In the next few installments, I'll be discussing other people's suggestions along these lines -- and also a few of my own, on the chance that the latter might possibly have at least some merit. (This may seem rather presumptuous of me -- but NASA's situation is currently desperate enough that ideas from anyone are likely to be welcome.)

And the first mission being suggested for possible modification is the very next planned Flagship mission: the "Europa Explorer". This would orbit Jupiter's moon, inspecting its surface in great detail both to see whether Europa really is a promising possible location for extraterrestrial life, and to find the best landing sites for a follow up mission that would actually land there and analyze ice samples for preserved biological evidence.

This mission was originally pushed last decade by NASA's eccentric then-Administrator Dan Goldin, who seemed to zigzag between very good ideas and terrible ones. Goldin became obsessed with the idea of flying it as early as 2003, despite the great amount of technological work that still had to be done on it. (In fact, he actually tried to cancel the far simpler Pluto flyby mission in order to make room for an early Europa Orbiter -- and while he ultimately failed in that, he did manage to delay the Pluto mission two years to a less favorable launch window, both increasing its cost and reducing its science return.)

Even more peculiarly, NASA at that time was obsessed with trying to launch the Europa Orbiter directly from Earth to Jupiter, despite the facts that: (1) any craft that brakes itself first into orbit around Jupiter and then around one of its moons has to carry a devil of a lot of braking fuel; and (2) the mass that can be launched to Jupiter by any rocket can be tripled by using gravity-assist flybys of Venus and Earth itself to catapult the craft into the outer Solar System -- the technique used by both the Galileo and Cassini missions.

The result was that the spacecraft had to be shrunk to such a small size that it could only carry a pathetic 27 kg of science instruments. And the technological difficulties of doing the mission -- and especially of finding ways to protect the probe's electronics from Jupiter's radiation belts, which are super-intense at Europa's distance from the planet -- quickly smashed Goldin's hopes of flying the mission for less than a billion dollars. So -- after its possible launch date kept getting bumped back -- the mission was finally dropped completely in 2001.

Now it has been reborn as a new, far better design -- Europa Explorer, which would use Venus and Earth gravity assists, and as a result can carry a hefty 180 kg of experiments, enough to do a comprehensive first survey of Europa. Moreover, a reexamination of the data from the Galileo spacecraft has shown that -- while Jupiter's inner radiation environment is still extremely nasty -- it's only about half as intense as previously thought.

This fact (along with the greater weight of aluminum shielding which the craft can now carry for its electronics) means that its reliable operating lifetime at Europa has been raised from only one month, up to 3 months minimum (with a 50 percent chance of holding out for eight months).

Moreover, it has another advantage. Its current design would provide a margin of at least 340 kg of extra mass that could be added to the craft if it's launched, as planned, on a Delta 4-Heavy booster. That would be assuming a fat mass-growth safety margin for the craft of fully 43% -- and it would also be for a relatively unfavorable launch window in mid-2015. NASA's current plan is instead to launch it in January 2017, for which the margin would increase to around 600 kg. What could one do with this extra mass that it may be possible to add to the craft?

There are several possibilities. One being seriously considered would be to add a tiny lander. This wouldn't be big enough to drill into the surface ice below the radiation-damaged surface layer, or to carry sophisticated analysis gear to look for possible biological compounds (that will have to wait until the later Europa Astrobiology Lander); but it could make some relatively simple surface composition analyses (the amount of salts and sulfuric acid in the ice, and maybe an overall measure of radiation-processed surface organics -- much of which may be dumped onto Europa's surface by infalling carbonaceous meteoroids), take surface photos during and after landing to find out just what the fine-scale surface roughness is and thus how the later big Europa Lander must be designed, perhaps make seismic and magnetic measurements to try and gain more data on the total thickness of the ice layer, and maybe gauge the amount of hard rocky and salt debris in the ice to see how hard it would be for a sampling cryobot on the later big Lander to melt its way down through the ice.

In order to brake itself by the 5400 km/hour needed to land on the surface, such a lander would have to carry enough fuel that it would necessarily be very small. So it would be a hard-landing capsule rather than a full-fledged soft-landing craft -- it might even be a cannonball-like "impactor" weighing only 10-20 kg, capable of surviving a crash onto the ice at several hundred km/hour and then peering out at the surface through a bunch of tiny viewing portholes and sampling ports in its surface.

So it's still uncertain just how scientifically useful such a tiny lander could be, although its biggest use (as hinted above) might be engineering measurements to determine just how to properly design a later big scientific lander.

But there are other possible alternative uses for that extra weight margin. You could raise the craft's data rate (which would involve little extra mass), or raise its current 180-kg science payload (although its current set of about 10 instruments will do a very nice job of studying Europa from orbit). But the best use for it might well be to prolong its operating lifetime in orbit around Europa by adding more radiation shielding.

The Explorer, as mentioned, is now designed to operate reliably for three months in Europa orbit. Every 20 kg or so of added aluminum shielding would allow it to survive another 3600 kilorads of total radiation exposure and thus to function reliably for another two weeks -- useful for surveying such a complex world. But such extra shielding could be used for another purpose: to prolong its initial tour of the other Galilean moons BEFORE braking into Europa orbit.

Besides listing five highest-priority middle-cost New Frontiers planetary missions, the 2002 Solar System Decadal Survey suggested about half a dozen second-priority (but still important) missions for later. And two of them were aimed at Jupiter's other three big moons, which are extremely interesting in their own right, if not as high-priority as the biologically interesting Europa.

These lower-cost Jupiter mission concepts included an "Io Observer" (which would go into a polar orbit around Jupiter to minimize its radiation exposure and make dozens of repeated flybys of that moon. (Jupiter's radiation at Io's distance is fully 20 times more intense than that at Europa, making an actual Io Orbiter impractical.).

Another suggestion was a similar "Ganymede Observer" -- now better regarded as a "Ganymede-Callisto Observer", since the Galileo craft revealed that the interior of the outwardly bland moon Callisto is radically different from that of Ganymede and almost as interesting -- which would make dozens of flybys of those moons.

But, as the Europa Explorer now stands, it will itself actually do a good deal of observation of Ganymede, Callisto, and Jupiter itself. The reason is that -- to minimize the huge bulk of fuel it will need to brake into orbit around Europa -- it would first brake into an elliptical orbit around Jupiter, and then spend 12-18 months it spends making repeated gravity-assist flybys of the three outer big moons to trim down its orbit so that it finally almost matches Europa's orbit before that final braking burn is fired.

As a bonus, it can thus make extensive observations of Ganymede, Callisto, and Jupiter itself, following up on the observations from the partially crippled Galileo craft. In particular, the Europa Explorer will be able to make the detailed, sweeping and long-term observations of Jovian weather patterns that had to be dropped from Galileo -- and also much wider-area surveys than Galileo could make of Ganymede and Callisto, during the Explorer's gravity-assist flybys of those worlds (probably about 8 or 9 flybys of Ganymede, and 2 or 3 of Callisto).

But there's one thing missing from the mission: it won't make any flybys of Io, and so is limited to very long-range observations of that fascinating violently volcanic moon to follow up on Galileo's seven Io flybys (during several of which that craft was temporarily knocked out of commission by the high radiation level).

Now, during each of the elongated equatorial Jupiter orbits during which it made an Io flyby, Galileo soaked up about 33 kilorads. So that same 20 kg of added shielding that would allow the Explorer to spend an extra month in orbit around Europa could -- as an alternative -- allow it to survive 10 Io flybys with no radiation damage at all! And the instrument payload currently planned for its studies of Europa would also serve, with virtually no modifications, to make excellent studies of Io, (judging from the science payloads suggested so far for the Io Observer).

The mission plan would of course be changed for such a setup. As things now stand, Europa Explorer is supposed to help brake itself into orbit around Jupiter when it first arrives by making a flyby of Ganymede -- after which it will make about a dozen flybys total of Ganymede and Callisto, both to sharply lop down the apoapsis of its initially very elongated orbit, and to lower its periapsis down from Ganymede's orbit until it can start making flybys of Europa itself. It will then make several Europa flybys over several months to further lop down its apoapsis until its orbit is almost identical with Europa's, thus minimizing the amount of propellant it will have to burn to actually brake at last into orbit around Europa.

In the revised mission plan, the Explorer would instead help brake itself into Jovian orbit by making a gravity-assist flyby of Io, rather than of Ganymede, upon arrival --which is the same thing that the Galileo spacecraft did, and in fact was also the first flight plan for the old cancelled Europa Orbiter concept.

It would then make its Io flybys -- and would then make about the same total number of flybys of Ganymede and Callisto as in its current plan, but this time they would be to both lop down its initial elongated apoapsis, and to raise its periapsis from Io's orbit to Europa's. (And, of course, the Io flybys themselves could serve to do a lot of the apoapsis-lopping.) After that, its flight plan would be the same as in the earlier version.

The Explorer would need added maneuvering fuel to set up those added Io flybys -- setting up 10 more flybys of any of the Galilean moons would probably require about 165 more kg of fuel (plus another 32 kg or so of added propellant in order to maneuver the spacecraft with its 20 kg increase in shielding mass). But all this is an increase of only about 200 kg in the total spacecraft mass, much less than the extra payload margin that we have if this mission is launched in 2015 -- let alone at the much more favorable 2017 opportunity now planned for it.

Virtually all the cost increase from augmenting the mission in this way would be in its operational costs, and specifically its longer period in orbit around Jupiter before entering orbit around Europa -- which would be only a small fraction of its overall operational costs, since as things now stand the Explorer is supposed to fly for about eight years between launch and its arrival in orbit around Europa.

The added risk of its failing before it achieved its main goal of orbiting Europa, due to that added flight time, would also be small, since the added shielding would fully protect the electronics from Jupiter's radiation itself. The only changes that would need to be made in the craft itself would be adding that extra thickness of radiation shielding and quantity of propellant -- for a major increase in science that could, to a large degree, fill the science goals of the proposed Io Observer.

Finally, this augmentation of the mission would be completely flexible in form. If the craft, during its development, turns out to run overweight and its spare payload margin shinks, the number of Io flybys that it would carry out (before settling down to its main task of using Ganymede and Callisto flybys to adjust its orbit nearer to Europa's) could simply be reduced -- right down to the point of being completely cut out if need be.

The only changes that would need to be made in the spacecraft itself would be removing some of the outer layers from the boxes of aluminum shielding surrounding its electronics modules. What would be necessary is developing, in advance, several alternative designs for the Explorer's tour of Jupiter's moons --with varying numbers of Io flybys in them -- before it finally enters orbit around Europa.

Without such a plan, it will probably be a very long time before we ever get any other closeup look at Io. The latest Solar System Roadmap recommends against adding any new candidates to the list for top-priority New Frontiers missions, except for a Saturn flyby-and-entry probe mission that I mentioned in an earlier chapter.

And if NASA sticks to the philosophy of launching all four of those remaining first-tier New Frontiers missions before moving on to any others -- at the currently planned rate of one every four years or so -- an Io Observer could not possibly be launched before about 2027, even if it was the very next mission chosen.

My proposed augmentation to the Europa Explorer mission would of course not be a full substitute for a dedicated Io Observer mission -- the amount of radiation shielding currently planned for Europa Explorer would also allow a Jupiter polar orbiter to make fully 100 Io flybys!

But beggars can't be choosers, and space scientists are clearly entering a period -- with no discernible end in sight -- in which they will be beggars. Even ten close Io flybys -- during each of which the Europa Explorer would be able to make far more sweeping observations of Io than Galileo was able to do during its own flybys -- would be enough to answer a huge number of our top-priority remaining questions about Io, at a cost that would be only a tiny fraction of the cost of a dedicated Io Observer mission, and much sooner.

The possibility of using the Europa Explorer's current extra weight margin in this way is therefore, I think, a perfect example of the sort of modification of NASA's current planetary program that is currently being sought to make it more cost-effective-- and so I believe it should be added to the list of other alternative ways in which Explorer's weight margin might be used (such as a tiny Europa lander or a longer stay in orbit around Europa).

But there is another possibility if NASA has to tighten its belt even further. That would be not to put a spacecraft into orbit around Europa at all before launching that eventual sophisticated Europa Astrobiology Lander, but instead having the next Jovian mission just observe Europa (along with Jupiter's other moons) during nonstop flybys of them -- as Galileo did, but with better instruments and much more data return per flyby -- in order to select a good site for the Lander. In my next installment, I'll examine this more drastic emergency fallback option.

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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