I concluded that most of those billion-dollar-or-less Titan and Enceladus missions would be better off being kept separate, because they really don't save much money by being combined into a single bigger mission -- and the scientific loss from losing such a mission because of a design error would be bigger.

But there's one possible exception: one pair of proposed cheaper Titan and Enceladus missions that might be combinable into a single bigger mission with a considerable total cost savings.

That possible combined mission would be a craft that:

(1) Skimmed through Titan's upper atmosphere on its initial arrival at the Saturn system in order to brake into orbit around Saturn;

(2) Retained its heatshield for several months while making repeated Enceladus flybys, observing that moon with instruments peering sideways from its rear section beyond the heatshield's edge;

(3) Finally skimmed by Titan again to brake into a polar orbit around that moon, and then

(4) Ejected its heatshield to map Titan, using some additional instruments peering out of its front.

Such a mission could use the same spacecraft for both the Enceladus and Titan portions of its mission -- something that isn't possible for any other combination of the lower-cost Titan and Enceladus missions being currently mulled over. And thus it might be able to save a considerable amount of money as compared to flying the Enceladus Multiple Flyby and Titan Orbiter missions separately -- its total cost, with luck, might be as low as $1.5 to $1.7 billion.

There are two possible technical difficulties with this mission. One is that there is only a limited degree of overlap in the two instrument payloads desired to observe Enceladus and Titan.

The sensitive mass spectrometer used to analyze Enceladus' plume gas would certainly be very useful for further analysis of Titan's organics; but a mass spectrometer to analyze the composition of Enceladus' solid plume ice particles -- by analyzing the puffs of vapor when they slam into a metal target on the spacecraft -- would probably be useless at Titan. (However, it might be possible to use the same single mass spectrometer for all gas and dust analyses at both worlds.)

The radar system used to examine the two moons would be totally different. The subsurface radar sounder used to probe beneath Enceladus' south polar ice for water pockets would, unfortunately, not be usable at Titan, for the unique reason I've mentioned previously: any Titan orbiter, to survive for any length of time, must orbit at 1200 km or more to avoid friction from the upper fringes of Titan's amazingly extended atmosphere -- and the horizontal spread-out of the footprint for the beam of any subsurface radar sounder at that altitude will be so wide that it will hopelessly scramble and blur the depth profiles of any subsurface echoes it detects.

And the radar that a Titan orbiter would use -- "Synthetic Aperture Radar" like Cassini's, to provide clear 2-D surface images of surface terrain features despite Titan's blurring atmospheric haze -- is not necessary at Enceladus.

And as for visual and infrared cameras: those useful at Enceladus would be either relatively short-wavelength visible cameras to take sharp photos of surface features, or a long-wavelength (10-20 microns) infrared mapper to observe the exact size and temperature of the vent spots.

But the most useful one at Titan would be an infrared camera working at about 2 microns -- which is the wavelength of that one of the half-dozen spectral "windows" through Titan's haze and the methane in its atmosphere that allows the clearest images of its surface. (Cassini's images of Titan's surface taken by its "VIMS" mapping spectrometer at that wavelength are far sharper than the seriously fuzzy views taken at the 1-micron window by its regular "ISS" cameras, although they cover a much bigger area.)

There are some other spectral windows that might also be useful at Titan, to obtain more mapping of differences in its surface composition -- notably at 5 microns, and at several points between 1 and 2 microns.

But their usefulness at Enceladus could be limited, although a VIMS spectrometer could probably provide some useful surface-composition data there too. (Water ice -- which almost purely makes up Enceladus' surface -- is dark at the 2-micron wavelength, making it much harder to see fine surface features there.)

As for the longer-wavelength IR thermal mapper camera used for Enceladus' vents: it could not punch through Titan's haze to the surface -- but it might provide useful meteorological information on the temperature and altitude of Titan's occasional liquid-methane clouds (which even seem to occasionally take on the form of flat-out thunderstorms dropping methane rain), as well as the temperature patterns of Titan's upper organic-smog haze.

The second problem for this mission is preserving the integrity of the craft's aerobraking heatshield between its first and second fiery skims through Titan's upper atmosphere, during the months when the craft would deliberately make repeated very low-altitude flights through the plumes over Enceladus' south polar vents at a speed of 14,000 km/hour or so.

Such a craft would ordinarily need some kind of light but multiple-layer "Whipple shield" -- like the ones on craft that make high-speed flybys of comets -- to avoid being fatally shot by any large particle of plume ice (as small as a millimeter across) that it ran into at such high speeds.

In the case of our combined-mission craft, the heatshield still fastened to its front might adequately shield it against such particles -- but in the process the heatshield's surface would probably be pockmarked with little impact craters, making it dangerously unreliable during the second fiery braking flight through Titan's atmosphere.

There's one possible solution to that problem: provide the craft with two heatshields, stacked one behind the other -- with the forward shield serving for protection during both the first skim through Titan's air and the flights through Enceladus' plumes, and then being ejected after the last Enceladus flyby to expose the undamaged smooth surface of the second heatshield during the second skim past Titan -- after which the second shield would then be ejected as well.

(The outer heatshield might be a bit bigger in diameter, allowing a space of several centimeters to exist between the two layers of shield -- which would allow the first heatshield to function as a true "Whipple shield".

These provide very effective shielding against the impacts of ice and dust grains despite being lightweight. Quite a few particles may punch through the first layer; but in the process any such particle itself shatters and vaporizes into a dispersing cloud of fine debris that spreads out by the time it reaches the second shield, and so has very little power to punch through or damage that layer of shield.)

Presumably most of such a craft's instruments would be located on its rear deck, peering sideways at Enceladus around the edges of the double shield (with the cameras using motorized tiltable mirrors to cancel out the motion blur during their fast flybys of Enceladus at an altitude of only 10-20 km).

The big rod like Enceladus radar-sounder antenna would be unfolded from the craft's rear, and later ejected itself before the craft braked into Titan orbit -- while the big SAR antenna for mapping Titan would later unfold from the spacecraft's front end after it had been exposed by the release of the second heatshield.

The craft's actual high-gain dish for radio communications with Earth would be fastened to its rear deck, peering backwards, with the craft slewing around to point the dish at Earth after each Enceladus flyby to relay back the recorded data.

After the final braking into Titan orbit, it might be possible to extend this dish on a tiltable mast to allow it to stay constantly pointed at Earth while the craft orbited and mapped Titan.

There's a distinct Rube Goldberg scheme to this mission concept -- but that doesn't necessarily mean it's impractical. Many spacecraft have had similarly complex designs and missions. And -- to repeat -- even given its complexity, it does seem to be unusually low-cost for a craft capable of carrying out the next desired stages in the exploration of both Titan and Enceladus.

But there is another possible spacecraft model that -- with very few modifications -- might be able to carry out both this combined mission and many other different types of outer Solar System missions, and which would therefore be a better investment than the highly specialized mission I've described.

That is one that would carry out its braking maneuvers with Radioisotope Electric Propulsion, rather than with aerobraking heat shields. In my next chapter, I'll describe this possibility for a "generic" Outer System spacecraft.

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