This would not only allow quick sprints to even the most distant planets without waiting for Jupiter to be lined for a gravity-assist flyby; it would then allow the craft to easily brake itself into orbit around its destination planet without the need for aerocapture - and, even after all that, it could have 20 km/second or so of maneuvering capability left, allowing it to cruise virtually anywhere within the planet's system of moons (and repeatedly enter or leave orbit around the moons themselves).

But NEP - quite apart from the fact that its development would take about a billion dollars by itself - has, of course, the huge disadvantage that people do not look fondly on putting large amounts of radioactive material into orbit where it could sooner or later sprinkle down on their heads.

The relatively small amounts of plutonium-238 contained in the electrical generators of present-day outer planet probes produce more than enough unease already, despite NASA's repeated attempts to assure everyone that there is no significant danger of a launch accident or accidental reentry scattering powdered plutonium into the atmosphere.

Actually, though, an NEP craft's reactor could be made much safer than the current-day RTGs. It could be fueled with uranium-235 - which is more expensive than plutonium, but fully a thousand times less radioactive when not piled up into a critical mass.

And such a craft could be launched, unfueled, into a high Earth orbit from which it would virtually never reenter, and only then be loaded with uranium fuel carried in two or more separate unmanned satellites that would automatically dock with it before it lit up its engines and left Earth. Still, NEP is clearly a project for the fairly distant future where NASA is concerned.

At any rate, there was general agreement at the group meetings that, given the extreme importance of these new technologies for a wide variety of missions, NASA needs to considerably increase its spending on a separate program to develop them - an enlarged version of its current "New Millennium" program - rather than trying to fund their development for any single mission.

JPL currently follows the latter strategem of trying to fund their development only when a specific mission first requires them, thus sharply reducing the odds that Congress will approve that particular mission with its high total cost.

But here the really central problem raises its head: limited funding.

At the meeting of the Decadal Survey's Steering Committee, NASA Solar System Exploration Division director Colleen Hartman set forth a model of the wonderful things NASA could do if it had $935 million per year to spend on Solar System exploration not connected with the planet Mars:

- a $360 million Discovery mission every 2 years, - a $640 million Medium-class mission every 4 years, - an $850 million large mission every 5 years - $155 million per year for Research and Data Analysis; - $70 million for development of deep-space propulsion systems such as SEP, NEP and solar sails; - and $200 million a year for other new technology development and maintenance of the Deep Space network.

There's only one tiny problem: NASA doesn't have $935 million a year to spend on these programs. In FY 2002, it will have only about $500 million. And, even in the unlikely event that NASA drastically shrinks its spending on the Space Station - perhaps by turning it from a permanently manned lab into an unmanned one periodically visited by humans - it's very hard to conceive of non-Mars Solar System exploration ever getting more than $600 million per year.

And - until those new technologies allow mission costs to start shrinking again - Solar System missions will continue to show a steady trend toward becoming more expensive, since most of the relatively cheap stuff has already been done.

That being the case, it's a pretty safe bet that for a long time to come, we will see only two Medium to Large class non-Mars Solar System missions per decade, along with several Discovery-class ones.

The clear frontrunners for the Medium to Large missions in the next 10 years are the Pluto flyby - whenever it finally does launch - and Europa Orbiter, with the Comet Nucleus Sample Return as a dark horse.

Still, that by itself allows a very active and productive program, provided the missions are wisely chosen. It will be extremely interesting to see the Decadal Survey's final recommendations for the Solar System exploration program when they're officially released next spring.

One remaining part of NASA's Solar System exploration -- the most important part of all -- will not be directed in detail by the Decadal Survey's report next year. NASA spends as much on the exploration of Mars as on all the rest of the Solar System combined. And a separate report recently issued by "COMPLEX" (the national Academy of Sciences' Committee on Planetary and Lunar Exploration) has already concluded both that that disproportionate chunk of the planetary budget is justified (given Mars unquestioned importance) and that the overall form of the program currently planned looks pretty good (although the COMPLEX committee did recommend some minor changes).

But the Decadal Survey committee does need to mesh its other recommendations with the spending levels and scientific results from the Mars program -- and so, at its November meeting, the Survey's central committee was given a quite detailed description of the U.S. Mars program. In the final part of this series, I'll describe the Mars program as it now exists, and the changes that have recently been made in it.

Back to Part One of this Report

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OUTER PLANETS

The Medium Cut Of Space Exploration
Los Angeles - Jan 23, 2001