So far, they've come up with two possibilities. One would be a Saturn orbiter -- much smaller and with fewer instruments than Cassini, but with those instruments better designed for studies focused on Enceladus. It would brake into Saturn orbit, and then make several gravity-assist flybys of Titan in order to put itself into an orbit allowing repeated Enceladus flybys. it might fire a small burn to make its orbital period synchronous with Enceladus -- say, flying by it every other Enceladan orbit. Or, alternatively, it might enter a "Titan-Enceladus cycler orbit" of the type studied by Ryan Russell, which would allow repeated flybys of both worlds with about one Enceladus flyby for every five Titan flybys.

Cassini has already made three close flybys which revealed the startling facts that Enceladus' powerful water-vapor and ice-particle vents are concentrated near its south pole, are much stronger than expected, and contain virtually none of the expected ammonia (which can tremendously lower the freezing point of liquid water). It is now scheduled to make eight more flybys during the next three years of its mission. During three of these, it will fly over the south polar vents at about 100 km altitude, and skim over Enceladus' equatorial regions at only 25 km above the surface.

Ironically, during these very low dips, it won't be able to obtain any closeup photos of the surface any sharper than those it has already taken. The reason is that, during all its Enceladus flybys, Cassini must fly with its big high-gain antenna dish pointed forward as an impromptu shield against any dangerously large ice specks that it might run into. But its cameras look out to one side. And so it cannot slew during the flybys to provide image-motion compensation to cancel out any of the motion blur that smears each photo of Enceladus' surface during the moment when the shutter is open.

But these close flybys will allow analysis of a much more concentrated dose of the vapor emitted from the Enceladan plumes. Cassini's mass spectrometer was originally designed to analyze the organic compounds in the more concentrated gases of Titan's upper atmosphere -- and so, up to now, its analyses of the more rarified Enceladan plume gases have been rather fuzzy, with an accuracy of only about 2%.

This has allowed it to determine that the plumes' water vapor also contains several percent each of carbon dioxide, methane, and a gas with the molecular weight of either nitrogen or carbon monoxide (which Cassini's ultraviolet spectrometer has found to be mostly nitrogen). But the expected ammonia seems mysteriously absent, although it may exist in smaller traces. And there are tantalizing indications of traces of more complex hydrocarbons -- acetylene, propane, and/or ethylene. Cassini's upcoming lower-altitude flights through the plumes will greatly raise its sensitivity, and allow it to look for other trace gases as well -- including more complex organics.

And they will also allow its "CIRS" infrared spectrometer -- which has a rather fuzzy spatial resolution -- to make much sharper maps of both the width and the actual warmth of the plume vents. Right now, they could be very narrow vents so warm that they must be gushing sprays of actual liquid water into space -- or they could be much wider and cooler patches of ice (or clathrate compounds) that are just warm enough to be violently boiling directly into vapor in the vacuum of space.

But Cassini -- despite the big haul of additional information it will soon give us on Enceladus -- simply isn't properly instrumented to study the unexpected plumes. A smaller, New Frontiers-class Saturn orbiter could do so. It could carry a much more sensitive mass spectrometer to analyze the trace compounds in the plumes' gas in far more detail -- as well as analyzing the ice and dust specks themselves when they slam into a metal target on the craft and are vaporized, as can also be done on comet and Europa probes.

It could, of course, also carry much shaper cameras and thermal mappers with image-motion compensation systems, and an ice-penetrating radar sounder to probe up to a few kilometers below the vents to see whether they are connected to actual pockets of liquid water. Finally, the craft could be equipped with thin multiple-layer "Whipple shields", like those on comet flyby craft, to efficiently shield it against dangerously large ice specks, allowing it to fly much lower over the Enceladan surface than Cassini can.

Such a craft could also use its instruments to examine other targets -- such as Titan during its flybys of that moon. In particular, its mass spectrometer would allow a much more thorough analysis of the exact makeup of the organics that are forming out of methane in Titan's upper atmosphere and then raining very slowly down onto its surface over the eons. (Unfortunately, its radar sounder could not make usable studies of Titan during these flybys -- as I mentioned in the last chapter, spacecraft that fly by or orbit Titan must do so at such a high ltitude that their radar-sounder beams are seriously spread out, which hopelessly fuzzes up any attempt to profile subsurface layers in Titan. Any such sounder must operate from a Titan balloon or airplane instead.) And there is interest in having such an Enceladus craft also make one or two flybys of Dione, where Cassini has found tantalizing hints of a very small outgassing equalling only about 1/300 of that at Enceladus.

The second type of lower-cost Enceladus mission that may be feasible is a craft that would make a single, nonstop flyby through the Saturn system and over Enceladus, and use an aerogel collector pad to actually scoop up particles from the plumes -- after which the craft would return to Earth with the tiny but valuable sample after a trip totalling 17 years, for very detailed ground analysis.

This looks very much like the Europa sample-return flyby mission that I described two chapters back. But it would actually be much simpler, since Enceladus is obligingly spitting particles of its material into space itself, without the need for a big separate impactor spacecraft to crash into the moon's surface and kick up a cloud of debris (as is the case at Europa, which seems to have no active vents, at least during the current "inactive" phase of its complex geological cycle).

As with the Europa sampling mission, the craft would also carry additional instruments: those cameras and thermal mappers, and the sensitive mass spectrometer to analyze the gas and dust particles of the plumes in order to augment the returned part of the sample. (Sounding radar probably wouldn't be worthwhile for a craft that would only make a single pass over Enceladus.) And -- since it wouldn't need a big load of fuel to brake into Saturn orbit -- it could be powered by big solar panels rather than a plutonium-fueled RTG, even at Saturn's distance from the Sun. (In this respect it would be like the New Frontiers Saturn entry-probe flyby mission that I described a while back -- although, as with the Stardust comet sample-return flyby, its solar panels would need Whipple shields on their leading edges to protect them during the Enceladus flyby.)

But there's a problem: the craft would have to fly by Enceladus at fully 51,000 km/hour -- as opposed to only 36,000 km/hour for a Europa sampling mission, and 22,000 km/hour for the Stardust mission. There is very serious question as to whether it would be possible to preserve any complex, biologically interesting organic compounds in ice grains that slammed into an aerogel collector at that high speed, since the sheer heat from the impact and from the friction as each grain plowed through the aerogel might break down such organics. (indeed, as I noted earlier, this could also be a serious problem for even the slower Europa sampling flyby. In fact, one recent study suggests that the organics found in the grain tracks of the returned Stardust aerogel sampler may actually have been manufactured out of the aerogel itself by the heat of impact, making it impossible to identify the comet's native organics in the sample.) So this mission -- potentially spectacular and valuable as it is -- remains a question mark at the moment.

In any case, the possible existence of relatively cheap missions to Titan and Enceladus raises another question. There's currently some worry as to how to fit a mission to the newly interesting world of Enceladus into the previous queue of big future Flagship-class Solar System missions -- and thus some thought about the possibility of turning the "Titan Explorer" Flagship mission that's scheduled to come second in the parade into a combined "Titan-Enceladus Explorer" instead. Could such a Flaghip mission be made by combining two of the lower-cost Titan and Enceladus missions that are now being studied -- such as an Enceladus sample-return craft that would first fly by Titan and drop off a cheap nonlanding Titan balloon, which would then radio its data directly back to Earth after the main craft had left the Saturn system and was on its way back to Earth? Alternately, could one of these cheaper Titan or Enceladus missions be combined with that New Frontiers Saturn entry-probe flyby mission?

This isn't as attractive as it first looks. Remember that the worthwhile part of Dan Goldin's revised planetary exploration strategy is "Smaller But More Frequent" -- that is, if you split up a big mission into two or more smaller ones, you will lose less if a design flaw in the first such mission causes it to fail. Thus there's something to be said for keeping a "Titan-Enceladus Explorer" mission broken up into smaller separate missions to Titan and Enceladus.

However, the advantage from breaking up missions is less for missions to the outer Solar System than for the inner System, for two reasons. First, it takes a long time to fly to a destination in the outer System -- so, if you're waiting to see whether one small craft to an outer planet will finish its mission before you dare to launch another one, exploring that planet will take a very long time. Second, the four giant planets are actually something close to miniature solar systems in their own right. They contain a dazzling array of phenomena to be studied: the weather patterns and magnetospheres of the planets themselves, their extraordinarily varied moons, and their rings -- and many of these phenomena benefit from being observed by several instruments simultaneously, which can't be done if the mission's instruments are spread out among several different spacecraft flown at different times.

On balance, it usually seems preferable not to try to combine two of the newly considered cheaper Titan and Enceladus missions into a single Flagship mission. But there's one possible exception -- one possible way to combine two of these missions which could lead to a single mission with a cost much lower than the combined cost of the two separate missions -- and I'll discuss this in my next chapter.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

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