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Cassini's Epic Tour of the Rings

Cassini drops off Hugyens
by Bruce Moomaw
Cameron Park - May 15, 2001
In our previous installment on Cassini mission to Saturn we looked at the activities the Huygens entry probe would conduct during its active phase as it parachutes into the atmosphere of Titan.

This brief visit to Titan of just few hours 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.

Coupled with a natural element of doubt as to whether the Hugyens probe will succeed given its high risk goals. Whereas Cassini's mission is a little more routine with the recent experience of the Galileo spacecraft to draw upon as its makes repeated close flybys of the planet's moons, and using their gravitational pulls to twist itself into drastically new orbits.

This is now a familiar technique for space exploration will enable Cassini to 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. Firstly, it has far more targets including 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. By 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.

Nonetheless 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. Cassini was launched by a Titan-Centaur, and consequently could be far heavier.

(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 -- including a second, wide-angle TV camera (Galileo only carried a single telephoto one), a long-wavelength IR spectrometer to study the gases and temperatures at different levels of Saturn's and Titan's atmospheres in detail (along with a short-wavelength near-IR spectrometer which, like the one on Galileo, will specialize more in analyzing the surface composition of the planet's moons and rings), and a novel instrument to map the flow patterns of Saturn's magnetosphere by actually constructing low-resolution image-type maps of the streams of ions and neutral atoms fired out of that magnetosphere toward the spacecraft (at the time Cassini was launched, this instrument hadn't even been used yet to study Earth's magnetosphere, although some satellites since have carried it).

Cassini carries a total of 12 experiments -- counting the "Radio Science" experiment that uses the spacecraft's own radio transmissions as a way of studying the atmospheres and gravitational fields of worlds -- and each of those really consists of a collection of two or three instruments aimed at the same goal.

And Cassini's instruments are more technically sophisticated than Galileo's -- after all, due to the 8-year launch delay that Galileo suffered in the Eighties thanks to the Shuttle's parade of serious problems, its instruments were built fully two decades ago! So even those of Cassini's instruments that are generally similar to those on Galileo have far greater capabilities.

The TV cameras on the two Voyagers that flew through the Saturn system two decades ago couldn't detect anly 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 filters, as against only 8 for Voyager's and Galileo's.

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.

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