The proposed Europa Titan and venus Explorers that I've described in previous installments were regarded as Small Flagships -- which NASA hoped to launch about once every five years -- while the Europa Lander and Neptune Orbiter that might follow them would fall into the "Large" category (missions that could be flown only about once per decade).

But it quickly became clear that any cost estimates of such missions, at this point, are both extremely fuzzy and very likely to rise. NASA, for instance, hopes to fly the Europa Explorer orbiter for around $1.3 billion -- but some of its planners wouldn't be surprised to see it rise to 2 to 2.5 billion dollars.

And -- given the revolutionary new cooling and electronics technology that any Venus Mobile Explorer will need to survive for long periods on Venus' furnace-like surface -- it is painfully obvious that there's absolutely no hope of flying it within the $1.5 billion cost cap. So the "Small" vs. "large" Flagship distinction has now broken down.

NASA's advisory groups had proposed spending about $8.5 billion per decade on the exploration of the Solar System aside from Mars.

This could allow about seven small "Discovery" class missions, four medium-sized "New Frontiers" missions, and about $2.8 billion for Flagship missions (of whatever size). But over the next decade, NASA actually plans to spend about $4.4 billion just for Discovery and New frontiers missions. Where can it find almost as much money again for Flagship missions?

On top of this, the list of desirable future missions identified by the 2002 Solar System Decadal Survey group was heavily weighted toward really big ones.

After the top-priority list of five missions that I've described previously -- some of which have already had to be descoped in order to stay under a $750 million cost cap -- the Decadal Survey group identified only about four or five more desirable New Frontiers-class missions for later (taking only about 10-15 years to fly), while it identified another half-dozen desirable Flagship-class missions (which, at their much slower launch rate, would be stretched out over 30-50 years!).

And, as I've also previously noted, the science community also seems to be running out of ideas for scientifically worthwhile but really cheap "Discovery"-class Solar System missions. To quote MIT's Maria Zuber: "The easy missions have been done."

So it's not exactly surprising that NASA's Solar System advisory groups-- along with its planning groups for other types of space-science missions, which are encountering the same kinds of problems -- are now carefully reexamining the prioritization of future missions, with one particular idea in mind.

To wit: to what extent can we identify really high-priority scientific questions about Solar System objects that could be answered by relatively cheap, near-future missions, leaving the others to be answered later by Flagship missions?

That is, how much can we "telescope" Solar System exploration -- by designing missions which give up a fair amount of scientific investigations, but which in the process can take such a big nosedive in total cost that they're actually more cost-effective than the originally planned bigger missions?

Several new science teams have recently been set up to try to identify such missions for various worlds, and some of their reports will be coming in over the next few months. But this is an area in which any suggestions from anyone may be welcome for NASA. So, in the remaining chapters of this series, I'm going to be presumptuous enough to describe several possible ideas that have occurred to me over the last couple of years.

I'd like to emphasize that I will not be saying that such missions are the most desirable ones -- even if my technical ideas are correct, it would still very likely be preferable to fly bigger missions if we can dig up the money for them -- but I am going to toss out a few ideas which might be worthy of further examination to see if they really are practical.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

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