Indeed, Doug Stetson - Solar System Exploration Program manager at the Jet Propulsion Laboratory - proposed just such a concept in a talk to the Committee. However, a reexamination of Stetson's presentation makes it clear that in that last piece, I seriously overstated the ease and cost of such an ion-drive Pluto mission - and that I did so because of a rather puzzling clash between Stetson's oral presentation and some of the information actually contained in his printed material accompanying it.

Specifically - while Stetson correctly said that the same "NSTAR-1" ion engine successfully used by the Deep Space 1 probe could be used to propel a Pluto probe - he did not make it verbally clear that such an ion-drive module would actually require multiple ion engines, as well as an entirely new, very lightweight and efficient solar-panel array to power them.

The fine print in his charts accompanying the talk shows that - while Deep Space 1's solar array generated only 2.5 kilowatts of power at Earth's distance from the Sun (45 watts of power for each kilogram of the array's weight) - the solar array necessary to launch a 350-kg probe on a 10-year voyage to Pluto would have to generate at least 15 kilowatts of power, with an efficiency of 170 watts per kilogram of array.

Such a system would presumably be very similar to the ion-drive module planned for the cancelled Deep Space-4 comet rendezvous mission, which would have carried four NSTAR-1 engines powered by a revolutionary new lightweight inflatable 12-kilowatt solar array. On that mission, only two of the engines would have been fired at a time (with the other two as backups) until the probe was 420 million km from the Sun, after which the array would produce only enough power to fire one engine out to 525 million km from the Sun before the module had to shut down completely.

While, as was pointed out at the Meeting, much of the new technology needed for Deep Space 4 was developed before it was cancelled, this still makes it clear that a good deal more spending will be needed to develop a Pluto ion-drive module - perhaps $100 to $200 million. And, of course, while such a module would also be useful for other Solar System missions later, its development will also force a further delay in a Pluto probe's arrival at the planet, perhaps seriously reducing the probe's scientific value as one of Pluto's poles tilts more into permanent shadow and its atmosphere starts to freeze out onto the planet's surface.

Another datum in Dr. Stetson's printed materials clashed oddly with the earlier statement to the Meeting by two OMB officials that the Bush Administration wants to cancel the 2006 Pluto probe because, after the initial $30 million provided for it by Congress in NASA's Fiscal Year 2002 budget, a sudden huge lump of fully $200 million in funding would be necessary for it in FY 2003 to get it launched on time.

Stetson's graph of JPL's cost estimate for a Jupiter-assisted Pluto probe indicates that its actual cost in FY 2003 would actually be only about $120 million - which is in agreement with the cost estimates for the "New Horizons" 2006 Pluto probe concept just selected by NASA for further feasibility study after the Decadal Survey Meeting had ended. The reason for the different cost estimate given by the OMB officials is unknown to this reporter. At any rate, the justification for launching the Pluto probe in January 2006 now looks a lot stronger to me than it did at the time of my last report.

Indeed, such a Pluto probe would fit in very well with the fact - made clear during the Decadal Survey meeting - that NASA is now leaning strongly toward starting a new line of "Medium-class" or "Discovery-Plus" Solar System missions costing $500 to $700 million each, as opposed to its current program combining relatively cheap Discovery planetary probes with occasional billion-dollar missions that now find it much harder to be funded.

Many of these Medium-class probes would have their overall scientific goal announced by NASA but would then be put up for competitive bidding by independent teams that would design the spacecraft and mission operations plan themselves. This is exactly the procedure NASA followed last year in issuing an Announcement of Opportunity for Pluto probe designs, with a cost cap of $500 million on the mission. Thus a 2006 Pluto probe (or a later ion-powered one) could easily become the first mission in the new Medium program line.

At any rate, if such a 2006 Pluto mission IS finally launched, NASA has just announced that it will take the form of the New Horizons probe, proposed by a team led by Southwest Research Institute's Alan Stern in collaboration with the Applied Physics Laboratory of Johns Hopkins University and Ball Aerospace. APL - which successfully flew the NEAR probe to the asteroid Eros - is already scheduled to launch the Contour flyby probe of several comets next year, and the Mercury Messenger orbiter in 2004.

This reporter was correct in suggesting last year that a Pluto probe could be based largely on the existing design and systems of either the Stardust comet probe or Contour. There were five proposals offered to NASA in response to its Announcement of Opportunity. Of the two finalists, POSSE (offered by a team involving JPL and Lockheed Martin, and led by the University of Colorado's Larry Esposito) was indeed based mostly on the Stardust design, while New Horizons is based mostly on Contour - although its triangular body looks nothing like Contour's drum-shaped one.

It's impossible for an outsider to know the standards on which NASA's appraisal committee finally selected New Horizons over POSSE - presumably an estimate of how likely each team would complete the probe on time and within budget was the main factor. The two proposals were strikingly similar in general details. They both involved direct launch of a spacecraft from Earth to a gravity-assist flyby of Jupiter; a total cruise time of about a decade to Pluto; a flyby of it at a range of several thousand kilometers; more distant but still extensive observations of Pluto's oversized and equally interesting moon Charon (including radio occultations of Earth and ultraviolet occultations of the Sun by both Pluto and Charon, to study their atmospheres); and a 4-year extended mission in which at least one smaller Kuiper Belt object - and probably more - would then be flown past by the spacecraft for similar study.

One thing that strikes one immediately on looking at these two probes is how cost-effective they are, compared to JPL's earlier proposal for a "Pluto-Kuiper Express" using unnecessarily miniaturized technology - a mission that was finally dumped after its cost estimate exceeded $600 million. Both replacement probes would have cost only about $420 to $500 million - but they would produce far more scientific return. PKE would have carried 15.5 kg of science instruments, as against 24.5 kg for POSSE and 22 kg for New Horizons. PKE would have recorded only 1 gigabit of data during its Pluto flyby, whereas POSSE's recorders could contain 34 Gbits, and New Horizons fully 96 Gbits (although current plans call for it to record only 16 Gbytes). And their communications bit rates are about twice as fast as PKE's - 760 bits per second in the case of New Horizons, which is much slower than Voyager's or Cassini's bit rate but still quite capable of sending back all that recorded Pluto data during the year after flyby.

This extra capability, in turn would greatly increase their scientific return. PKE would have carried a package of 4 experiments - a camera, near-IR mapping spectrometer, UV spectrometer, and radio occultation experiment - to deal with the "Category 1" science goals regarded as crucial by the Pluto mission's original scientific design team: the surface features of Pluto and Charon, mapping of the composition of their ices and other surface compounds, and analysis of the composition and physical structure of Pluto's faint but very interesting atmosphere.

Again with Posse and New Horizons similar instruments would be carried, but with much better data capabilities, capable of dealing with the Category 1 questions in much sharper detail, and also dealing with all the committee's "Category 2" science questions and even with all or almost all of the "Category 3" questions. (The only exception is that they lacked a magnetometer capable of looking for any unlikely magnetic field for Pluto or Charon their designers finally decided that this sensor wasn't worth the cost.)

Their lists of optional instruments did contain some differences. They both carried solar-wind and high-energy radiation sensors, but POSSE would have carried an imaging IR radiometer to map Pluto's and Charon's surface temperatures in sharp detail, an ion mass spectrometer to directly analyze gases escaping from Pluto's thin atmosphere, and a sensor to measure the distribution of fine dust particles throughout the Kuiper Belt. New Horizons lacks those - although a recent scientific advisory group has urged that a dust sensor be added to it - but it does carry one important extra instrument I'll describe shortly.

At any rate, the details of the 2006 New Horizons mission are as follows. Its flight time to Pluto would depend on whether it was launched by an Atlas 5, - in which case it would reach Pluto in July 2017 - or a heftier Delta 4, which would cost $30 million more but allow it to reach Pluto 12 months earlier. In either case, the probe would be given a final boost by a STAR solid kickstage attached to its base, and fly by Jupiter only 14 months later.

The Jupiter flyby for a 2006 launch to Pluto would be moderately distant, but much closer than Cassini's - about 2.75 million km away for a Delta launch, and 3.6 million km for the slightly slower Atlas launch. (Callisto orbits about 1.9 million km from the planet.) This would allow New Horizons to make short-duration but detailed observations of Jupiter's weather patterns, as well as observing Jupiter's magnetosphere and (from a distance) Io's volcanic activity. Indeed, a workshop to plan useful science observations to be made of Jupiter and its moons will be held in 2003.

The spacecraft would then simply cruise all the way to Pluto - traveling most of the time in "hibernation" mode in which it would roll slowly around its axis for stabilization and communicate with Earth only about once every two weeks.

It is powered by a 219-watt "F-5" nuclear RTG - in fact, the only RTG the U.S. currently owns that is actually fueled with expensive radioactive plutonium-238.

The U.S. owns a second, larger "E-8" RTG - but, to fly the more ambitious Europa Orbiter mission (which needs two RTGs) the U.S. must either resume its own manufacture of Pu-238 or buy it from the Russians at $2 million per kilogram. And to fly both the Pluto and Europa missions, it must both build and fuel one more new RTG.

As New Horizons finally approached Pluto, it would start making an impressively detailed set of science observations, much better than Voyager 2 made of Neptune's moon Triton. Its main instrument package is "PERSI" (Pluto Express Remote Science Investigation), provided by Southwest Research Institute and Ball Aerospace, which - as mentioned - includes a 4-color camera, near-IR mapping spectrometer and UV spectrometer, all impressively miniaturized into a 10-kg package. The other primary instrument is "REX", the radio science experiment from Stanford University used during its radio occultations by Pluto and Charon to obtain more data on Pluto's atmosphere and any possible (though unlikely) atmosphere Charon may possess.

But New Horizons also carries two more experiments, to contribute more and better data beyond the minimal Category 1 requirements. One - from Southwest Research Institute - is "PAM", which combines a solar-wind plasma analyzer and a high-energy radiation detector. The other is "LORRI" (Long-Range Reconnaissance Imager) from the Applied Physics Lab - specifically added to cope with one problem the Pluto mission has always had. Scientists would like to get as good a map as possible of Pluto's entire surface, since it will be some time before they go back and one of the planet's poles is gradually moving into very long-term full-time shadow as it moves around the Sun. But Pluto rotates only once every 6.4 days - so, if you can only afford one probe, its best photos of the side of the planet that it will not fly by will be taken days before it reaches the planet, at a great distance. (The same problem applies to Charon, which revolves around Pluto at the same 6.4-day period.)

LORRI is a separate black-and-white camera with a much longer telescopic focus, so that it can take photos of Pluto's and Charon's "other" side 3.2 days and 8 million km out with a resolution of only 40 km per pixel. With it, the craft will be able to start returning pictures better than the fuzzy views taken by the Hubble Space Telescope when it is still fully 5 months from the planet. This is one instrument not carried by the proposed POSSE Pluto probe, and it may have been one factor in the rejection of that proposal.

During the last stage of the approach, the instruments on PERSI will also begin mapping Pluto's surface - including MVIC, a 4-color camera (red, blue, green and the near-IR band that identifies methane frost), and LEISA, a miniature near-IR mapping spectrometer copied after one on the current EO-1 Earth resources satellite and covering the 1.25 to 2.5 micron spectral range. The third PERSI instrument - ALICE, a UV spectrometer copied after one that will be flown on Europe's 2003 Rosetta comet probe, covering 500 to 1850 angstroms spectral range - will start observing Pluto's airglow to analyze its atmospheric composition.

When the probe is still 12 days out, these instruments will all start mapping Pluto and Charon every half-day with slowly growing resolution - but the busy main phase of the encounter will occur during the 24-hour period centered around closest approach.