My possible combined mission would start as a Saturn obiter making repeated low-altitude flybys of Enceladus, skimming over that little moon's strange eruptive plumes to try and determine what actually drives them -- and whether they are connected to a subsurface liquid-water ocean that might be a valid environment for the evolution of microbial alien life.

After completing this, it would move on to the second phase of its mission by braking itself into polar orbit around Titan, to completely map that big moon's great surface complexity in sharp detail using radar and infrared cameras -- and also to carry out some limited mapping of Titan's surface composition using the half-dozen infrared spectral "windows" through which a Titan orbiter can peer through that moon's methane-laced atmosphere and organic smog to observe its surface.

This might allow it to locate spots where liquid-water "cryovolcanoes" have erupted flows of water-ice "lava" onto the surface -- which later Titan landers can sample to look for evidence of microbiological evolution in Titan's own probable deeply buried subsurface liquid-water ocean.

This mission would require differing sets of instruments to study these two moons of Saturn -- but the same spacecraft could carry out both missions, making it much more cost-effective than flying these two missions as two separate billion-dollar spacecraft.

It would carry out both its major braking maneuvers -- first slowing down into orbit around Saturn, and later braking again into orbit around Titan -- by skimming though Titan's upper atmosphere behind a protective heat shield, slowing itself down by thousands of km/hour with no fuel usage at all.

Actually, it would carry a double-layered heatshield -- with the first layer protecting it not only against the heat of its first brush with Titan, but then against the ice specks that would pelt it at high speed during its brushes through the upper part of Enceladus' plumes. That layer would later be shucked, exposing an undamaged second heatshield for the aerobraking maneuver into orbit around Titan itself.

But there's another possible way to carry out this combined mission -- using a different new technology than aerobraking, and one that may also be applicable to a wide variety of other outer Solar System missions, making its technological development by NASA more cost-effective than the development of aerocapture.

That technology is "Radioisotope Electric Propulsion" (REP), which I've mentioned previously as a possible alternative to aerocapture in carrying out a mission to orbit Neptune and make the same kind of detailed study of that distant moon and its moons and rings that Galileo and Cassini have done for the Jupiter and Saturn systems.

One of these two technologies is flatly necessary for any mission to orbit Uranus or Neptune. That's because those two planets are so distant that -- to reach them in an acceptable time of less than two decades -- any spacecraft must be hurled outwards from the Sun so fast that it will be hurtling rapidly past its destination planet upon arrival, thus requiring a prohibitive fuel weight to brake into orbit around that planet if it tries to slow down using a conventional chemical-rocket engine.

In one version, upon arriving at its "ice giant" planet destination, it screams through that planet's upper atmosphere on a carefully controlled path to aerobrake into orbit around it -- although, to do this for a giant planet as opposed to Titan or one of the small planets, it needs a new bullet-shaped heatshield and a more quickly responsive autopilot system.

In the other version, the craft is equipped with a REP system -- a package of small ion engines powered by a plutonium-fueled Radioisotope Thermionic Generator bigger than those which are already used to power the systems of outer-planet spacecraft, turning out 750-1500 extra watts for the ion engines.

When you consider both total flight time and spacecraft weight, the most efficient way to use a REP system for this purpose turns out to be to use it only modestly during the initial ramming of the spacecraft away from the inner Solar System; most or all of that initial boost is provided by the launch booster itself (or perhaps by a separate solar-powered ion-drive package that would be shut down and ejected after the craft passes through the Asteroid Belt). Instead, the REP system is used mostly to gradually slow down the hurtling craft during the last months of its approach to its distant planet destination.

The original plan was to have it use the very efficient but very low-thrust REP drive to do all the work of braking into orbit around Uranus or Neptune. But this would require a very gradual, in-spiraling approach from the outer zone of the planet's gravitational attraction, millions of kilometers from the planet itself -- an approach that will likely take a extra year before the craft is close enough to start its studies.

By one of the curious paradoxes that riddle the field of orbital mechanics, it turns out that it's actually more effective to instead add a chemical rocket engine with a very small set of fuel tanks to the craft, and briefly fire it at a moment of close approach to the planet to carry out the very last part of braking into an elongated orbit around it -- after which the REP system (and gravity-assist flybys of the planet's moons) can be used to quickly trim that elongated orbit into a scientifically proper one.

Amazingly, the total weight of such an auxiliary chemical-rocket engine and its fuel is hardly greater than the weight of the additional xenon propellant that the ion engines would have to carry to spiral the craft gradually into orbit around the planet -- and the total flight time, before the science can start, is a year less.

But whether you carry out the braking into orbit with just a REP drive by itself or with a combination of REP and chemical-rocket braking, such a system has one advantage that aerocapturing a craft into orbit around Uranus or Neptune does not.

Namely: after its science-observing orbit is achieved, a REP-powered craft is still carrying its package of ion engines, which (if a modest amount of extra xenon propellant is added for them) can allow it to do some very impressive maneuvering around the planet -- even spiraling gradually into orbit around one of the planet's moons, and perhaps even later departing from that moon again to enter orbit around a second one.

An aerocaptured orbiter has to carry a small chemical engine of its own, to raise its periapsis safely up out of the atmosphere as soon as it's slowed down to orbital speed around the planet -- and such an engine can carry extra fuel to allow some orbital maneuvering of its own, as Galileo and Cassini have done -- but when it comes to actual orbital maneuvering, or spiralling into and out of orbit around the planet's moons, the REP ion drive is vastly more efficient and acceptably quick.

Now, a REP-drive Uranus or Neptune orbiter does have two disadvantages compared to an aerocaptured one. First, since aerocapture allows a much more dramatic final slam on the brakes without requiring any fuel weight at all, an aerocaptured craft can hurtle out to its distant target planet faster than a REP-drive craft -- it could reach Neptune in about a decade, two or three years less than a REP craft even with a chemical-engine aid.

Perhaps more important, a REP craft needs plutonium-238 to fuel its RTG -- a fact that enrages some people and unnerves many others, given the possible consequences of a launch accident. It's also expensive.

However, the REP craft as conceived by NASA could, if chosen, use a new "Stirling engine" to convert its plutonium's heat into electrical energy -- fully four times more efficient at this than the thermocouples used by current RTGs, and thus capable of producing comparable power with only one-fourth as much plutonium. (Stirling engines involve moving parts -- a notorious bane of spacecraft -- but the models NASA has already developed have run up to eight years with no problem.)

And -- besides the fact that they can be very easily modified to carry out major maneuvers in orbit around a planet -- a REP-based craft has one other advantage over an aerocaptured one: it's applicable to far more types of outer Solar System missions.

Aerocapture cannot be used at all (obviously) to slow down into orbit around any world without a significant atmosphere -- which, where the outer Solar System is concerned, includes a multitude of scientifically important targets: all of the "Trojan" asteroids that share Jupiter's orbit (and seem to be transitional between the rocky asteroids and the largely-ice comet nuclei and Kuiper Belt objects); those Kuiper Belt objects (including Pluto, which has only a very faint trace of atmosphere); and every moon of every giant planet, except Titan. (Neptune's Triton -- like Pluto -- has a tiny trace of atmosphere which cannot be used for aerobraking, unless a possible complicated system is developed involving a huge inflatable "ballute" as a drag brake.)

And even with the worlds that can use aerocapture, the design of the heatshield for the purpose varies greatly from world to world -- it's pretty easy for Titan, but the four giant planets require a much more streamlined bullet-shaped aerobrake that is much more quickly responsive, to autopilot commands, and the characteristics of their own aerobrakes will differ from world to world.

However, a REP-driven spacecraft can, with only small changes, be used to orbit around or rendezvous with all sorts of worlds -- big and small, with or without atmospheres.

NASA's Glenn Research Center has developed a design for a "standard" REP-drive planetary probe which would weigh about 480 kg (only a little more than the current New Horizons craft heading for a Pluto flyby), carry 50 kg of science instruments (twice as much as New Horizons, which itself carries fully eight experiments), use two ion engines (with a third as backup) powered by an 830-watt Stirling-based RTG system -- and carry out a wide variety of missions.

It would take about five years not just to fly by a Trojan asteroid, but also to rendezvous with and orbit two of them. The same craft, with almost no changes, could instead rendezvous with and orbit a "Centaur" (a Kuiper Belt object that has wandered into the space between Saturn and Neptune), in only a slightly longer flight time -- or with Pluto, or another full-fledged KBO, in only about 14-20 years, depending on its distance from the Sun. (It could reach Quaoar, a big KBO fully 6.5 billion km from the Sun, in about 19 years.)

And such missions are themselves important; the Solar System Decadal Survey ranked a craft just to fly by both a Trojan and a Centaur as an important second-rank New Frontiers mission, and we will need to take a look at a side variety of KBOs besides Pluto and whatever other one or (with lots of luck) two other small KBOs that New Horizons can fly by.

With a very small chemical-rocket braking stage added -- weighing only about 100 kg total -- this craft could also enter orbit around any of the four giant planets and start exploring their moons in an acceptably short time -- if launched by the same Atlas 5-551 booster that launched New Horizons; it would take only about seven years to reach and orbit Saturn (even without any gravity-assist flybys of Earth and Venus to catapult it outwards, as Galileo and Cassini did), nine years to orbit Uranus, or 12 to orbit Neptune.

And all of these missions would require it to carry only 200-300 kg of xenon propellant for its ion engines. If it carried more propellant, after entering orbit around any of the giant planets it could cruise widely around the moon system of any such giant planet, and even enter orbit around one of the moons as a final stage of the mission.

In the case of the "Titan-Enceladus Explorer" that I've mentioned, that final destination could be Titan, which it would finally brake into orbit around using its REP thrusters rather than the complicated rigmarole of a second aerocapture heatshield.

It would instead carry a very lightweight ejectable Whipple shield to protect it from Enceladan ice grains -- and after entering Titan orbit, it could use its thrusters to counter the drag of Titan's extended atmosphere for a long period, thus orbiting closer to Titan than it could otherwise.

It could, alternatively, enter obit around any of the four big but atmosphere-free moons of Jupiter, around Triton, or around any of the smaller moons of the four worlds. In the peculiar case of Uranus -- a planet which lies on its side, with its moons' orbits thus often tilted into a plane which a simple dynamically coasting Uranus orbiter cannot easily match -- it could use its REP drive to tilt its orbital plane to match them.

The Glenn Center foresees the possibility of augmenting this craft with more ion engines and a more powerful Stirling RTG system (up to 1500 watts) to power them, in order to trim a few years off its flight times -- but, strictly speaking, even this would not be needed; all it really needs for any of these missions is appropriately sized xenon tanks (and, sometimes, that clipped-on small chemical braking stage).

While NASA is considering an aerocapture system as one of the five candidates for a new space technology that will be tested in Earth orbit on its "ST-9" mission in 2012, it might be wiser to put aerocapture development on hold and instead concentrate on developing REP, which looks like a very flexible "Swiss army knife" technology for exploring the vast ranges of the outer Solar System economically. It is less efficient in flying bigger spacecraft; but, thanks to the miniaturization of science instruments (as in New Horizons), these are becoming less necessary.

Its one disadvantage seems to be that it does require the launch of plutonium -- but, even here, the Stirling generator massively cuts the amount it would need to carry. The 830-watt version would need only about 11 kg of plutonium -- the same amount that the simpler thermocouple RTG on New Horizons uses to crank out 200 watts. (Cassini carries three of the same units.)

Indeed, a properly efficient and durable Stirling generator seems to be the only significant piece of new technology necessary for such missions -- the ion engines intended for them have already been largely developed.

However -- even without such a system -- there are still a few more tricks we can use to try to make the near-future Solar System exploration program cheaper and more scientifically cost-effective than it currently is. In my final chapter of this series, I'll describe two such tricks -- one applied to Uranus and Neptune, the other to the entirely different fiery world of Venus.

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