Sacramento (SPX) Feb 05, 2007
Following the highly successful three phase extended mission by NASA's Galileo Jupiter orbiter, scientists working with NASA's Saturn orbiter Cassini are looking at how an extended mission would work for Cassini. From the time Cassini first entered orbit around Saturn, they have been working on the design of a possible extended tour for it as well, re-planning it on the basis of the craft's new discoveries about the Saturn system, as well as computer calculations of just how many interesting destinations they can reach using more Titan flybys.
While Saturn lacks the savagely intense radiation belts around Jupiter that gradually fried Galileo's electronics, there are some limits on Cassini's lifetime.
In particular, it is running out of the remaining supply of maneuvering fuel that allows it to keep tweaking its trajectory to make its Titan flybys at just the right distance and angle.
After the maneuvering fuel runs out, mission control will be unable to set up Cassini for any more Titan flybys to readjust its orbit and it will have to settle for some final orbit. And while it could potentially go on making detailed observations of Saturn and its rings from that final orbit for years, scientists are increasingly concerned about even the slightest risk that the unsterilized spacecraft may crash into either Titan or Enceladus and contaminate either world's potential biosphere with Earth germs.
It was that same concern that Galileo might contaminate Europa that led mission control to eventually order its final suicide dive into Jupiter's atmosphere after they were satisfied that they had squeezed as much additional science out of it as they could -- and it's possible that Cassini's controllers may decide to end its career in the same way as the very last adjustment to its orbit that they can make.
However, the hope at this point is that there may be still another two-year extended mission after the first one, taking the mission all the way into mid-2012. However, such a third orbital tour would have less frequent moon flybys than the first two tours.
In any case, on Feb. 1, Cassini's "Project Science Team" finally reached consensus on what that two-year extended mission will be. In the meantime, NASA headquarters will review the choice and is not expected to officially declare the decision for about another two weeks.
The multitude of potential tours of the Saturn system has various names. The design finally chosen for Cassini's 4-year primary tour was "T18-5". The one that has now been chosen as the extended mission is "PF6-h9".
While it doesn't achieve all the goals the different scientific groups would have liked -- in particular, it lacks a second close flyby of Iapetus -- they are nonetheless extremely pleased by the proposed extended tour.
As one might expect, its most important targets are Titan and Enceladus, which have turned out to be the surprise stars of the Saturn system.
The swarm of questions raised about both worlds makes them the center of the chosen tour. During its 60 more orbits around Saturn during the "PF6-h9 Extended Tour", Cassini will make 26 close flybys of Titan for a mission total of 72.
The flight paths past Titan during the extended tour have been mostly chosen to keep adjusting its orbit to enable other Saturnian science goals; but they also allow low flights over many regions of Titan that could not be well-observed by the close-range radar during the primary tour.
Cassini, during the first half of its extended mission, will make repeated polar flybys of Titan to gradually tilt its near-polar orbit back into Saturn's equatorial plane, allowing it to make more flybys of Saturn's smaller moons.
As for Enceladus: that little moon is one of those that must be specifically aimed for -- and Cassini will do that seven more times during the extended mission, observing different parts of Enceladus, and mapping its gravity field to get some idea of its internal structure and thus of the still-mysterious mechanism that drives violent geysers of water vapor (and maybe even liquid water) from a single small region on an icy moon only 500 km in diameter. Two more of those flybys, like the one in March 2008, will be breathtakingly close: only 21 to 25 km altitude.
The other moons of Saturn -- except for the almost-inaccessible Iapetus -- are much less interesting, but that's a far cry from saying that they aren't interesting at all.
The "PF6-h9" tour includes one more close, targeted flyby each of three other moons.
+ Cassini will fly by Dione once at 500 km altitude: a moon which is long-dead now, but shows clear signs of violent tectonic stretching during the Solar System's ancient days -- and which also seems to be still releasing a tiny trickle of water vapor, only about 1/300 as much as Enceladus, from someplace on its surface.
+ The spacecraft will fly within 1500 km of Helene, one of Saturn's recently-discovered tiny moons that seem to be loose clumps of icy rubble stuck together only loosely by gravity. Helene, discovered in 1980, is one of two moons that share Dione's orbit at its gravitationally stable "Lagrange points". (Two more tiny Lagrange moons share Tethys' orbit.)
+ In addition, Cassini will skim the surface of Rhea, Saturn's second-biggest moon, at 100 km altitude. Unfortunately, Cassini's lack of a tiltable scan platform seriously limits its ability to cancel out the motion blur that fuzzes up its images during its really close flybys of various moons.
During close flybys of Enceladus -- which is located in the middle of the vast "E Ring" of microscopic ice particles that are sprayed into orbit around Saturn by Enceladus' plumes -- it has to keep its high-gain antenna pointed ahead as an impromptu shield against any rare big E-ring ice grains that might damage the craft, which means that its cameras (fixed rigidly peering off to the craft's side) cannot have their motion blur reduced at all by tilting the entire craft.
During its close Rhea flyby, Cassini is free to tilt itself and thus its cameras -- but at its closest approach it will be flying over Rhea at a speed much too great to be canceled out even by the craft tilting itself at its fastest possible rate.
Since Rhea is not very interesting geologically, the purpose for this close flyby is different. Scientists would like to know the rate at which small meteoroids and chunks of ice in the outer Solar System are being drawn toward Saturn and cratering its moons.
This rate is thought to have been the same for a long time, and knowing the count of small meteoroids will allow for an estimate of the rate of bigger ones that produce visible craters on the various moons' surfaces.
This will enable an estimate to be made of the time since the last crater-erasing geological processes on the various parts of the moon's surfaces to be estimated.
Cassini's main goal at Rhea will be to use its "Cosmic Dust Analyzer" -- which counts and gauges the size of microscopic dust particles that hit it -- to actually try to count the spray of additional dust particles thrown up just above Rhea's surface as ejecta by the small meteoroids currently crashing into that moon. Rhea is considered the best possible moon to use for this purpose.
Simply in the process of getting from one intended target to another, Cassini will also continue to make occasional "nontargeted" flybys of moons -- many of them close enough to still be scientifically useful. For instance, during the Primary Tour Mimas is the only one of Saturn's nine biggest moons not lucky enough to have any close flyby during the Primary Tour -- Cassini has flown only within 63,000 km of it. During the Extended Tour, however, it will be able to make one nontargeted flyby within 9700 km of Mimas.
But Saturn's moons, of course, are not the only subject of the Extended Tour. Cassini will continue to observer Saturn's weather patterns and its magnetospheric phenomena, as well as of its rings.
For one thing, it can make further occultation observations of the rings' very fine structure and the sizes of its particles -- both by observing the light of various stars twinkling through different parts of the rings, and by slicing its own radio beam to Earth through the rings.
The new tour allows an especially good combination of such occultation profiles of the rings at various angles, which should provide even finer knowledge about the super-fine structure of Saturn's hundreds of thousands of ringlets -- and even individual temporary clumps or flocks of particles.
And in August 2009, Saturn will undergo one of its equinoxes -- the moments, separated by 14.7 years, at which the tilted planet's equators, and thus its rings, are precisely edge-on to the Sun. Cassini's orbit will still be mildly tilted out of the equatorial plane at this point, allowing it to view the rings' structure.
Most formation theories of the "ring spokes" -- the remarkable, semi-linear clouds of extremely fine dust particles that are sometimes seen in the rings -- predict that they appear in large numbers only near equinox. This is because at such times the intensity is at a minimum with ultraviolet sunlight hits Saturn's ring particles and drains off the electrical charge they've picked up from Saturn's own magnetophere, lofting the very tiniest ones out of the ring plane.
Also, Cassini's "CIRS" longer-wavelength infrared spectrometers will map the temperature patterns of different parts of the rings on both the side of them lit by the Sun and the shadowed side - which will change after equinox. Mapping the rate at which the temperatures change on the newly lit and the newly unlit faces of the rings will provide more data on the size of the ring particles and the rate at which they tumble as they revolve around Saturn.
Finally, in late 2009, Prometheus -- the innermost of the two tiny rubble-clump moons on either side of Saturn's thin F Ring just outside the edge of its A Ring -- is expected to do something very dramatic.
It was originally thought that Prometheus and Pandora served as "shepherd moons" whose gravitational tugging keeps the F Ring particles perpetually herded into a narrow zone between the moons' orbit; but Cassini has confirmed that the real situation is far more complex.
The two small moons actually interact with the particles of the F Ring in very complex, chaotic, almost impossible-to-predict ways -- herding the F Ring particles into complex waves and even into short-lived clumps which then dissolve again -- and the constantly-changing mass distribution of the F Ring in turn keeps nudging the orbits of Promethus and Pandora in complex ways.
In 2009, it's predicted that Prometheus will -- in a very un-shepherdlike way -- actually brush through the outer part of the F Ring, disrupting its structure in unpredictable ways.
It now seems likely that the F Ring, Prometheus and Pandora actually form a unified, ever-changing entity that keeps radically changing its total nature over fairly short periods of geological time, with the two current "shepherd moons" being only recently formed from big clumps of ring rubble that will finally break up and be replaced by other new clumps.
Many details of this Tour have not yet been publicly announced -- and, indeed, its acceptance by NASA HQ is not an absolutely sure thing. Butit's obvious that -- even after its breathtaking low buzz over Enceladus south polar plumes and the end of its Primary Tour in July 2008 -- its important scientific revelations will be far from over.
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Sacramento (SPX) Feb 05, 2007
Cassini is now two and half years through its official "primary tour" of the Saturn system - and which is scheduled to last another 17 months before ending on June 30, 2008. During this primary tour Cassini will have made 46 close flybys of Titan (during one of which it dropped off the European Space Agency's Huygens Titan lander), four close flybys of the unexpectedly fascinating moon Enceladus, and one each of Tethys, Dione, Rhea, Hyperion, Iapetus and Phoebe.
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