NASA's schedule of missions to explore the outer Solar System was in serious trouble at the last meeting, and the situation hasn't improved.

Missions to outer Solar System are by nature complex and expensive, and it's now become clear that the funding now scheduled for the program is totally inadequate to launch the series of missions NASA had hoped for during the next decade.

For that reason, NASA's Pluto-Kuiper Express - planned for launch in 2004 to make the first flyby of the last unexplored planet - has had a remarkable on-again off-again career over the last six months.

Last fall, NASA announced that - because both it and the Europa Orbiter mission to follow it were now certain to cost almost twice as much as the Jet Propulsion Laboratory had first suggested - the 2004 Pluto mission would be cancelled, to ensure that there were enough funds to launch the Europa mission in 2007.

Europa is the subject of great scientific interest because of the possibility that it may hold life, but the Pluto cancellation nevertheless stirred up a first-class stink in the planetary science community.

As most planetary science watchers now know, if the 2004 launch opportunity is missed there can be no gravity-assist flyby of Jupiter to catapult the probe out to Pluto.

All other possible techniques to reach Pluto with a later launch would delay the probe's arrival at Pluto for years after its planned 2012 date, making it highly likely that Pluto's thin but scientifically interesting atmosphere would have completely frozen out on the planet's surface by then as it moves slowly away from the Sun in its eccentric orbit.

The SSES members, indeed, were furious at NASA's move - and they and the Division of Planetary Sciences of the American Astronomical Society both issued unusually forceful statements denouncing it, and declaring that the 2004 Pluto launch should be kept on schedule even if it meant that further delays were required for the Europa mission (which can tolerate a launch delay without any loss of science on arrival at Europa).

NASA and Congress responded to this (and to a surprising degree of pressure from the general public) by tentatively rescinding the cancellation in December and announcing that the Pluto mission design would be thrown open to competitive proposals by various scientific and engineering organizations, rather than giving JPL an automatic monopoly on it.

The hope was that a simpler proposal would emerge, using less innovative technologies than the original Pluto Express called for -- which were to be partly and unnecessarily based on technologies developed for the Europa Orbiter. This would then allow a Pluto mission to be flown for under $500 million dollars, while still making sure that the craft would arrive at Pluto by 2020 at the latest.

Last week, however, the Bush Administration unexpectedly announced the re-cancellation of the Pluto mission. In addition, the Solar Probe mission that was to skim by the Sun was also being cancelled for the time being, as like Europa Orbiter it too can tolerate a launch delay without impacting its science returns. Though it does need to be launched inline with the Solar Maximum cycle to enable it to flyby at minimum on one one pass and at maximum on the second flyby.

This news arrived as the SSES committee was meeting and immediately threw the meeting into great turmoil.

But, on reexamination, the news was not entirely hopeless - for the Administration also announced that it intended to sharply increase research spending on "alternative propulsion" systems, such as solar or nuclear-powered ion drives and even solar sail concepts that might allow a Pluto probe to be launched after the 2004 Jupiter flyby window but still race directly from Earth to Pluto fast enough to reach the planet before 2020.

Congress Weighes In
However the next day, both the Republican and Democratic members of the Senate Appropriations Committee stated quite forcibly that they did not necessarily agree with Bush, and did not want NASA to cancel the mission - or retract the already issued Announcement of Opportunity for companies to submit Pluto mission design proposals - until they themselves had had a better look at NASA's budget and the overall situation in a few months.

NASA obliged, and so once again this planetary Perils of Pauline has had yet another eleventh hour reprieve. But it's far from certain what if anything will finally be flown, or whether it will be launched in 2004.

The Bush Administration had intended to cancel it largely to allow for an increase in funding of the Mars program; and with another round of cost overruns again plaguing the Space Station, it seems highly unlikely that NASA's total Space Science budget can be increased by any significant degree. But at least the survival of a Pluto probe is now possible.

Indeed, Drake thinks its overall odds for survival are not that bad - largely because of the planned increase in funding for deep-space propulsion technologies. As such, it may be possible to launch a probe to Pluto as late as 2009 and still reach Pluto by 2020.

There is no doubt that -- by attaching a Pluto probe to a solar-powered ion engine module that would run continuously until it ran out of solar power partway through the Asteroid Belt -- the spacecraft could travel directly from Earth to Pluto in about a decade.

The cost of both the spacecraft and its launch vehicle would be no more than for a 2004 Jupiter-flyby mission design; the only added cost would be for the ion module - and thanks to the successful test of a solar-powered ion engine on the Deep Space 1 probe,designing and building such a module, which could be used for a wide variety of other Solar System missions, now seems to be straightforward given the additional funding now being slated for deep space propulsion technologies.

In fact, one of the three finalist candidates for the next of NASA's low-cost Discovery missions for inner Solar System exploration - the "Dawn" spacecraft that would visit and orbit the big main-belt asteroids Vesta and Ceres - uses a three-engine ion module.

And in February, NASA held a workshop on "Innovative Approaches to Outer Planetary Exploration", in which its Glenn Research Center delivered two talks claiming that a small Pluto spacecraft could use its own RTG nuclear power source to run the Center's newly developed small ion engines - using only a few hundred watts of power - for years on end, and ram itself to Pluto in 11 years at considerably lower cost than that for a solar-powered ion module.

Of course, given the uncertainty about exactly when Pluto's atmosphere will freeze out, the arrival of a Pluto spacecraft must still be as soon as possible - but it appears that the Pluto-Kuiper mission is far from dead.

Drake says that seven design proposals for a Pluto mission will be submitted by the mid-March deadline in response for that Announcement of Opportunity, and he thinks it likely that at least a few of them will be plausible under the new funding regime.

Part Three of this report will be published on Tuesday.

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