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Planetary Scientists Looking Forward To Saturn's Splendor

Cassini dropping off a vistor to Sol's biggest Moon Titan
by Bruce Moomaw
Cameron Park - Jan 27, 2001
The Cassini spacecraft has now successfully rounded Jupiter, and used a gravity-assist from that world to catapult itself on its way outward to Saturn -- a voyage of over six and half years of which it is now approaching the halfway point.

Cassini is not only the biggest deep-space probe the U.S. has ever launched (over 6 tons), but the most expensive at over 3 billion dollars budgeted so far. Moreover Cassini is sometimes referred to as "the last of the dinosaurs" with individual Solar System spacecraft from now on being deliberately made smaller and less expensive, with their experiment payloads shrunken accordingly. Happily, all that expenditure for Cassini seems to be paying off.

Up to now, Cassini has operated with near-perfection that is almost eerie -- its only onboard malfunctions have been a leaky pressure regulator in its main engine, and an attitude-control reaction wheel that showed temporary signs of sticking, both of which were easily worked around.

In addition, all of its 12 scientific experiments are working perfectly, which is also rather eerie -- usually, when a spacecraft carries that many scientific instruments, at least one of them can be relied on to develop some kind of significant problem.

However one potentially serious problem does lurk and could have a serious impact on the radio link with the European Huygens probe that it is supposed to drop into the atmosphere of Titan. But that problem is due to a design error rather than an actual malfunction, and (as we'll see), it now seems likely that it too can be completely worked around.

Cassini lacks the folding high-gain antenna and the tape recorder that both stuck on Galileo. Instead, it has a rigid already-unfolded antenna and two solid-state recorders.

Adding further confidence to the mission is that the main engine has already worked perfectly during a deep-space maneuver burn almost as long as the one it will perform during Saturn Orbit Insertion. Furthermore, it carries a complete backup engine, which will switch on instantly if the first one fails during the Saturn orbit insertion burn!

That good behavior on Cassini's part has now paid off.

Originally, the need to reduce the mission's huge cost had forced NASA to plan for absolutely no scientific observations during its long voyage to Saturn, although it would be making two gravity-assist flybys of Venus and one each of Earth and Jupiter.

But its flawless performance -- and the fact that its interplanetary navigation has been so perfect -- has saved enough operating funds and maneuvering fuel that NASA was able to carry out full-scale scientific observations during every flyby except the first one of Venus.

Indeed, its Venus flybys have already provided the first important new scientific information from the mission: Venus -- contrary to some earlier studies -- apparently does not produce any lightning at all in its clouds.

And its images of Jupiter's overall cloud patterns (which the Galileo spacecraft couldn't provide, due to its antenna problem) have been far higher in resolution than those from the Voyagers, revealing a strange beauty of fine detail in Jupiter's vast cloud landscapes.

But the main course is yet to come.

With so many years and flybys between launch and arrival at Saturn, it's easy to forget about what Cassini will actually do when it reaches Saturn in June 2004, and drops off the Huygens Titan probe, before spending some four years surveying the planet, its rings and moons during its primary "orbital tour" of the Saturn system.

There is a surprising shortage of information on this subject on the Web -- for example, almost nothing is available on its planned orbital tour, although its detailed design has been firm for almost two years -- and as such I hope these reports will help fill in some of the details that may have been rather obscure up to now.

Cassini will start making photographic observations of Saturn and its rings in January 2004, fully 6 months before reaching the planet -- and when it's still a couple of months out, these should start to surpass observations from Hubble. This observational phase will reveal both Saturn's cloud patterns and its dark "spokes" of fine dust unexpectedly discovered in Saturn's rings by the twin Voyagers in 1980/81.

Although these initial observations will be breathtaking, eye candy its first major new observation will occur on June 12, 19 days before reaching the planet, when Cassini buzzes Saturn's outer satellite Phoebe.

Phoebe -- which up to now has been only fuzzily photographed at 2 million km range by Voyager 2 -- is a dark little world, 220 km wide, which orbits fully 13 million km from the planet, in an orbit tilted 30 degrees to Saturn's equator -- and backwards.

It is, in all likelihood an asteroid -- or a dirty-ice planetesimal left over after the formation of the giant planets -- which was captured by Saturn during its formation days. This capture was made possible when there were still enough clouds of gas and debris far from Saturn capable of braking a passing object into orbit around Saturn. (Only in the past two months have a dozen much smaller similar captured moons been discovered orbiting the planet.)

Not surprisingly, planetary scientists would like to take a close look at Phoebe, but it's very hard to set up a flyby because its revolution around Saturn takes fully 550 days, and its orbit is tilted 30 degrees relative to Saturn's equator.

By pure luck, though, it happens to be at such a point in its orbit that, just by fiddling with Cassini's arrival time at Saturn by a few months, flight controllers could arrange for Cassini to fly within 56,000 km of Phoebe.

This is still a pretty distant view of the little world -- but because Cassini's precise navigation has allowed it to save significantly on maneuvering fuel, an additional course correction has been added that will allow it to fly within 2000 km of Phoebe, enabling extremely detailed images to be taken, along with IR surface composition spectra readings, and density measurements to tell if it's rock or ice.

To do so, flight controllers slightly changed its flyby path past Jupiter, aiming its final arrival point slightly further away from Saturn. Then a few days before reaching Phoebe, they will make a 27 meters/sec burn to angle it back toward Saturn on just the right path to fly immediately past Phoebe.

Cassini will then resume making routine closer and closer observations of Saturn itself, until -- on July 1 -- it swoops past Saturn's northern hemisphere, crosses the ring plane a short distance beyond the visible "F" ring, and swoops down to within only 6,000 km of Saturn's cloud tops.

95 minutes before making that closest approach, it will start firing its main engine to brake into Saturn orbit, shutting the engine down just at periapsis.

During the SOI burn, Cassini will make measurements of Saturn's magnetic field and magnetosphere -- as this is by far the closest it will ever come to the planet -- but its cameras and spectrometers are rigidly fastened to its side, its scan platform having been removed back in 1996 to save money, so the spacecraft can't slew around to point its camera at any target until the engine has shut down.

After that, however, it will immediately start slewing around to point them for closeup views of Saturn's cloud features and rings.

Cassini cannot transmit back pictures live as it must then slew around again to point its big high-gain antenna dish at Earth and deliver data at 170,000 bits per second.

But its two solid-state recorders can store a total of 4 billion bits of data at a time -- almost four and half times as much as the Galileo spacecraft's tape recorder -- which it will then play back to Earth over a period of just a few hours.

Its typical mode of operation will be to spend 16 hours each day slewing around to point its instruments at various targets and recording data, before pointing its antenna at Earth to spend eight hours playing all the recorded data repeatedly back to Earth. During both periods, it will often roll slowly around its main axis -- once every 23 minutes -- so that its fields and particles sensors can monitor the flow of radiation and charged particles in different directions within Saturn's magnetosphere.

After braking itself into Saturn orbit, Cassini will soar away from the planet -- passing again through the ring plane all the way out at Dione's orbit -- in a huge, elongated orbit that takes it 10.4 million km out from Saturn.

On Sept. 25, shortly after passing that apoapsis, it will fire its main engine again to raise its next periapsis all the way out to Titan's orbit 1.2 million km from the planet -- thenceforth staying safely away from the rarified disk of dust particles which spreads some considerable distance beyond the edge of Saturn's visible rings.

While this material is much too rarified to be visible from Earth without extreme light amplification instruments, it's dense enough that each pass through it provides a very slight threat that Cassini might be crippled by hitting a "grain of sand" -- and so, while that first close pass just beyond the edge of Saturn's visible rings is necessary, the flight controllers want to avoid flying through the danger zone any more.

With the spacecraft now in its primary orbit attention will then turn to Cassini's second most important target: Titan, which is also the target of the Huygens entry and landing probe provided by the European Space Agency.

Next Week: Cassini at Titan

Related Links
Main Cassini Portal NASA/ESA
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Cassini Snaps Io Among The Clouds
Tucson - Jan. 22, 2001
The Galilean satellite Io floats above the cloud tops of Jupiter in a stunning image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), and only two days after Cassini's closest approach. But the image is deceiving with over 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's cloud tops.

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