Also, comets contain complex organic compounds formed in the System's early days - and there is much speculation that impacting comets may have supplied the inner planets with much of their water supply, and early Earth with most of the organic compounds that later evolved into life.

But in-situ analyses --by either flybys or landers on the nucleus - contain serious limits on the sensitivity and variety of such analyses; and while scientists are very much looking forward to the tiny sample of dust and gas that will be collected by the Stardust spacecraft during its comet flyby and then returned to Earth in 2006, it will be both small and seriously modified by the fact that its dust particles and gas molecules will have smacked into an aerogel collector at 6 km/second.

Thus the actual return of a large, intact sample of material from the surface of a comet nucleus is a very high priority. The classic design of the CNSR mission is a bit unusual as "outer Solar System" missions go; it will be examining an object originally from the outer System, but that object will be one of the "short-period" comets that was jockeyed by a chance Jupiter flyby a short time ago into an inner-System orbit with a period of only a few years. Thus this craft doesn't need a nuclear power generator, but can rely on solar cells.

It would, in fact, use an enlarged "Solar-Electric Propulsion" (SEP) module of the same type NASA is currently advocating for the Pluto probe: a module with multiple ion engines, powered by a new, inflatable giant solar-panel array which generates 10-20 kilowatts but is very lightweight. The current mission plan is very similar to the canceled 2004 "Deep Space 4" mission that would have rendezvoused with and returned a sample from comet Tempel 1.

The craft would use its SEP system to match orbits with the comet and go into a slow, very close orbit around its tiny nucleus, then release a separate lander that would lightly touch down on the nucleus (hopping to a second landing site later), drill up a core sample a meter or more long from each site (to make sure it had punched down below the comet's sun-baked and modified surface crust), and then loft itself off the surface to rendezvous and dock with the main craft and pass the sections of core sample to the main craft - which would then use its SEP thrusters to return to Earth and land the samples in a small entry capsule, after a total flight of 7-8 years.

There are variations now being considered on this plan - for instance, given the nucleus' very feeble gravity, the separate lander might be omitted, and the main spacecraft itself might touch down on the nucleus to collect the samples itself (perhaps hovering just above the surface and extending the sampler on the end of a telescoping boom).

This would allow omission of the complex and risky automatic rendezvous and docking; but it would also mean that the craft's solar panels might be seriously coated with the dust naturally jetting off the comet's surface, interfering with its ion drive for the return to Earth. Thus the panels might be designed for temporary refolding during their near-comet period - or the craft might use a separate chemical rocket engine for its return to Earth.

This mission is scientifically important - it's currently ranked Number 3 in urgency among outer System missions, behind the Pluto flyby and the Europa Orbiter. But it's also complex and expensive - the very reasons why Deep Space 4 got the knife.

Its current cost is pegged at about $1.1 billion. And so one intriguing new idea - described by both JPL's Doug Stetson and APL's Stamatios Krimigis during their presentations to the Decadal Survey committee - is to drastically cut its cost by "bifurcating" it.

One of the biggest elements in making CNSR so expensive turns out to be the requirement to keep those comet nucleus samples cryogenically cold during their return to Earth to preserve their ices - which include not only water ice, but other, much lower-temperature ices of such things as carbon monoxide and methane.

But Stetson and Krimigis reported that their agency's study teams had both concluded that these chemically simple ices and simple, volatile organic compounds can be very well analyzed with the improved in-situ instruments available now, allowing both their chemical and isotopic makeup to be analyzed by the lander on the spot without returning them to Earth at all.

Other in-situ sensors would record the way in which the dust-ice mixture of the comet's surface crust changes physically with depth - and after the ices were thus vaporized out of the original sample, the only material that would actually be returned to Earth for super-detailed study would be its mineral dust grains and more complex, non-volatile organics, which don't need refrigeration.

This changed design could apparently, by itself, lower CNSR's cost to the "Medium-class" range of $600-700 million. One in-situ lander instrument described by Krimigis was a miniature laser-equipped mass spectrometer, already developed, that would vaporize ices and organics and analyze them down to a part per million (or, in some cases, a few parts per billion), and which can identify the molecular weights of complex organics of up to tens of thousands of AMUs.

Some other important outer System missions, however, are so complex that they still resist any attempt to cut them down below the billion-dollar range. For instance, the obvious next step in Europa exploration is a lander, equipped with instruments to analyze the moon's surface and look for chemicals indicting the possible presence of life or of "prebiotic" compounds - but even a preliminary, simple Europa lander, described by Stetson at the conference as "Europa Pathfinder" - would be in the billion-dollar range.

Europa Pathfinder would be a 220-kg lander vehicle carried on a main craft modeled after Europa Orbiter - which would release it after two weeks of mapping Europa's surface from orbit to refine the search for a good landing site.

The lander would deorbit itself with a small solid fuel motor, then use a much bigger one to brake itself to a landing speed of about 100 meters/second , after which the 30-kg lander itself would use three airbags to bounce to a stop on the surface.

The lander this time, though, would be a hockey-puck-like disk with antennas around its edge, capable of operating just as well upside down, or even on its edge should it become wedged into a crevasse.

It would use batteries to operate for only one Europa day - 3 1/2 Earth days - although a miniature nuclear RTG could allow it to work much longer. And it would be very extensively sterilized, since a National Academy of Sciences group concluded in 2000 that the danger of contaminating Europa's subsurface liquid water deposits with Earth germs is even greater than the risk of contaminating Mars (although Jupiter's radiation does serve as a powerful natural sterilizer for any spacecraft's outside parts).

Its instrumentation, as its name suggests, would be not so much for a detailed search for Europan life as for an initial appraisal of the nature of Europa's surface that could guide the design of such future bigger landers.

It would use multiple fiber optic ports, gazing out of its hull in different directions, both to photograph Europa's surface in detail and to utilize a laser Raman spectrometer to analyze the compounds on the surface, including organics.

It would also measure the radiation level at the surface, and carry a seismometer to monitor the tidal creakings of Europa's ice crust, and any meteor impacts on it, to determine both the thickness of the ice layer above the Europan ocean (if it's too thick for Europa Orbiter's radar sounder to do so) and the extent to which such tidal stresses might open up near-surface cracks.

But its scientific limitations would be severe - it could not even probe below the top meter or so of Europa's ice in which Jupiter's intense radiation has very extensively modified it chemically, breaking down any biologically interesting compounds into an uninformative organic sludge.

And - given the fact that the cost of this mission would likely be in the $850 to $900 million range - it might be better to simply jump directly to a more expensive but much more informative large Europa lander of the sort JPL has developed a preliminary design for, in which the entire orbiter would lower itself to a soft-landing on Europa's surface and then drill down several meters below the surface to acquire samples for really detailed chemical and organic analysis.

Such a more sophisticated mission would probably be around the $1.5 billion range. But some tentative consideration is even being given to whether -- given both NASA's overall funding limitations and the very long gaps in arrival times between successive Europa missions -- it might be better to simply skip Europa Orbiter completely and jump directly to such a Europa lander, targeted to some particular promising spot picked using the rather limited photos and near-IR composition maps that "Galileo" has already collected.

Even if NASA did jump directly to such a Europa lander mission, it couldn't possibly be launched before 2012 or so -- and the majority consensus is still that we need Europa Orbiter to pick out the best possible landing spot for it, since Europa lander missions will be few and far-between.

But the possibility of such a change in NASA's current plans can't be ruled out -- and the fact that there is still a first-class scientific wrangle going on over even what the first Europa mission's basic scientific goals should be would seem to be a further argument for postponing the official initiation of Europa orbiter, and instead flying the Pluto flyby (whose goals and design are now firnly established) first.

Another billion-dollar mission would be necessary to study Titan in more detail than Cassini and the Huygens atmospheric probe it carries (which may or may not survive after its actual hard landing on Titan's surface in 2005).

Titan, thanks to its shroud of orange haze, is arguably the most mysterious world remaining in the Solar System - more so than Pluto - and Cassini/Huygens' purpose is to provide us with our first decent scientific understanding of this strange world (including such an elementary question as whether it has large lakes or seas of liquid ethane and methane on its surface).

But after those craft have given us a detailed analysis of Titan's atmosphere, and the first decent view of its surface features through the haze layer, a detailed exploration of Titan will still remain to done.

While the design of a Titan lander must remain uncertain until we have that first overall appraisal of Titan's surface characteristics, some initial ideas are taking form - and one basic idea embraced by the Decadal Survey group is that, given the likely vast variation in Titan's surface features, a lander in one place won't suffice.

Its surface may include everything from those liquid hydrocarbon lakes and rivers, to plains where a glacially slow "snowfall" of solid organic haze particles have been raining down for eons to form a layer hundreds of meters deep, to mountain ranges topped with methane frost, to volcanoes erupting liquid water.

Moreover, Titan's biggest point of interest is the fact that - while its cryogenically cold surface temperature almost always keeps liquid water from existing on its surface, and therefore rules out life - complex organic chemical reactions have nevertheless been occurring there in huge amounts for billions of years, providing information which no Earth-based lab ever could on the variety of complex natural reactions that led to the formation of life on Earth.

And this will be especially true in those isolated spots on its surface where either subsurface eruptions or the heat from giant meteor impacts have occasionally allowed liquid water to exist for years before refreezing, and thus allowed still more organic reactions to occur.

So we must have a vehicle ("Titan Explorer") capable of aerial travel, which can both survey the surface from just a few kilometers up with cameras, a near-IR surface composition mapper and a radar sounder to probe the layering of the subsurface, and make multiple landings at the most interesting places it thus identifies.

A powered robot helicopter has been seriously suggested - for, given Titan's strange combination of a surface gravity seven times weaker than Earth's and an air density four times greater, it would need only a tiny fraction of an Earth helicopter's power to stay aloft.

But such a vehicle is probably too complex in design for the first Titan Explorer. Thus mission planners are currently oriented more toward an "aerobot", a balloon which could vary its buoyancy to touch down periodically on the surface, take pictures and collect samples, and then quickly float back into the air to analyze them at its leisure several kilometers up as Titan's high-altitude winds blew it along to the east for a total mission of about a month. (It might be equipped with motorized propellers, like a blimp, to swerve toward new latitudes to the north or south.)

The strangest Titan Explorer concept of all is an "aerover" - a vehicle with three gigantic inflated tires, looking rather like the bicycles in the 1960s TV series "The Prisoner".

It would use those tires as helium-filled balloons to carry itself as a multiple-landing aerobot during the first part of its voyage; then it would finally land again, deflate them, refill them with Titanian air, and use them as motorized wheels to continue its surface studies in a more detailed way as a rover slowly rolling along Titan's surface for another few weeks.

This giant inflatable wheel concept has also been proposed for future Mars rovers; instead of utilizing a complex computerized navigation system to plot a path around boulders, the rover would simply roll right over most of them without stopping.

At any rate, a Titan aerobot - given the very long radio-signal time lag between Saturn and Earth - could not depend on orders from Earth to land when its instruments saw an especially interesting site from the air. It would have to have a great deal of onboard computerized autonomy to identify promising features and phenomena itself from its instruments' aerial observations and land immediately, as close to them as it could manage. And - like the Europa lander - it must have miniature but highly sensitive and flexible chemical analysis equipment to identify as many complex organic compounds as possible.

Titan Explorer will probably also need a small Titan orbiter to serve as a radio relay to Earth, which might also carry more science instruments of its own (although that dense atmosphere, in Titan's weak gravity, towers up so high above its surface that any such orbiter will have to stay more than 1200 km up to avoid burning up in just a month).

The plan would be for the orbiter to separate from the lander just before arrival at Titan, and then "aerocapture" itself into orbit around the moon by deliberately skimming through its extended upper atmosphere, thus braking itself without the need to carry a large load of fuel.

JPL engineer Spilker, however suggested to this reporter that - if we can develop a new, high-speed radio communications link directly between the Titan lander and earth, eliminating the need for such an orbiter - it might be possible to lop the total cost of the Titan mission well down below a billion dollars.

Again, however, any such Titan mission has virtually no chance of being flown before the 2013 date that marks the far deadline for the current Decadal Survey's recommendations. The most important near-term goal for Titan exploration is to provide enough funds to make sure the Cassini spacecraft works as long as possible beyond its current planned 4-year minimal lifetime in Saturn orbit.

Another mission concept would go much farther a field: Uranus and Neptune orbiters. The logical next step after the Galileo and Cassini missions would be similar orbiters to study the two smaller giant planets and their moons and rings in detail.

Neptune, despite being farther away, has always been recommended ahead of Uranus. One reason for this - the high-altitude haze layer that blanketed out Uranus' cloud patterns to the cameras on Voyager 2 - is no longer applicable, since near-IR cameras on Earth telescopes have turned out to punch easily through the haze to reveal Uranian storm patterns as spectacular as Neptune's.

But the other factor still applies: Neptune's big moon Triton, which (having a thin atmosphere and giant liquid-nitrogen geysers) is not only much more interesting than Uranus' moons, but - unlike them - is massive enough that a spacecraft can use flybys of it to drastically shift the shape and tilt of its orbit. Uranus is the only one of the four giant planets to lack any moon big enough to do this; any Uranus orbiter will have to depend almost entirely on its onboard fuel supply to change its orbit.

Unfortunately, any Uranus or Neptune orbiter will have to use the complex and as-yet untested new technology of "aerocapture" to brake itself into orbit around either world.

The reason is simply that - in order to get to either planet in a time shorter than decades - it must be flung out into the outer Solar System very fast by either an ion drive or a Jupiter flyby, which in turn means it will hurtle past either planet much too fast to be slowed down by a simple chemical rocket without a huge, prohibitive weight in fuel.

Thus the spacecraft would have to fly all the way to the distant planet huddled behind a heat shield, and then make a fiery pass through its upper atmosphere to brake by as much as 20 km/second - and that, in turn, requires a pretty sophisticated autopilot system to instantly tilt the craft upward or downward if sensors indicate that it's either decelerating too fast - putting it in danger of plunging into the planet -- or too slowly, which would cause it to fly by the planet without stopping.

Given the desirability of other new technologies for it - such as miniaturized components, and autonomous programming (like that on the Titan Explorer) to allow it to immediately start making observations of interesting new scientific phenomena rather than waiting for lengthy instructions from Earth, Spilker concluded that such a mission would unavoidably be in the billion-dollar range.

The prototype Neptune orbiter his JPL group studied (which also drop two entry probes into the planet's atmosphere) would weigh fully 3300 kg, fly to Neptune in 12 years (with the aid of a 23-kilowatt ion drive), and then have to aerobrake itself by fully 13.4 km/second.

But a simpler nonstop Uranus or Neptune flyby, making brief observations of the planet and its moons like Voyager 2 and dropping off two atmospheric entry probes, could be flown as a Medium-cost mission. A good launch opportunity to reach Uranus by way of a Jupiter gravity-assist flyby (without the need for an ion engine package) opens up in 2015, and a similar one to reach Neptune in 2017.

It's open to question, though - given their much smaller scientific return - whether such a mission would be considered worthwhile, even if it continued onwards to fly by several Kuiper Belt objects. In any case, there is again no doubt that the next mission to Uranus or Neptune won't be flown until the late 2010s at the earliest.

One thing that is abundantly clear is that all the new Solar System missions I've talked about in these articles - Medium-class or bigger, and to both the inner and outer System - will not only benefit hugely from new technology, but in all likelyhood depend on new technologies.

At the November meeting, former NASA Space Science head Wesley Huntress discussed a recent NASA workshop - attended by representatives of all the major NASA branches and laboratories interested in planetary exploration - that tried to list the most useful new technologies for it.

They concluded that the top-ranked one is indeed an improved SEP ion drive system powered by inflatable, very weight-efficient big solar arrays. Whether it's finally used by the first Pluto probe or not, this will certainly have to be developed in the next few years.

Other especially valuable new technologies would be improved nuclear power generators - capable of producing the same amount of power as a current-day RTG with only about one-third the amount of expensive and medically dangerous plutonium-238 - and a laser-based optical communications system which (using a spacecraft mirror only a fraction of a meter wide, and an Earth-based network using mirrors meters wide) could relay back scientific data at hundreds of times the rate achievable by today's radio com links.

Ranked a bit lower, but still very important, would be aerocapture braking (which is currently scheduled to be tried for the first time by France in its large 2007 Mars orbiter). Additionally we need all sorts of new miniaturized spacecraft components and instruments, and complex computer software for autonomous operations.