For these, NASA headquarters would itself define the mission description from the start, and select and recruit the very large team of scientists and engineers needed to carry it out. The Cassini Saturn orbiter was the last such Flagship-class mission flown so far.

NASA, at the moment, has no more such missions officially planned; but its scientific advisory groups, between 2003 and 2006, came up with a fairly firm list for the first few missions. The first would be "Europa Explorer", to orbit Jupiter's moon Europa at low altitude, and examine and map its entire surface in detail. It would include not only high-resolution cameras, but also:

+ Gravitational and magnetic experiments to nail down once and for all whether Europa really does have a subsurface liquid-water ocean beneath its icy crust (although the Galileo mission, during its Europa flybys, has alreaady proven that fact almost conclusively).

+ Near-infrared and mass spectrometers to identify and map the various other substances mixed with Europa's surface ice, some of which may be biologically important.

+ A thermal mapper and an ice-penetrating subsurface radar sounder to look for regions where pockets or fissures of liquid water may have recently existed near the surface. These -- along with spots of particularly concentrated non-ice substances -- would be the most promising landing sites for the later follow-up mission to Europa Explorer: a "Europa Astobiological Lander" to touch down, extract ice samples anywhere from several to several hundred meters below the surface, and analyze them for any actual evidence of microbial life in Europa's subsurface ocean that might have been slowly transported from there up to the near-surface by the slow geological processes in the ice crust.

NASA has suggested this mission several times before in the last decade, only to keep retracting the suggestion due to funding problems -- but the newest version of the mission looks more scientifically productive and cost-effective. At the moment, NASA is leaning toward requesting the first funds in 2009 to launch the Explorer in 2017.

NASA hopes to spend about $3 billion per decade on these Solar System Flagship missions, enough to launch one or two of them per decade depending on their (still very uncertain) cost.

At the moment, it's suggesting the next two at intervals of about five years after Europa Explorer. The first one would be the "Titan Explorer" -- a hot-air balloon dropped into the atmosphere of Saturn's strange organics-drenched moon, which has now been shown by Cassini and its Huygens piggyback lander to be even more complex and interesting than had previously been thought.

Besides observing the geological, meteorological and even liquid-related processes on Titan, it would try to identify complex, naturally-created organics on Titan similar to some of those that are thought to have existed on ancient Earth as the intermediate steps in the evolution of Earth life from nonliving compounds -- a process which is still mostly a mystery. Indeed, one Solar System science goal considered every bit as important as the actual discovery of life on other worlds would be the discovery of such "prebiotic" compounds.

Their nature has been lost forever on Earth simply because living things, once they did evolve, ate them all up -- but they may well exist on Mars and other cold worlds in the Solar System if the process of the creation of life began on those worlds but then stalled out before it was completed, giving us major and otherwise impossible insight into the creation of life on our own planet.

The Titan balloon might be accompanied by a separate module to orbit Titan, which would serve as the balloon's communications relay, and also map Titan's entire surface far more completely than Cassini will be able to do during its repeated brief flybys of Titan. In any case, the balloon would be blown along for thousands of kilometers by the gentle winds a few kilometers above the surface, sending back images amd composition maps of that vastly varied surface.

When its controllers concluded that it is flying over a particularly interesting region, it could be commanded to lose buoyancy and drop down to within about a hundred meters of the surface, and then drop tiny harpoons that would then be reeled back up into the gondola carrying samples of the local surface -- which the craft would then analyze in detail, looking for those complex hoped-for organics. In particular, it would look for frozen flows of mixed water and ammonia ice, erupted by cryogenic volcanoes from Titan's deep subsurface -- a realm where the relative warmth and liquid water could allow far more complex prebiotic chemical reactions to occur than anywhere else on this world.

The next Flagship in the originally identified series would be the "Venus Mobile Explorer" -- another balloon which would similarly be blown along by Venus' own gentle near-surface winds, occasionally dropping down to sample the surface. Its goal would be to understand the complex and still-mysterious geological history of that world.

One of its goals would be to look for patches of granite and hydrated minerals that might confirm the current suspicion that Venus had a liquid-water ocean of its own during its earliest days -- before its greater proximity to the Sun set off runaway physical processes that destroyed the planet's water and turned it into the scorching world it is today, covered by an extremely dense blanket of carbon dioxide that would have been turned into surface minerals had the surface liquid water still existed (as on Earth), and which now generates enough greenhouse effect to raise the planet's surface temperature to a savage 480 degrees C.

It's even possible that -- if Venus did have an initial water ocean that lasted for hundreds of millions of years -- microbial life might have begun to evolve on that world as well. (There is even a recent theory that such an ocean might have survived for up to 2 billion years, perhaps even providing time for primitive multicelled plants to start to evolve there before the planet committed suicide.) Looking for any fossil evidence of life on Venus will be a stupendously difficult undertaking -- if it's possible at all -- but clearly the first step is trying to determine whether the planet ever possesed a water ocean at all.

This mission would definitely occur after the Titan balloon, because it calls for much more advanced technologies that mostly don't yet exist. It's much easier to keep electronics warm on a cryogenically cold world than it is to keep them cool on Venus' roasting surface by using a very high-powered refrigeration unit (or to develop revolutionary new electronics that can operate at Venus' native temperature).

Even the balloon itself would probably not be made not of plastic but metal -- it would actually be a steel bellows, a grotesque accordion-like affair that could be quite small while still generating enough buoyancy to keep the craft suspended in Venus' 90-bar surface atmosphere. (It is, in fact, hard to say whether to compare this craft to a balloon or to a research submarine, with the "balloon" being regarded instead as a ballast tank.) It seems virtually certain that this mission could not possibly fly before about 2025.

Initially, the next Solar System Flagship was recommended to be one of two alternate choices -- but that was before 2006, when Cassini's second really major discovery in the Saturn system put the cat among the pigeons by revealing still another world that may well be worthy of detailed near-future study. And, on top of that, NASA's slowly worsening financial problems are compelling a reinspection of the overall design of all these Flagship missions. That will be the subject of my next chapter.

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