But the sessions open to the press contained much information on mission candidates under serious consideration, and on the criteria by which a program should be judged -- and this, by itself, revealed what looks like a major change in the form of the future U.S. Solar System program. That change is the rebirth of "medium-class" missions.

There was universal agreement at the Meeting that the "Discovery" Program of Solar System missions costing less than $300 million -- and selected by NASA from batches of ideas periodically sent to it by independent groups of scientists and engineers -- has worked very well indeed.

Its first three missions have been completed with splendid results, the next two (Stardust and Genesis) are already well into their flights, three more are scheduled for 2002-04, and the ninth is scheduled to be picked in just a few weeks from three finalists (including a mission to explore the main Asteroid Belt in detail and orbit two of its biggest members, and an astronomy satellite using planetary transits to provide a statistical survey of the numbers of Earth-size planets existing as inner planets of other stars).

The Bush Administration's spokesmen -- in the same phone call to the Meeting in which they announced plans to cancel both of NASA's next two possible bigger missions to the outer Solar System -- praised Discovery heartily. Even though its missions are pretty clearly limited by cost to the inner Solar System and (at a stretch) Jupiter, it can by itself clearly fill in a huge number of the remaining gaps in our knowledge of the Solar System.

Indeed, the only quibble about Discovery at the Meeting was that we may not be spending enough on it -- there's a strong feeling that the cost cap for each Discovery mission should now be raised from $300 million to $350 million, to cope with the fact that NASA is facing a shortage of "small-medium" boosters capable of launching such missions.

America's manufacture of launch vehicles is overwhelmingly governed, not by the needs of space science, but by the far greater needs of the military and commercial buyers -- and they want bigger boosters, along the lines of the coming Delta 4 and Atlas 5. But these have greatly excess capability for the small Discovery spacecraft.

A threat earlier this year to shut down, in a few years, production of the Delta 2 booster which has been the mainstay of Discovery launches has now been deferred -- but it could still happen later this decade. And the stripped-down "Medlite" Delta that launched five of America's last six deep space probes really is being cancelled -- the launch of the "CONTOUR" comet probe next year will be its last.

So -- unless the U.S. does keep producing a small-medium booster, starts allowing the launch of some U.S. payloads on foreign boosters, or works out a way to consistently carry Discovery craft as piggybacks on multiple payloads -- they'll actually be launched on rockets considerably bigger and more expensive than they need. This bizarre twist is what may soon require that $50 million hike in Discovery's permissible mission cost.

The real danger, though -- as former NASA Associate Administrator for Space Science Wesley Huntress pointed out -- is that, the way things are going, Discovery may soon be the ONLY remaining part of the U.S. Solar System Program, except for the big missions linked with the Mars program and its ultimate goal of returning Mars samples to Earth.

Most of the other missions NASA has been planning for years now -- in which the agency itself would define the mission from the start -- have been huge things costing around a billion dollars each. And it's now clear that the U.S. government simply is not willing to shell out that kind of money more than rarely.

The Cassini Saturn/Titan probe -- a behemoth costing close to $3 billion -- has no future counterpart being planned anywhere; and the Europa Orbiter is now teetering on the edge of cancellation because the Jet Propulsion Laboratory's estimates of its cost have now mushroomed from an initial $400 million all the way up to $1.2 billion. Congress has already ordered its cancellation if it exceeds $1 billion -- and the White House now seems unlikely even to wait that long.

NASA's Solar System Exploration Division director Colleen Hartman said at the Meeting that NASA is no longer willing to tolerate a Solar System program centering around billion-dollar missions launched only once each decade or so -- instead, it wants the Decadal Survey to recommend a "scientifically integrated" program of less expensive missions which would be launched often enough for the agency to be able to present to the OMB each year both a list of new successes and a list of high-priority science goals that have yet to be achieved.

For the same reason, she said that NASA would like to shorten the total flight time of Solar System missions -- a president is less likely to approve a mission that won't reach its target before the end of his second term in office -- and this is one reason why the agency is placing increasing emphasis on deep-space propulsion systems, such as ion drives, that could considerably shorten the flight time of spacecraft to the distant outer planets.

Congress took $32 million out of development funding for deep space propulsion -- but $20 million still remains for it this year, and NASA still wants to greatly expand that figure from now on.

So perhaps the most important new idea to infuse all the committee's deliberations was a virtually unanimous agreement that the U.S. needs to start a new line of "Medium-class", or "Discovery Plus", Solar System missions costing $500-700 million.

The National Research Council's recent list of recommendations for U.S. astronomy research over the coming decade reached exactly the same conclusion: U.S. astronomy satellites are now falling into two extremes of low-cost competitive Explorer missions and high-cost mammoths, and it's time to resume the nation's earlier habit of frequently flying medium-cost space astronomy missions.

Some of the Medium planetary missions could be planned in detail by NASA itself from the start. But many would be picked by a modified version of the competitive Discovery scheme.

In it, NASA (unlike in Discovery) would itself announce the overall target and scientific goal of a Medium mission, but would then put out a competitive Announcement of Opportunity for individual science-engineering teams to propose spacecraft and mission designs capable of carrying out that mission.

Its request last December for competitive designs for the Pluto flyby mission -- with a cost ceiling of $500 million -- was in fact the very first possible mission of this type, and there are rumors that the Decadal Survey Committee would like to fend off the planned cancellation of the 2006 Pluto flyby by recommending that NASA should actually label that mission as the very first of the new Medium-class Solar System missions.

There was some apprehension expressed at the meeting that, given the greater cost of Medium missions, individual teams would be less willing to propose them than with Discovery -- for instance, there are usually over two dozen proposals received for each new Discovery opportunity, but there were only five for the Pluto mission.

Since NASA itself would be setting the overall goal of each such medium mission, though, five proposals are likely to be enough -- and there was also a suggestion to encourage such proposals by turning the current "two-step" Discovery selection process (in which the government itself offers additional funds for further development of the half-dozen best proposals) into a three-step one (with the government reducing the number of finalists at each stage, and perhaps financing the more detailed studies in two of the three stages).

Whether this "semi-competitive" concept for Medium missions is finally accepted or not, there were many very interesting and fresh suggestions being floated for Medium missions at the Meeting.

Some were proposed in "white papers" recently written by various groups of planetary scientists affiliated with the Division of Planetary Sciences (a subbranch of the American Astronomical Society which serves as the nation's central organization of planetary scientists) to explore specific targets in the Solar System, which were summarized at the meeting.

Others were described by Douglas Stetson (manager of the Solar System Exploration Program Office at the Jet Propulsion laboratory) and Stamatios Krimigis - head of the Space Department at Johns Hopkins University's Applied Physics Laboratory, which managed the NEAR asteroid mission and is starting to establish itself as a potential rival for JPL.

Exploration of the long-neglected inner world Mercury is now moving into high gear as the 2004 launch approaches for NASA's Mercury orbiter "MESSENGER", and the more ambitious European Space Agency orbiter "Bepi Colombo" which will be launched to Mercury, carrying two orbiters and a lander, around 2009.

Although the ESA's current money problems now seem likely to force a moderate delay in the latter mission, these two spacecraft between them should do a good job of exploring Mercury for some time to come, and so the Decadal Survey committee has decided to pass on any further Mercury missions for the 2003-13 period.

But then there's Venus. Given the great success of "Magellan", there's been much debate about what to send there as an encore -- especially given the extreme difficulty of using landers to explore its roasting surface. As it stands, we are nowhere near having the high-temperature electronics needed to put long-lived seismometers there, for instance.

There has been much interest in next launching a Venus weather satellite, given its low cost and the surprising number of major mysteries that remain about Venus' atmosphere.

The U.S. has seriously considered launching one as a Discovery mission, and the European Space Agency will decide in a few weeks whether to launch a "Venus Express" (based on a copy of Mars Express) in 2005.

While it now looks as though ESA will lack the money for this, it might be launched later this decade. And Japan has already decided to launch a less ambitious "Venus Climate Orbiter" in 2007, which would focus on using multispectral cameras to photograph Venusian cloud patterns at different altitudes to solve the mystery of the planet's "superrotation", in which Venus' upper cloud patterns whirl around the planet in the same direction that Venus rotates, but 50 times faster.

An American or European Venus weather satellite would carry as its most important instrument a near-IR imaging spectrometer to take advantage of the fact that near-IR light in many wavelengths actually pierces part of Venus' cloud layer to reveal the different cloud patterns, winds, temperatures and trace gases underneath -- and some wavelengths even pierce all the way to the surface, allowing such a craft to look for active volcanoes, map some surface minerals, and perhaps even detect ripples produced in the thick lower air by earthquakes. It would also use other IR or microwave instruments to profile the higher-altitude weather, and perhaps also study upper atmospheric processes.

But the new white paper by the Division of Planetary Sciences actually ranks such a craft a bit behind another cheap mission: an entry probe to analyze Venus' numerous and important trace atmospheric gases in greater detail than ever before - with some possible surface imagery on the way down.

Where more expensive Medium-class Venus missions are concerned, however, there are several new possibilities. The DPS also recommends a follow-up to the Magellan radar orbiter -- which mapped all the planet at 1-kilometer resolution -- that would map it at a really sharp 25-50 meters and use either stereo or interferometry to build an equally sharp topographic map.

And, at the Decadal Survey meeting, Doug Stetson described a sort of half-billion-dollar "Venus Pathfinder" that would both analyze the surface at one spot and test some of the technologies needed for an actual sample-return mission from Venus' surface.

The latter, as you might expect, would be a gigantic undertaking, costing billions of dollars, and unlikely to be funded for decades yet.

Separate U.S. and European groups working on its overall design have found that the best technique is to land a craft briefly on the surface to drill up a 1-kg sample, then inflate a helium balloon made of the super-tough plastic polybenzoxasole (which doesn't soften at all in Venus' savage surface heat) to loft itself back into the cool cloud layer, from which it would launch the sample canister into Venus orbit on a 3-stage solid-fuel rocket. The canister would be then retrieved in Venus orbit by a separate orbiter, before heading back up the gravity well to Earth.

Stetson's "Venus In-Situ Exploration (VISE)" mission would test some of this. It would land a small craft using a skirt-like "drag brake" in Venus' thick air (taking photos during and after landing), hastily drill up a surface sample into a small airlock -- and then, after only an hour on the surface, inflate a small balloon to loft it above any surface hills and then a bigger one to soar back to the much cooler clouds 60-70 km up. There it would float for about 3 days, analyzing the surface sample at its leisure by using some instruments that require lengthy analysis times.

Scientists would like VISE to "age-date" the Venusian rock -- determine the time at which it last condensed out of lava -- to within about 250 million years, given the current theory that Venus' entire surface may have been completely engulfed by lava during an age of catastrophic eruptions half a billion years ago.

Age dating is hard to do, but some small instruments have now been developed that may be able to do a fairly good job of it without returning a sample to Earth. One of these -- which compares rubidium and strontium isotopes -- would be carried on the proposed "Urey" Mars Scout mission, and could also be used on VISE.

For Want Of A Full Moon

Moving on further from the Sun, there's our own Moon. The very cheap Clementine and Lunar Prospector orbiters did a superb job of starting to map the composition and the magnetic and gravity fields of its entire surface, and the low-cost European "SMART-1" in late 2002 and the much bigger and more ambitious Japanese "Selene" in 2004 will follow up.

But there seems to be disagreement about what to do next. One obvious target for the next unmanned Moon lander is the gigantic "South Pole-Aitken (SPA) Basin" on the farside -- the biggest impact crater known in the Solar System, fully 2600 km across and 12 km deep, but not firmly known to exist until 1990.

It seems to have punched all the way down through the Moon's crust into its mantle, and thus contains perhaps the deepest rock samples we can hope to access for a very long time.

It's also the oldest surviving impact crater on the Moon, and so determining its formation age could tell us whether there really was a brief but "cataclysmal" increase in the number of impacts in the Solar System about 3.8 billion years ago (possibly due to the migration of Uranus and Neptune into the outer System) strong enough to obliterate all earlier features on the planets and moons.

But the DPS white paper on the Moon, oddly, deemphasizes an unmanned sample return mission from the SPA Basin, claiming that it would be too hard to distinguish which rocks came from which depths, and from which different-aged lunar impacts, without a full-scale manned landing there by geologists.

Instead, the DPS recommended that the next small unmanned Moon lander touch down on one of the apparent deposits of ice located by Lunar Prospector at the Moon's pemamnently shadowed poles, and analyze it in-situ to confirm that it IS ice and what its detailed chemical composition is.

However, that DPS community panel was chaired by Paul Spudis, who has long been a fervent advocate of manned planetary exploration -- and other scientists claim that an unmanned mission to the SPA Basin is a very good idea. One unchosen proposal in the last round of Discovery selections, "Moonraker", would touch down there, simply scoop up about 1 kg of pebbles from its immediate vicinity, and then rocket back to Earth.

But Stetson described a more ambitious, Medium-class SPA lander. Its orbiter section, before the landing, would fly past the Moon and hang itself at the L-2 Lagrange point 65,000 km above the Moon's farside to serve as the lander's radio relay to earth. The lander itself would drill up samples from 2 meters depth, and also dispatch a rover to collect carefully chosen samples for about two weeks.

Then the lander would launch 4.6 kg of samples -- but, to save fuel weight and thus the mission's launch cost, the sample container would NOT be launched back to Earth. Instead it would just be boosted barely to lunar escape velocity, leaving it in an Earth orbit just beyond that of the Moon -- and the orbiter would leave the L-2 point, rendezvous with and collect the sample canister, and then use its own engine to return to Earth.

In today's report, I'll skip Mars -- as America's Mars program, now costs as much as all the rest of its Solar System program combined and is very much a separate entity within today's space program.

Indeed, the Decadal Survey Steering Committee was not given responsibility to recommend the form of that program. But it was given a detailed review of the current form and recent developments of the Mars program -- and the separate NASA advisory committee assigned to review that program has just published their appraisal of its scientific appropriateness. So I'll deal with their revelations later, in the last part of this series.

Moving out to the fringes of the inner Solar System: Both Stetson and Krimigis, in their presentations, gave great attention to medium-class missions to asteroids and comets, with asteroid missions being foreseen in both the Discovery and Medium classes.

One recent, but unsuccessful, Discovery proposal - "Hera" saw a return in an expanded form. Hera as initially proposed would have used a solar-powered ion drive to rendezvous with three separate near-Earth asteroids, hover just above the surface and use an extendable boom to collect 300 grams from each asteroid and return to Earth within 5 years. But not only is it still a very promising candidate for the near future, but higher-cost versions of it, extending the spacecraft's range with more efficient ion drives, would be able to collect samples from any near-Earth object or Main Belt asteroid.

Interestingly, one target high on the list would be Eros, the same asteroid studied by the NEAR spacecraft. Samples returned from several preselected types of surface features on it, when combined with NEAR's detailed map of the entire asteroid, could tell us firmly whether our current geological interpretations of NEAR's data about Eros are correct, and thus allow similar interpretation of photos and in-situ data from probes to other asteroids.

The DPS panel on Main Belt asteroid exploration recommended a mission to rendezvous with Vesta and perhaps also Ceres -- just what the current Discovery finalist mission "Dawn" would do.

But its only other recommendation for Main Belt asteroid spacecraft this decade is that we start work on the technology for a swarm of hundreds of tiny super-miniaturized probes that could be dispersed at some later time to fly by or rendezvous with great numbers of asteroids -- since, given the differences in asteroid makeup, we must look at a great number of them to fully understand the flotsam and jetsam of Sol.

As we move into the vast reaches of the outer Solar System, missions naturally become more expensive -- and some of the billion-dollar class spacecraft NASA has envisioned for that purpose still look necessary at some point. But recent ingenuity has also allowed a reduction of cost in some aspects of outer system exploration that could bring them within the realm of Medium-class missions. In my next report, I'll bring you details on some of these proposals.

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