They had also decided, however, that the fourth one should be one of two alternatives.

One choice would be the "Europa Astrobiology Lander" that I've also mentioned above: a craft to soft-land on Europa, use a mechanical or an ice-melting thermal drill to collect ice samples from at least several meters below the surface for detailed analysis to look for possible remnants of life that have been transported from Europa's subsurface water layer upward to the top of its ice crust by the slow geological processes in the ice. (In the topmost meter or two of Europa's surface, Jupiter's intense radiation has scrambled biologically interesting organic compounds almost beyond recognition).

This craft must be able to brake itself to a soft landing on a surface whose small-scale roughness is currently unknown except that it seems to be greater than that of the Moon or Mars -- and it must be able to function reliably for a long period in the very intense radiation environment of Europa.

It may also be that the only way it can collect enough interesting material to be properly analyzed is by using a "Cryobot" -- a torpedo-like probe with a heated tip, attached to the main lander by a power cable, that can melt its way down several hundred meters through the ice, and filter the large amount of meltwater that it produces as it goes to extract compounds of interest.

In short, it's complex and expensive enough that there is no point in even starting to fund it unless the earlier Europa Explorer finds more evidence from orbit that Europa is indeed of intense biological interest.

The alternative choice would be a Neptune orbiter, similar to the Galileo and Cassini orbiters of Jupiter and Saturn. These two more smaller and more distant "ice giant" worlds -- which have much less hydrogen and helium wrapped around their inner ice-and-rock cores than the "gas giants" Jupiter and Saturn -- are extremely important for further study, as are their moons and rings.

A Uranus orbiter would take less time to reach its destination planet -- and Uranus, if you peer through its concealing upper haze layer with near-infrared cameras, has cloud patterns just as dramatic as those on Neptune -- but Neptune is definitely regarded as scientifically preferable for the first ice-giant orbiter because such a mission could also make repeated flybys of Neptune's big and very interesting moon Triton, which has huge liquid-nitrogen geysers and a very thin atmosphere of its own.

Most current designs for the Neptune orbiter again call for it to drop off one or two Galileo-type entry probes into Neptune's atmosphere just before braking into orbit.

Galileo and Cassini, of course, were very complex and expensive missions -- and any similar orbiter that we try to send to Uranus or Neptune has a major additional problem. Those two planets are so distant that -- for any craft to reach them within an acceptable time of 10-20 years -- that craft must be launched outwards from the Sun so fast that it will have to carry a very large supply of braking fuel in order to slow down into orbit around the destination planet as it hurtles past it.

The feeling is that one of two new entirely new techniques must therefore be used to carry out such orbital braking. One would be "aerocapture" -- leaving the craft tucked behind a heat shield all the way to its planet, and then having it make a fiery skim through the upper fringes of the planet's atmosphere before soaring back out into space, thus slowing itself down by several thousand km/hour without spending a drop of fuel.

After it had thus braked itself into an elliptical orbit around the planet, the craft would eject its heat shield -- and then, on reaching the first apoapsis of its orbit, it would immediately fire a modest-sized burn on its engines to lift its orbit's periapsis safely up out of the atmosphere, and then set about its business.

While aerocapture has not yet been tested, there's a strong feeling that it will be a major near-future technology in Solar System exploration. Using it at Mars, Venus or Titan should require only a normal-shaped heat shield, along with an autopilot that can sense whether the craft is slowing down at precisely the desired rate during its plunge, and tilt its nose slightly up to bend its flight path a bit deeper into the atmosphere if it isn't slowing down adequately, or tilt its nose down to loft it a bit higher if it is slowing down too much.

NASA's engineers feel that a single test of the technique in Earth orbit will probably be enough to okay it for those three other worlds -- and such a test is one of the five candidate new technologies that could be tested in Earth orbit by NASA's upcoming "Space Technology 9" satellite. Aerocapture, in fact, is regarded as pretty much a necessity for any future mission that brakes itself into orbit around Titan, as the "Titan Explorer" flagship mission will likely do -- and, in fact, aerocapture is easier for Titan than for any other world, even Earth.

Titan's unique combination of low gravity with a dense atmosphere, which thus towers up to a great height above its surface while only slowly decreasing in density, gives any craft trying to aerobrake there a particularly fat margin of non-fatal steering error. (It's very likely that even future Cassini-type missions to orbit Saturn itself will also use Titan aerobraking to brake themselves.)

Using any of the four giant planets themselves to aerobrake into orbit around that planet is trickier. Their higher gravity makes the density of their atmospheres increase more rapidly with depth -- and at the very high speed with which such a probe will plunge into such an atmosphere, any small initial aiming error must be very quickly corrected to avoid being fatal.

Thus any craft that uses a giant planet's atmosphere for aerocapture must have a radically different shape for its heat shield: a bullet-like shape with a high "lift-drag" ratio, so that even a slight change in the craft's tilt, when commanded by the autopilot, can make it veer very quickly further upwards or downwards.

There's no reason to think this will be impossible -- indeed, there's some feeling that even a second test in Earth orbit of this second aerocapture system may be unnecessary. But aerocapture is a new and as-yet untested technology. And NASA's new 2008 fiscal year budget would delay the launch of the Space Technology 9 craft by fully two years -- to say nothing of the fact that there are four other alternative new space technologies that may be chosen for ST-9 to test, instead of aerocapture, when it finally does fly in late 2012.

The second possible braking technique for a Uranus or Neptune orbiter would be ion engines, since their very high exhaust velocity allows them to carry out very big maneuvers with a relatively small weight of propellant. They have such tiny thrust levels that it would take an ion engine-equipped craft months of firing them to gradually brake into an initial very high orbit around a planet, and then slowly spiral down closer and closer to the planet to its scientifically proper orbit.

But such a craft has one advantage that an aerocaptured orbiter doesn't: since it would retain its ion engines after entering orbit around the planet, it could then -- with only a modest additional weight of propellant -- do a great deal of really major maneuvering, even later braking itself into orbit around one of the planet's moons. Indeed, it might be possible for the craft to later accelerate itself away from that moon and cruise to another moon to orbit that one as well.

But thus using ion engines in the outer Solar System requires a high power source other than the huge solar panels that can power such engines in the inner System (such as those on the Deep Space 1 craft and the upcoming "Dawn" asteroid mission). That source must be nuclear. Until recently, it was thought that we would actually have to develop a very small but efficient and reliable spacegoing nuclear reactor for such a mission, which would be very expensive. But recent studies indicate the feasibility of replacing such "Nuclear-Electric Propulsion" with "Radioisotope-Electric Propulsion".

That is, if your craft weighs only a few hundred kilograms, you can power it instead with a large RTG (a plutonium-fueled radioactive battery), which -- despite being less efficient than an actual nuclear reactor -- could generate enough power to run several small ion engines that would be sufficient to brake the probe into orbit around a giant planet, and do considerable maneuvering later as well.

Such a system would require a new, more efficient "Stirling engine-based" RTG system -- which would convert the plutonium's heat into electric power about four times more efficiently than current RTGs, thus greatly cutting the amount of plutonium needed. This is very important both because of the high cost of plutonium-238, and the desirability of carrying as littleof it as possible to minimize any health dangers to humans from a launch accident. No Stirling-engine RTG has yet flown, but NASA has done considerable work on developing one.

Recent NASA studies indicate that a craft with a dry weight of 500 kg could be delivered to Neptune by a combination of an aerocapture heat-shield with a Venus gravity-assist and "Solar-Electric Propulsion" (an ion -engine package powered by big solar panels, which would initially help ram the craft outward from the Sun but then shut down partway through the Asteroid Belt due to decreasing sunlight) in only about 10 1/2 years. (Such a craft would be much smaller than the Cassini orbiter -- but the continuing miniaturization of instruments means that it could carry out comparably good studies of the planet and its moons. The current "New Horizons" Pluto flyby craft weighs 412 kg, and carries fully eight experiments.)

A similarly sized Neptune craft using a 750-watt REP system would weigh only about 1000 kg total -- one-third as much as the total weight of the SEP-and-aerocapture version. Even assuming that it did not use a Venus gravity-assist flyby (which it might) -- and that it was instead rammed directly from Earth to Neptune by its launch booster, with the effectiveness of its REP drive being maximized by only using that drive to slow the craft down as it approached Neptune in the outer Solar System, and then to make it slowly spiral into an orbit around Neptune -- it would cost about as much as the aerocaptured Orbiter. And it would take a lot longer to reach orbit around Neptune -- about 19 years. But if we added a very small chemical-rocket braking stage (only about 100 kg total weight) to help it first enter Neptune orbit, this could be lopped down to only 15 years travel time. And (as I've said) an REP craft could then use its ion thrusters to also make very extensive maneuvers around Neptune, perhaps ultimately even entering orbit around Triton -- and, if we were willing to spend extra money, maybe even carrying a small Triton soft-lander.

NASA's Solar System Roadmap advisory group concluded that, whether a Europa lander or a Neptune orbiter was chosen as the fourth new Flagship mission, it couldn't be launched before 2030 at the earliest -- and since either mission would probably cost somewhat more than any of the first three Flagship missions, it might be delayed until more like 2035. In addition to the problems NASA already had with both the cost and the timing of such big missions, however, Cassini has now seriously upset the planning applecart with its spectacular discovery of giant water-vapor geysers from the south polar region of Saturn's tiny moon Enceladus -- which may thus end up crashing the previously planned line of Flagship missions. This will be my next subject.

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