The possibly life-bearing moon Europa is so intensely interesting that it will have space missions to itself (as we'll see) - but the other worlds in this miniature solar system have also turned out to be intensely interesting.

Perhaps the most important is Io, which Galileo was originally designed to fly past only once because of the savagely high radiation levels at its orbit, but whose ever-changing volcanic surface makes it of intense geological interest, Galileo has actually ended up making seven Io flybys and gathered a great deal of data - but its instruments were originally designed clear back in 1977, before Io's volcanic nature was even suspected, and they've also suffered some radiation damage. (For instance, its near-IR spectrometer cannot map Io's rock and mineral deposits in closeup detail.)

That vicious radiation - over a dozen times more intense than at Europa - will make any actual orbiter of Io itself impractical for a long time, but a Jupiter orbiter as radiation-resistant as the planned Europa Orbiter could make at least 50 close flybys of Io before failing.

Like Galileo, it would be in an equatorial orbit, allowing it to make repeated close flybys not only of Io, but also of Ganymede and Callisto, which Galileo has also revealed to be far more interesting than initially thought.

All three moons - along with Jupiter itself - could be studied very nicely, despite their widely differing natures, by the same set of cameras, IR and UV spectrometers, thermal mappers and magnetospheric sensors.

However, the craft might start out in a polar orbit around Jupiter - allowing it important new views of the weather patterns, auroras and magnetosphere around Jupiter's poles - and then use its early flybys of Io to tilt its orbit toward the equator in order to slow down the speed with which it flew by Io later.

JPL engineer Thomas Spilker, who has studied this type of mission in detail, has concluded that this "Galileo 2" mission could be done for somewhere around $700 million - expensive, but still much cheaper than the original Galileo.

These Medium-class mission proposals, however, have been floating around for a while. The real problem comes in trying to develop stripped-down, lower-cost versions of the very expensive billion-dollar missions that have up to now been considered critical in exploring the outer Solar System, while still retaining most of those original missions' scientific return.

And the most famous example of the latter is Europa Orbiter (or "EO") - whose cost estimate, optimistically pegged at only $300-400 million a few years ago by the Jet Propulsion Laboratory, has now mushroomed to $1.2 billion.

EO's problem is that the scientific mission that seems necessary for it is also very difficult. It would be the first spacecraft ever put into orbit around a moon of another planet - which will require fully 2.5 km per second of delta-V, even given an intricate series of flybys of other Jovian moons and Europa itself to let the spacecraft almost match orbits with Europa before it finally brakes into orbit around the moon.

Thus half its weight will be composed simply of propellant - and the spacecraft's dry weight must be reduced as much as possible to prevent the need to launch it on a big and expensive booster.

Moreover, its electronics must operate in by far the most hostile radiation environment any spacecraft has had to endure. Galileo, skimming rapidly through the intense inner zones of Jupiter's radiation belts on its elliptical orbits, was designed to endure a total of only 150,000 rads during its two-year primary mission - and while it's endured over three times that dose so far in its 4-year extended mission, it's already showing significant ill effects.

But EO - which must first match orbits with Europa and then permanently orbit that moon in the inner parts of the radiation belts - must endure fully 4 million rads (half of it just during its brief one-month operating lifetime in orbit around Europa). And the thickness of the spacecraft's metal radiation-shielding layer is, in turn, seriously limited by that need to keep weight down.

Thus JPL, which has always been in charge of this mission, has always insisted that sophisticated new electronics technology, both miniaturized and very radiation-resistant, must be developed for this mission - but, of course, that development effort is itself expensive, although it also has application for later missions. And it's hard to think of ways to scale down this mission scientifically without seriously harming its science return.

Indeed, it was made clear at the "Large Satellites" Subcommittee meeting that EO's basic scientific goals are still in considerable dispute. Initially, its main purpose was to simply prove whether or not Europa still does have a liquid-water layer below its surface ice by measuring the slight tidal flexing of the ice, which can only be done from orbit around Europa rather than during flybys.

A "white paper" by half a dozen members of the Division of Planetary Sciences (America's main society of planetary scientists), presented by John Cooper at the Subcommittee's meeting, concluded that this is still EO's most important goal: "The central theme of... missions in the next decade should be to determine whether Europa's ocean really exists, and to what degree. Are we talking about slushy ice, some scattered pockets of brine, or a liquid ocean greater in volume than that of Earth?"

Other scientists at the meeting, however, felt that this question has now pretty much been answered by Galileo itself during its final flybys of Europa, during which it confirmed that the moon has an "induced" magnetic field which is probably generated by Jupiter's own magnetic field flowing through a unified, global subsurface liquid-water ocean.

But EO's other goal - to identify the places on Europa where liquid water is closest to the surface as the best landing sites for later life-seeking and drilling missions - requires very thorough mapping coverage of the moon's surface with an orbiter.

A Jupiter orbiter making repeated Europa flybys - even dozens of them - would be much more limited in its ability to map the moon even with cameras and surface-composition mappers, let alone the deep radar sounder which would be used to try to punch through kilometers of ice to map the underlying liquid water layerand find the spots where the ice is shallowest.

Up to now, determining the makeup of the non-ice substances in Europa's surface, and mapping local concentrations of them, has been considered a secondary goal for Europa Orbiter - but the DPS group report said flatly that it now appears that this may be even more important than the radar sounder in understanding the satellite and picking good landing sites.

Quite apart from the search for organic substances in the ice, we don't even have an answer yet for such a fundamental question as whether the main contaminant of Europa's ice is sulfuric acid or magnesium sulfate (Epsom salt).

One theory which has gained increasing prominence over the past year is that Jupiter's deadly radiation belts may be manufacturing large amounts of free oxygen and other chemicals that could serve as nutrients for Europan microbes, which may then perhaps be transported downward into its buried ocean by the geologically slow churnings of its thick ice crust.

In fact, the DPS panel report suggested that this might actually cause Europa's ocean to be richer in oxygen than Earth's - which in turn raises the astonishing possibility that Europa's ocean might be able to evolve and sustain not just microbes, but large multi-celled animals!

So the exploration of Europa remains a matter of great scientific urgency; the place simply continues to look more and more promising as a possible abode for alien life. But we should never forget just how hard and how expensive it will be to explore Europa.

The Europa Orbiter has been caught between a fiscal rock and a technological hard place, and this year Congress flatly placed a cost cap of a billion dollars on the mission, declaring that if JPL can't design the mission for that price, it will be farmed out to competitive bidders like the Pluto probe.

And the White House then shocked the latest Decadal Survey meeting by telling it flatly that it intends to cancel the mission totally for the time being, even if it doesn't get fiscal competition from a near-term Pluto probe.

However - as with the much easier Pluto mission - some suspicions are now starting to arise that it just might be possible to do EO for a good deal less money than the current JPL design suggests.

As I reported earlier, Johns Hopkins' Applied Physics Laboratory was ordered by the Decadal Survey committee to do an initial cursory study of whether money might be saved with a really startling proposal: modifying the design of APL's "Messenger" Mercury orbiter for the far different mission of orbiting Europa. And the results - presented at the meeting - tentatively suggest that such a mission might be plausible.

In some ways, Messenger is already surprisingly well adapted to the role. While it lacks those revolutionary miniaturized electronics, its body is made of lightweight composite materials, so that - while it weighs only 1100 kg, about the same mass desired for Europa Orbiter - it's already capable of a delta-V of fully 2.3 km/second for its Mercury mission, almost as much as required for the EO mission. And its current instrument payload - 47 kg - is already much more than the 20 kg planned for the current version of Europa Orbiter.

But it has one big problem: its electronics are not designed to endure anything remotely like the radiation dose it would need for the Europa mission. That very cursory initial study by APL suggests that such modifications would probably be possible; but, for some parts of the craft, they would be difficult.

However, there is a possible alternative. Both the current JPL Europa Orbiter and the Messenger-based version that APL studied would be launched directly from Earth to Jupiter - but propelling the craft to Jupiter indirectly with gravity-assist flybys of the inner planets (the technique that Galileo and Cassini both used to get into the outer Solar System) would allow a spacecraft twice as massive to be sent to Jupiter without enlarging the size of the launch booster.

NASA wants to avoid close flybys of Earth with plutonium-fueled spacecraft from now on; but one mission design using three flybys of Venus would get the job done nicely - albeit for an extra 3.6 years of flight time, which seems an acceptable price to pay.

And this, in turn, would allow a Europa orbiter's mass to be increased enough that a much thicker layer of radiation shielding could be wrapped around its vulnerable electronics sections, greatly reducing the need for expensive development of new radiation-hard electronics.

Whether the actual Messenger design is valid for a Europa mission or not, it provides an encouraging indication that some less expensive, less technologically sophisticated version of Europa Orbiter could be designed for a price tag of more like $600 to $800 million.