chunks of material in the faint, translucent inner "C" ring are mostly no more than about 2 meters across, while the medium-bright outer "A" ring and the "Cassini Division" (which, despite its name, still has a fair amount of material in it) have many fragments up to 10 meters across. (Saturn's widest and brightest ring -- the "B" ring -- has such a dense population of ice fragments that a radio beam can't even pierce it.)

We don't understand what kind of process is causing these differences in fragment size -- it may be due to the fact that the A Ring was formed only a few hundred million years ago and its fragments haven't been so thoroughly ground up by collisions -- and so we want Cassini to carry out a whole series of such radio occultations, at different times and in different locations around the ring to study detailed changes in its complex cross-section structure.

But since Saturn's spin axis is tilted 27 degrees to the ecliptic, its rings -- surrounding its equator -- are similarly tilted relative to Earth, with their maximum tilt visible to Earth only twice during each of Saturn's 29-year orbits.

Scientists naturally want Cassini's radio occultations to occur during these maxium-tilt periods -- reducing the amount of ring material the beam will have to slice through on its way to us, and maximizing the clarity of the results.

Well, Cassini will arrive at Saturn soon after such a brief moment of maximum tilt -- with the tilt then shrinking until the rings are almost edge-on to us by the end of the 4-year orbital tour -- and so it will carry out a series of seven radio occultations of the rings during the earliest part of the tour, the 14-month-long Phase I.

Tour Phase I will end with Cassini's sixth Titan flyby on August 22, 2005, and the spacecraft will then move on to the remaining three phases of its four-year tour, each with its own particular set of scientific goals.

I'll describe those in the final part of this series.

  • Part One - Two - Three

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    In the first of our series on the activities of the Cassini spacecraft at Saturn, we ended with the spacecraft having entered orbit and firing its main engine to fly by Saturn's moon Titan, where it will parachute its European-constructed probe Huygens into that moon's atmosphere, giving us our first close up look at Saturn's most enigmatic moon.

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    In our continuing series on the upcoming Cassini mission to Saturn and Titan, Bruce Moomaw provides SpaceDaily readers with an insight into the science and technology that will make or break our first expedition to the surface of a gas giant's moon.

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