At the moment NASA has three teams considering the proper design of the Flagship-class missions to these three moons that I've mentioned before -- plus another working on the design for a Flagship-class "Jovian System Observer" that would probably end up in orbit around Ganymede. They will issue their reports this fall.

But it also has two teams considering the question of whether it may be feasible to fly worthwhile missions to Titan or Enceladus at a cost of less than a billion dollars -- which is on the bottom fringes of the "Small Flagship" cost range, just a short distance above the current $750 million cap for New Frontiers-class missions. (There are also rumors that the New Frontiers cap may be raised before the next mission solicitation goes out in 2008.)

The Titan team is led by the University of Arizona's Ralph Lorenz; the Enceladus team by the Southwest Research Institute's John Spencer. They are scheduled to issue their final reports later this month, but they've already announced a good many preliminary conclusions -- specifically, that there are several concepts for such missions that might be feasible (although we will have to be very careful not to overestimate their scientific return, or try to make them overly ambitious).

In the case of Titan, one such mission could apparently be a Titan orbiter -- without the additional lander or balloon that would accompany it in the Flagship-class "Titan Explorer" concept.

Although Cassini is currently scheduled to make fully 71 Titan flybys during its first six years -- and might even make a few more later -- its Synthetic Aperture Radar (SAR) system will still by then have mapped only about half of Titan's surface, and then only at a top resolution of about 400 meters. And its radar-altimeter profiles of Titan's topography -- which have turned out to be very important scientifically -- will have covered a good deal less.

Its cameras can cover much wider areas of Titan -- but only in the 1-micron spectral window through Titan's methane atmosphere and orange organic smog. And their views at that wavelength are extremely fuzzy. Its Visual and Infrared Mapping Spectrometer (VIMS) can actually double as a camera operating at several other longer-wavelength spectral windows that provide a much sharper view -- notably the one at 2 microns, where its photos are almost as sharp as Cassini's radar maps -- and comparison of them can provide some limited data on Titan's surface composition. But VIMS can only take photos in a "pushbroom" mode during Cassini's flyovers of Titan, which means that its surface area coverage is as limited as that of the SAR system.

A Titan polar orbiter could use improved SAR and a dedicated, wider-field 2-micron camera (probably along with cameras at a few of the other spectral windows) to map most of Titan's surface, just as the Europa Explorer is supposed to do for Europa. It could thus provide far more data on Titan's surface, which Cassini has revealed as amazingly varied -- both in physical features and in composition. And it could also observe Titan's complex, highly changeable meteorology continuously for years. (Just as one example, it now seems that liquid methane rains copiously onto whichever of Titan's poles is undergoing winter, filling up huge methane lakes -- at the same time that the lakes at the other pole are completely drying up. And one of the big questions is just how rare methane cloudbursts are in Titan's equatorial-latitude "desert" zone.

We know they do occur there occasionally -- the Huygens probe, after all plopped down in one of the muddy methane "playas" left behind by such a cloudburst at some unknown earlier time -- but do a few of them occur there every spring and fall during Titan's 29-year seasonal cycles? Or are they much rarer, perhaps limited only to those rare occasions when a water-ice volcano on Titan belches a major overload of methane into the air?)

Such a Titan orbiter might be a good deal cheaper than the Europa orbiter. Its electronics would not have to endure a viciously intense radiation environment, as in the Jupiter system. And it might be far lighter than the Europa orbiter (and thus launchable by a cheaper booster) if it could brake itself into orbit around Titan not with oversized fuel tanks, but with an "aerocapture" system like the one I described in an earlier chapter -- with the craft, upon first arriving at the Saturn system, skimming through Titan's upper atmosphere behind a heat shield and thus slowing itself in a single step from orbit around the Sun to orbit around Titan.

Titan happens to be the easiest world in all the Solar System to do such an aerocapture, because of its dense atmosphere coupled with its low gravity, which makes that atmosphere tower up to an amazing height above Titan's surface and tapers off in density only slowly, making any navigational error by the probe during its initial plunge through Titan's atmosphere much less dangerous.

Another possible cheap Titan mission could be a stationary lander to analyze the very complex organics and other substances on Titan's surface, which may be of genuine biochemical interest -- something the Huygens probe was not equipped to do despite the fact that it sent back telemetry for 72 minutes after its landing -- as well as photographing the local landscape. Such a lander would just plunge into Titan's atmosphere immediately on arrival, after detaching a small rear cruise stage that would have allowed it to fly from Earth to Saturn (like the cruise stage on the 2003 Mars rovers).

But there's a Catch-22 associated with such a lander mission. The study group has concluded that the dazzling amount of variation in Titan's surface means that such a mission could be scientifically worth the cost only if it consisted either of several short-lived landers touching down on different types of Titanian terrain, or if it was a single lander equipped with a small RTG nuclear power source allowing it to operate for months or years on the surface and thus also obtain seismic data on Titan's internal structure and level of activity, as well as on the surface weather.

Unfortunately, Cassini is unlikely to be able to provide enough data on Titan's surface even to pick out the best possible landing site for a single lander -- and the multiple-lander mission would certainly bust the billion-dollar cost cap, while the science return from even a long-lived single lander would be somewhat limited.

So the one other type of low-cost Titan mission that the group thinks might very be worthwhile is, instead, a balloon -- which, unlike the more ambitious Titan Explorer balloon, could not vary its buoyancy to land repeatedly on the surface and sample it. Instead, it would just be blown along a few kilometers above the surface for months by Titan's gentle winds at that altitude, doing an aerial survey of thousands of km of the surface at very high resolution.

The balloon vehicle (like the alternative Titan lander) would be delivered to Titan by a small detachable cruise stage, and would then communicate directly back to Earth throughout its flight (maybe with the aid of a big antenna built directly into its balloon envelope). To do so, it would be dropped into Titan's atmosphere fairly near whichever pole was tilted toward Earth at the time, and would then be blown around and around Titan by its mostly east-west winds -- although at some point the lesser north-south winds would probably pull it around to the hidden side of Titan and thus restrict its data flow. (It's possible that this same technique could also be used to lower the cost of the bigger Titan Explorer balloon mission by eliminating its Titan-orbiting com relay.)

Like a long-lived Titan lander, this balloon would require a small RTG nuclear power source, but the study group seems to think at this point that its total scientific return would be more valuable than that for a one-point lander. It would, of course, carry weather sensors (including for clouds and rain), and cameras to photograph the underlying landscape with a resolution of only a few centimeters.

Like a Titan orbiter, it could also carry a near-IR spectrometer to provide some data on the composition of the underlying surface, although the usefulness of the instrument would again be seriously limited by the fact that sunlight only punches down through Titan's methane-ridden air and smog in half a dozen narrow spectral bands. All this would help us pick out promising types of terrain for any future Titan lander (or multiple-landing balloon) to touch down upon.

And it could also carry a long-wavelength radar sounder to probe several hundred meters down below Titan's surface, gauging the depth of methane lakes and Titan's vast fields of cryogenic sand dunes, looking for any subsurface "water table" composed of liquid methane rather than water, and examining the subsurface structure of possible water-ice volcanoes. Scientists would love to be able to do this for Titan from orbit, as with Mars and Europa, covering much broader areas of the surface -- but there is a unique problem preventing such subsurface radar sounding from being done from orbit.

That same towering Titanian atmosphere that makes aerobraking so easy at that world will also drag any spacecraft out of orbit around Titan in only a month or so unless it orbits more than 1200 km above the surface -- and at that altitude, the horizontal footprint of even the narrowest possible radar-sounder beam spreads out so much that the echoes from near-surface features off to either side of the spacecraft's path will get seriously mixed up with the echoes from the deeper layers directly below the craft. So radar sounding at Titan must be done from an aerial vehicle, not an orbiter.

Meanwhile, John Spencer's simultaneous Enceladus team has come up with two possible concepts for exploring that intriguing little moon at less than a billion dollars cost. In my next chapter, I'll describe them -- and also how a single mission might perhaps be able to explore both Titan and Enceladus.

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