Cameron Park - May 15
In my last installment, I described the complex activities and studies which the Huygens entry probe will carry out after the Cassini spacecraft parachutes into the atmosphere of Titan.

Huygens' studies, however, will take only a few hours -- and that action is only the beginning of Cassini's epic survey of Saturn, its rings, and its moons as it orbits the planet for (at least) the next four years.

Moreover, there is a natural element of doubt as to whether the Hugyens probe will succeed (although its systems seem to be performing well when turned on during the occasional, necessarily limited tests conducted up to now) -- whereas virtually all of Cassini's systems and scientific instruments have by now been fully activated and seem to be functioning perfectly, making the odds very good that it will successfully complete most of its observations of the Saturn system.

So what will it do during those four years?

Well, it will be doing just what the Galileo spacecraft has been doing during its past five years in orbit around Jupiter -- making repeated close flybys of the planet's moons, and using their gravitational pulls to twist itself into drastically new orbits.

This now-familiar technique for space exploration -- used so often with spacecraft flying by either a planet orbiting the Sun or a moon orbiting a planet -- allows a craft to not only greatly bend its orbital path, but also greatly change the speed with which it is orbiting around the primary body, while expending only a small amount of propellant.

In this way it can greatly raise or lower its periapsis or apoapsis, change its orbit's "line of apsides" (that is, swivel the long axis of its orbit around the primary body like the hand on a clock), or -- by flying over one of the poles of the smaller body -- tilt the inclination of its orbit.

Galileo used gravity-assist navigation to a degree greater than that in any previous space mission; during its two-year primary mission, it made 11 close flybys of Jupiter's four big "Galilean" moons (one during each orbit around Jupiter), and during its three-year extended mission it has made fully 18 more moon flybys -- with four more hoped for during the planned final two years of its life (one of Callisto and three of Io).

Cassini will carry out the same kind of deep-space flying trapeze act around Saturn -- but its gravity-assist "orbital tour" of the Saturn system faces problems which Galileo didn't have.

For one thing, it has far more targets. It is supposed to make detailed, long-term observations not only of Saturn itself but also of its huge, spectacular ring system -- which requires that during much of its lifetime, its orbit must be highly tilted relative to Saturn's equator, since otherwise it would be looking at the rings edge-on.

In contrast, Galileo's controllers have been able to leave it close to Jupiter's equatorial plane during its entire mission. And while Jupiter has only four big moons worthy of closeup flyby inspections in the range of orbital distances from Jupiter available to Galileo during its tour, Saturn has fully eight -- Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion and Iapetus -- all of which scientists would very much like to get a close look at.

But at the same time, Cassini will have far fewer "trapezes" available to it than Galileo has. All four of Jupiter's big moons are massive enough to twist Galileo into a drastically new orbit during flybys -- but Saturn has only one really big moon useful for that purpose: Titan.

Its next biggest moon, Rhea, has less than 1/30 the mass of Titan, and thus virtually no ability to redirect Cassini into a usefully new orbit -- and of course Saturn's other moons are even worse.

So Cassini will have to use Titan flybys for ALL of its complex orbit changes during its orbital tour -- and will thus have to make a close Titan flyby during most of its orbits around Saturn.

Cassini, however, does have one compensating advantage over Galileo. Jupiter's huge radiation belts are so intense that Galileo's controllers have been racing against the clock to complete Galileo's studies before its electronics are seriously damaged by the radiation, although the spacecraft has held up to it much better than predicted (it still has only a sprinkling of radiation-related malfunctions after five years in Jovian orbit).

But Saturn's radiation belts are far feebler than Jupiter's, both because the planet's magnetic field is a good deal weaker, and because Saturn's rings are obligingly placed to soak up particle radiation in just those zones where it would otherwise be most intense.

So Cassini is completely safe from this problem.

It does run the risk of being hit at some point by a tiny but fast-moving stray particle of debris from the rings; there are unquestionably a lot of those orbiting around Saturn even far beyond the visible edge of the rings.

But Cassini's orbital tour has been carefully designed to stay far enough beyond the rings' visible edge to reduce the odds of a crippling dust-particle impact to less than 5 percent during those four years.

Cassini has far more scientific instruments to make those studies -- 362 kg of them, fully three times as much as Galileo.

This is largely because NASA insisted that Galileo must be launched by the Space Shuttle, whose design problems placed very serious weight limits on Galileo, forcing its designers to reject many instruments they very much wanted to carry. Whereas, Cassini was launched by a Titan-Centaur, and could pack more payload.

It's also because Saturn is smaller than Jupiter, and its gravitational field is a good deal less powerful -- so Cassini can carry a lot less weight in fuel in order to brake itself into orbit around Saturn. This also means that Cassini's orbits around Saturn - vast and sweeping as they are -- are much smaller than Galileo's orbits around Jupiter.

In five years, Galileo has completed only 29 Jupiter orbits and an equal number of close flybys of Jupiter's moons -- with the most, 12, around Europa. But Cassini, in its first four years, will complete 74 orbits around Saturn, including fully 44 close flybys of Titan.

Thus Cassini carries many more instruments than Galileo --

In total, Cassini carries 12 experiments -- including the "Radio Science" experiment that uses the spacecraft's own radio transmissions to study the atmospheric and gravitational fields of worlds. The other 11 experiment packages are a collection of two or three instruments aimed at the same goal.

From top to bottom Cassini is far more technically sophisticated than Galileo. This largely reflects the 15 year gulf between Galileo and Cassini thanks to the Shuttle's parade of serious problems.

The TV cameras on the two Voyagers that flew through the Saturn system two decades ago couldn't detect any light at wavelengths shorter than 0.64 microns -- and thus could not pierce Titan's orange haze to view its surface -- but Cassini's operate all the way up to near-IR wavelengths of 1.1 micron, and so should definitely have at least some ability to pierce the haze.

And its cameras carry 18 and 24 color filter sets, as against only 8 for the twin Voyagers' and Galileo.

Its near-IR mapping spectrometer has far greater spectral range than Galileo's instrument - in fact, it can also take visible-light spectra and is therefore called a "VIMS" (Visible and IR Mapping Spectrometer).

It also has far sharper spatial resolution -- and, in fact, besides making high-resolution full-spectrum maps, it can be programmed to instead make images at two or three times higher spatial sharpness in several dozen of its spectral frequencies simultaneously, thus serving as a third camera for Cassini.

Cassini's UV spectrometer to study the upper atmospheres of Saturn and Titan, and the faint gaseous emissions of the other moons and rings, can -- unlike Galileo's -- also be set to make low-resolution 2-D maps at a given wavelength, rather than just taking total spectra of a single point.

Its dust-particle detector can analyze the chemical composition of dust particles, by using a mass spectrometer to analyze the puff of gas produced by a dust particle when it slams into the detector and vaporizes.

And its instruments to measure the direction and energy of both low-energy plasma atoms and higher-energy charged particles in Saturn's magnetosphere also have an ability to analyze the composition of the atoms far beyond Galileo's limited capabilities along those lines.

So what will do Cassini actually do with all these capabilities during its four-year initial orbital tour around Saturn? Well, the tour design officially selected in early 1999 -- "Tour 18-5" -- is divided into four overall "Phases", with each Phase identified by its overall goals and the general nature of its orbits.

Phase I begins with Cassini's insertion into Saturn orbit on July 1, 2004, followed by its first elongated 5-month-long orbit and its release of the Huygens probe.

During Cassini's first Titan flyby on Nov. 27, the Huygens probe plunges into Titan's atmosphere, and Cassini spends threee hours of its approach to Titan simply recording Huygens' signals for repeated transmission to Earth during the remainder of the next orbit.

(That Cassini Titan flyby -- at an altitude of 1200 km -- will also greatly trim down the apoapsis of Cassini's next orbit, so that it lasts only 48 days instead of 148.)

While it is still an hour out from Titan, Cassini will stop listening for Huygens with its big high-gain antenna, then slew around and spend the remaining hour observing the weather patterns around Huygens' landing area with its side-mounted cameras and spectrometers.

During that next 48-day orbit, it will repeatedly transmit Huygens' recorded data back to Earth.

Then, during its second Titan flyby on Jan. 14, 2005, Cassini will use its onboard radar system for the first time to probe beneath Titan's haze -- which it couldn't do during its first flyby because the big fixed high-gain communications antenna dish on its top also doubles as both the antenna to receive Huygens' signals and as Cassini's radar antenna, and naturally can't be used for any two of these functions simultaneously.

This system -- built largely by Italy -- is strikingly similar to the radar system that the Magellan spacecraft used to map Venus, and has several different modes.

During Cassini's initial approach to Titan, from a range of about 22,000 km in to about 9000 km, Cassini will tilt back and forth and use its radar to map Titan's surface radar reflectivity ("backscatter"), which can provide data on its surface texture and composition.

(For instance, water ice is far more radar-bright than liquid hydrocarbons -- which was one of the first clues that allowed Earth-based astronomers to conclude in 1990 that most of Titan's surface was NOT covered with a liquid ethane-methane ocean as they had thought.) During all its operations, the radar will also frequently listen to the microwave emissions of Titan's own surface rather than its own pulses; the strength of these emissions depends both on the surface temperature and, again, on the types of substances making up the surface.

(For instance, it may be able to distinguish liquid ethane from more complex solid hydrocarbons, and water ice from frozen ammonia.)

Then, from 9000 km range in to 4000 km, Cassini will start pointing its antenna constantly at the point on Titan's surface directly under it, and make high-quality profiles of the altitude of Titan's surface features, with a spot wideth of about 25 km and a height accuracy of only 90 to 150 meters.

This technique allows it to study very subtle and wide altitude variations -- such as the difference between continents and seas on Earth -- which can't be detected well by photograph-type imaging techniques, but are invaluable for understanding a world's geology.

For instance, Titan -- for reasons we don't understand -- has a slightly eccentric orbit; it's about 6% farther from Saturn at its apoapsis than at its periapsis.

And if, as some speculate, Titan does have a liquid layer of water and ammonia under 30-100 km of its solid surface ice, the slight tidal flexing of its surface produced by those slight differences in Saturn's gravitational pull on it will probably be big enough for the radar altimeter to detect.

Finally, from 4000 km all the way in to the closest point of the flyby, Cassini's radar will switch to its "synthetic-aperture imaging" mode, in which it will send two pairs of radar beams off at angles to the left and right sides of its orbital track, and precisely record both the timing and the Doppler frequency of the complex return echoes from their pulses. The SAR will provide data which can be used by its Earth controllers to reconstruct "photographic" images of Titan's surface, complete with bright hills and slopes facing the "light" source and shadowed slopes pointing away from it.

The difference, of course, is that the "light" in this case will be radio waves, and its source will be the spacecraft itself rather than the Sun.

This is the technique Magellan used to construct photographic maps of almost all of Venus' surface with a resolution of a few hundred meters.

Cassini's radar resolution will be similar -- its radar echoes will map the surface at about 1.5 km resolution until the craft is within 1500 km of Titan; and for the short period in which it's closer than that, it will switch to a high-resolution mode in which it can detect features only 500 meters wide.

After the closest approach, as it pulls away from Titan again, it will run through the whole radar sequence in reverse. But since Cassini makes only fast flybys of Titan -- rather than orbiting it and mapping every bit of it as Magellan did -- the area of its mapping coverage will be far smaller.

On each Titan flyby, it can map less than one percent of Titan's surface using the synthetic-aperture technique, and so even at the end of that 4-year orbital tour only about 35% of Titan's surface will have been covered.

And when it comes to mapping Titan, the radar has competition.

The TV cameras of the Voyagers couldn't detect any longer-wavelength light than orange, and thus were hopeless at trying to pierce Titan's smog.

But longer-wavelength light -- on the border between red and infrared -- punches through the smog fairly well; both the Hubble Telescope and earth-based telescopes have made fuzzy but intriguing maps of Titan's surface features by using light at 0.94 microns and longer wavelengths.

And since Cassini's cameras can detect near-IR light all the way up to 1.1-micron wavelengths, they will probably have the ability to get moderately good visible-light photos of Titan's surface.

Without the smog, the narrow-angle camera could take pictures with only 6-meter resolution at 1000 km altitude -- but the blurring from the smog will probably reduce its sharpest pictures to only 100 or 200 meters resolution, and the fact that the smog diffuses the sunlight hitting the moon will blur the shadows of its surface features to the point that in many ways the radar pictures will probably be better.

There's also Cassini's VIMS -- which can not only take imaging maps of Titan's surface, but visible and near-IR spectra of it from which its surface composition can be mapped.

The smog particles don't fuzz up IR light at all at wavelengths of more than about 1.2 microns -- but when it comes to studying Titan, VIMS has another problem.

The methane gas itself in Titan's atmosphere absorbs all the sunlight streaming down onto Titan's surface except in half a dozen wavelength "windows" in VIMS' spectral range, so it can only take spectra of Titan's surface color in those limited-width bands, which greatly cuts down on its ability to identify different substances there.

(This is the same reason that the Huygens probe -- which has its own near-IR spectrometer -- will turn on a searchlight during the last 200 meters of its descent, to provide the spectrometer with full-spectrum lighting for a more complete analysis.)

Still, VIMS should have the ability to distinguish several different types of frozen ices and organic substances -- and its surface resolution will be as little as 500 meters for complete spectra, and only about 200 meters if it uses its "high-resolution" mode for sharper photographic maps in selected wavelengths.

The problem, however, is that all the cameras and spectrometers point off to Cassini's side, so it can't use them and the radar to map Titan's surface simultaneously -- and during each flyby of Titan, ground controllers will have to choose which type of mapping they want to do.

The current plan calls for most of the 44 flybys to be used for radar mapping, but there will likely be some changes made as a result of the early scientific results.

As for Cassini's mapping targets, they are fairly rigidly set (especially for the radar) by its mandatory flyby paths past Titan -- but they cover a wide range of likely features.

(On its first radar-mapping flyby Cassini will map the general area in which the Hugyens probe landed.)

That second Titan flyby will shorten Cassini's orbit to only 32 days, so it comes swooping back in for its third Titan flyby on Feb. 15, 2005.

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.

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