New Frontiers was initiated in 2003, stimulated by NASA's success in selecting a relatively cost-effective design for a Pluto probe by asking for competitive proposals that resulted in the selection of the Applied Physics Laboratory's "New Horizons" craft based largely on the design of a pre-existing spacecraft.

The Space Studies Board of the National Academy of Sciences, in its 2003 "Decadal Survey" recommending a program of near-future planetary exploration, suggested choosing a series of other middle-cost Solar System missions in a similar way -- with NASA providing a list of several acceptable overall mission concepts (whereas Discovery, Mars Scout and Explorer mission competitions are open to literally any mission idea that anyone is willing to propose), and then accepting the best proposal to meet the goals of one of those overall mission concepts, provided that the proposal's cost was less than $700 million.

The already-selected Pluto probe would, in fact, be considered the first such "New Frontiers" mission.

But, once again, that old devil Excessive Cost promptly reared its head in the very first New Frontiers proposal competition. Since it's harder for a group of scientists and engineers not directly created by NASA to get together and design such a proposal for a mission much more complex and expensive than a Discovery mission, when NASA asked for proposals for a list of four different possible mission concepts, they got a grand total of only five proposals (whereas they usually get 20 or more for every proposal request for the cheaper missions).

Only one of the four overall mission ideas -- returning a sample to Earth from the giant Aitken Basin on the Moon's far side -- was able to get even two competing proposals.

And the proposals that were actually received for two of the NF mission concepts were in fact a good deal more scaled down and less ambitious than the original mission concepts had actually been -- because it turned out that some of the concepts proposed by the Decadal Survey were too ambitious, and simply could not be done at all within that $700 million cost cap.

One concept was for a "Venus In-Situ Explorer" that would land, hastily grab a surface sample, and then inflate a balloon system that could operate in Venus' 480-degree C surface temperature and loft the lander back up into Venus' low-temperature cloud layer, where the lander could analyze the surface sample at its leisure while also studying Venusian wind patterns -- with the mission also being an engineering test of such a high-temperature balloon system, which will be a necessity if we ever do try to undertake the staggeringly expensive task of returning a Venusian surface sample to Earth.

But there was simply no way to fly such a complex mission at $700 million, so Univ. of Colorado scientist Larry Esposito instead proposed a more modest mission that would just plop down two small landers on different parts of the Venusian surface to each scoop up a sample and analyze it during the two hours that each lander could survive on the surface before its electronics roasted. (This proposal might have become a finalist except for a chance problem with its launch window, and it may very well be selected during a future New Frontiers competition.)

And the mission proposal that was finally officially selected as the second in the New Frontiers series -- JPL's "Juno" Jupiter polar orbiter now scheduled for launch in 2011 -- was also greatly scaled down from what NASA had originally envisioned.

The original Jupiter mission concept called for the craft -- just before entering orbit around Jupiter -- to release three entry probes into different zones of Jupiter's atmosphere that would be much lighter-weight but also tougher than the 1995 "Galileo" entry probe, equipped with pressure hulls and temperature insulation that would let them survive down to the roasting Jovian depths at 100 bars (Earth atmospheres) of pressure (as opposed to only 22 bars for the Galileo probe).

But it almost instantly became apparent that there was no way even to come close to flying such an ambitious mission for only $700 million -- right now we seriously lack the engineering knowledge to design such tough, complex probes at such a light weight; and in fact there isn't even any manufacturer or test facility in business at the moment that could build another thick heat shield for a 180,000 km/second Jovian entry like the one that had been built for the Galileo probe back in the 1980s.

So Juno just skipped the entry probes and settled for what it could do -- a lightweight, relatively simple polar orbiter of Jupiter -- and even then its estimated cost came close to the New Frontiers cost cap.

New Frontiers' overall cost problems, in fact, are such that the selection of the third NF mission won't be initiated until 2008 (with the mission to be flown in 2015), and the Space Studies Board no longer even recommends that Jupiter entry probes or a Venus lander capable of taking off and lofting itself back into the clouds should be included on NASA's list of acceptable mission concepts next time.

Instead -- besides again recommending the Aitken Basin sample-return mission, that new less ambitious Venus surface lander concept, and the fourth original New Frontiers concept of a craft that would briefly touch down on the surface of a comet nucleus to grab a sample and then return it to Earth -- it has proposed adding a substitute giant-planet entry probe mission, in which a relatively simple craft would fly by Saturn without stopping and drop two entry probes into its atmosphere to relay back data like that planned for the deep Jovian probes, but from a maximum depth of only 10-20 bars, and with a slower entry speed at Saturn that would allow both probes' heat shields to be constructed just using the material that's still left over from the building of the Galileo probe's heat shield.

And the still more expensive "Flagship" class Solar System missions that the 2003 Decadal Survey also recommended -- that would cost between $700 million and $3 billion, only be flown once or twice per decade, and be designed from the start by a single dedicated team directly picked by NASA headquarters -- also continue to wrestle with similar ballooning-cost problems.

This is particular important since, up to now, every concept for exploring those moons of the giant planets that may be of actual biological or biochemical interest has required such a hulking, expensive mission. This problem -- and possible ways of trying to deal with it -- will be the subject of my next chapter.

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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