Monday, January 3, 2011

Collection of Helium-3

One theme that occurs frequently among subjects that I discuss is the matter of conservation and renewal of resources, including energy. The following essay urges the importance of development of energy storage on medium to large scales. Apart from the importance of various applications of energy storage, it is vitally important to the application of "renewable" or "sustainable" energy technology. In particular it renders the exploitation of intermittent sources of energy worth while, and secondly it permits drastic reduction of the scale of base load energy that utility companies must budget for. Short of the question of actual generation of energy, this is arguably the most important theme in a broad and vexed range of subjects.

If you find irritating errors of format and presentation, I apologise; this is my first attempt at using the blog facilities; forgive!

 

      Practical options for Collection of Helium-3


Table of contents

Abstract
Before reading on, please note:
Why 3He, or Why Not?
Where 3He, and Where Not?
Cut Out The Middleman?
Jupiter, Titan or Oort?
Crafty?
Commuters' parking
Driving the bus?

Abstract
Helium-3 has been proposed as such an exceptionally valuable source of fusion power that it should be worth mining the entire surface of the moon for it. Irrespective of the possibly genuine value of Helium-3, such a suggestion is radically wrong-headed and short-sighted. This essay proposes less vandalistic, more practical, larger scale, more economical, and more open ended means of obtaining as much Helium-3 as humanity is likely to need in the foreseeable future; say the next few hundred million years.


This is not a document cast in stone, but a non-technical draft proposal.  Anyone with doubts, queries, suggestions, objections, or corrections should please feel welcome to contact me. 
If you do so, please let me know whether your correspondence is in confidence, or you would like to be credited with any changes or additions that you might inspire.  
Of course there also would be no hard feelings if you preferred to write an independent document of your own without consulting me. 


Of all the prospects for power generation by nuclear fusion, Helium-3 (3He) has been touted by some as the best possible and most desirable nuclear fuel available.  This is simplistic because the only clear advantages of 3He fusion are that, suitably burnt, it produces mainly energetic protons instead of neutrons. It thereby reduces problems of radioactivity, and also should permit direct generation of electricity by exploiting those protons as moving electric charges.  This should be more efficient than generation of electricity from heat.  Of course, any such process produces some heat as well, but the principles, as far as they go, seem sound and the advantages attractive.  
Unfortunately, to fuse 3He is more difficult than to fuse deuterium and tritium, and it is not yet clear how practical any power generation technology based on nuclear fusion will prove to be at all. Partisans do not let such considerations discourage them and perhaps they are right to stand by their commitment. 
And perhaps not.  
Another advantage of 3He is that, unlike tritium, it is not in itself radioactive or otherwise toxic.  In the broad scheme of things that is at most a minor advantage, but for some purposes it certainly is a bonus.  
In all it seems quite reasonable to explore the practicality of 3He fusion, but more in reasonable hope than in strident confidence.  Some processes such as Bussard mechanisms look promising, so it is reasonable to hope.  Note that there are increasingly many promising nuclear transformations that could yield tempting amounts of power; we may yet find ourselves burning Boron-9 plus 3He to produce 4He plus clean power. Be all that as it may, for the purposes of this discussion I shall assume that the usefulness of 3He may be taken for granted.  The question I address here is different, namely its availability. 


However daunting the technical challenges of 3He fusion might prove, a far more serious problem for the foreseeable future is the disappointing shortage of accessible 3He for fuel. If the fuel is not available, it hardly matters how easily it could have been ignited if we had had enough to burn.  On Earth 3He is present only in traces. Some of those traces came from the solar wind and some from tritium decay.  
The enthusiasm for 3He as a fuel is so passionate that it has given rise to some positively harebrained suggestions for how we might go about obtaining enough to meet our power needs for the foreseeable future.  Probably the wildest, but so far the most popular, proposal has been to mine the surface of the moon for 3He embedded in its dust and  rock.  At best that suggestion is unpractical; more realistically it is a proposal of futile vandalism.  
It also is deeply, quite perversely, illogical.  The mere presence of a substance and even of its theoretical accessibility does not demonstrate that it is worth extracting.  We do not mine gold or plutonium from the sea, even though both are present (yes, even the plutonium!)  3He in the crust of the moon originated from tritium and 3He in the solar wind.  Moon surface rock certainly seems to be a relatively rich source of 3He, but the operative word is "relatively".  The concentrations, quantities, and most obvious mining options are unattractive. 
One problem with the lunar mining suggestion is that the 3He seems to be very widely distributed in the top few metres or so of the lunar material.  Presumably it originated from that portion of the solar wind that was intercepted by the top micrometre or so of the moon's surface, but then got kneaded more deeply into the surface layer by the impact of meteorites, which themselves presumably contain their share of 3He as well. Or perhaps the 3He atoms passively diffused inwards. The true reason makes little obviously practical difference in this connection. In either case, to mine lunar 3He  would require heavy equipment crawling over the whole lunar surface and leaving it in a tremendous mess.  One would have to heat a great deal of rock to a temperature of several hundred K at least. 
Supporters of such a scheme will object that all their heavy equipment need do is heat the surface enough to boil out to 3He, collect it, store it, and ship it back to Earth. They seem oblivious of the fact that any such process, even pure, hands-off microwave heating,  still would merit the term "vandalism";  there is no way to leave the surface in anything like its original form.  Even on Earth the soil would take centuries, in some places millions of years, to recover from such extraction; on the moon the marks would remain till the sun swallowed both moon and Earth. 
At the very least, not to mention any aesthetic effects, this would destroy most of the information that we could get from the moon.  It would be rather like ploughing up the Parthenon on the principle that no harm would be done because the calcium from its marble would still be in the soil for future generations of archaeologists to try to interpret.  The main difference is that the surface of the moon is both older and less replaceable than the Parthenon.
And perhaps more persuasively for persons of entrepreneurial spirit, whatever value lunar 3He might have for humanity, it only would be relevant if there were no simpler, cheaper, more effective, and generally profitable alternative means of obtaining 3He than by moon-grubbing.  
There are such alternatives, and they are superior by several orders of magnitude in several dimensions.  Some of them certainly are more elegant.
In this recommendation I suggest that we certainly should consider how to obtain as much 3He as we can.  After all, who knows?  If not in fusion power stations, it might at least prove to be useful in balloons.  But let us avoid nonsense such as suggestions that we should get it from the moon! 
Apart from the ridiculous cost of heating so much rock, there also is the considerable cost of fighting the high lunar gravity, both on landing and lifting off.  Granting that that sounds less costly than fighting terrestrial gravity, the fuel and equipment necessary on the moon still come mainly from Earth and accordingly are more expensive than lifting anything from Earth.  At last telling we had not managed to fabricate a worthwhile hand-axe from lunar material, and yet protagonists of the lunar mining scheme assure us that the way to proceed is to fabricate catapult launchers on the moon, largely from lunar material, to return the mined gas to Earth! 
Even if they achieved some such sort of success, the whole idea would still be far more expensive than some alternative schemes that could be operated in space, largely or completely unmanned, while conserving most of the energy invested in navigating and running them.  
Just for example I suspect that specially designed spacecraft skimming the atmospheres of the giant planets would be far more practical for gathering 3He than moon mining ever could be.  And yet, although I do consider such craft to be an improvement on the lunatic proposal, I also suspect that we would do still better to get 3He more directly from the source.  An immediate consideration is that that source offers usable energy in incidental and convenient form.  Such energy could drive our equipment adequately to power the collection of the desired fuel, and probably at an energetic profit. 
In sum, assuming that it ever is profitable to burn 3He (which I hope it soon will be) the right place to find that 3He is where you don't have to dig for the stuff, don't have burn high-grade fuel to fight high gravitational gradients, don't have to ruin anything irreplaceable, don't have to send people expensively to do the dirty work, and don't have to worry about limits to supply or access in the foreseeable future of humanity. 


Obviously that ultimate source of 3He in our solar system is the sun.  The sun emits a continuous though variable flow of charged particles.  The solar wind is mainly a plasma of hydrogen, deuterium, and 4He, but it also contains some tritium and some 3He.  From some points of view tritium is at least as good as 3He, because it is easier to store, and in any case it decays into 3He with a half-life of about 12 years. Accordingly, given a constant supply of enough tritium, we could rely on an adequate supply of 3He. Tritium of course has other uses as well, but those are not relevant in this context. 
Some people point out the difficulties of storing large quantities of tritium, but compared to storing 3He that problem is positively trivial. 3He is if anything even worse in this respect than 4He.  Bulk tritium can be stored as solid compounds, for example as metal hydrides, whereas helium is a very fugitive gas and forms no chemical compounds with half lives of the order of even a nanosecond.   If we can collect a large enough store of tritium, it might be worth collecting the decay energy as well as the 3He.  After all, that energy mainly takes the form of energetic electrons.  
However, the main theme of this essay deals with 3He collection.  
The amount of solar wind that strikes the moon is derisory.  For one thing the average distance of the moon from the Sun is roughly 150,000,000 km.  Per unit of intercepted area an object roughly 1,000,000 km from the centre of the Sun would intercept something like 20000 to 25000 times as much solar wind as anything near the orbit of the moon, and more energetic solar wind at that.  For our purposes we could estimate that the moon intercepts roughly as much solar wind as an optimally oriented 10,000,000 km² disc, though in practice it distributes the captured particles unevenly over an area roughly four times as great, and it is not clear what proportion of the intercepted solar wind is actually captured rather than wasted.  If we constructed a spacecraft bearing a collector effectively intercepting an area of just 10 square kilometres, we could keep it commuting between Earth's orbit and the peri-solar plasma.  At perihelion it would pass through the corona, or possibly even the chromosphere.  
A fleet of a hundred such craft could intercept as much solar wind as the entire moon, and retain the desired materials, not just 3He, a good deal more efficiently into the bargain. 
At aphelion, the craft could pass through a reception area to rendezvous with servicing craft and shuttles that deliver the collected material to Earth.  
Note that in one way the comparison is unfair.  The moon has been accumulating 3He for some billions of years, whereas I am talking of transient supplies.  However, the difference is not as great as that would suggest.  For a long, long, time the levels of 3He in the surface of the moon probably have been increasingly close to equilibrium, losses balancing gains.  Secondly, it is not clear how much of the accumulated material is accessible.  Certainly, the amount of 3He reaching the moon annually would exceed our requirements, no matter what the accumulation might amount to by now.  


The other major options for gas harvesting are the gas giants, their moons, and smaller bodies such as asteroids.  They all have attractions in comparison to the moon, and on average have been collecting  for at least as long.  They are further away, both from us and from the sun, but on the other hand many probably are better than our moon at holding on to the 3He that they have collected.
The only rational way to collect gases of any sort in large quantities from either the gas giants or those of their moons that have atmospheres, would be by skipping collector craft round their atmospheres.  There are alternatives involving balloons and unmanned surface craft, but those are at present too speculative to be worth discussion in the present context.  
How much easier skimming the gas giants would be than skimming the outer plasma layers of the Sun, I cannot say. I suspect that the boundaries between the atmospheres of gas giants and outer space will be a good deal narrower than the circumsolar plasma. This should make it more demanding to navigate round gas giants, steering a path between destruction on the one hand and on the other, ignominious and expensive failure to collect useful quantities of anything. 
Another question is whether the concentration of 3He in such atmospheres would be worth collecting at all.  I am unaware of any work to determine their concentrations of 3He experimentally.  Personally I suspect that any gas we collect that way is likely to contain a large proportion of useful material such as hydrogen and hydrocarbons, all desirable in their own ways, but not foreseeably profitable to collect from space. I suspect that only the gas giants would retain much helium three in their atmospheres. 
There are other problems. Distance is one of them.  The journey to Jupiter not only is some 5 to 10 times as long as the journey to the Sun, but there are no energetically useful gravitational gradients between Jupiter and Earth. The journey would be expensive in several ways, not only the sheer cost of fuel, but because a round trip would be slow, and would take so long that the effective yield from invested capital could not compete with the rapid turnaround of journeys to and from the Sun. 
Next consider the asteroids.  Though it is conceivable that some of the asteroids might be rich in 3He, and though such asteroids should be far easier to mine than the surface of the moon would be, there might well be serious conservationist objections to despoiling them. 
Exploring the Kuiper Belt or Oort cloud might similarly be very rewarding, and less likely to attract objections from conservationists, but the energetic and capital requirements are immense.  Also, the distances are many times worse than the remoteness of the asteroids or even the gas giants.  Simply prospecting the Kuiper Belt for 3He at all would be a challenge.
Furthermore, any exploration for 3He or any initiative to mine or extract it from resources that are further from the Sun than the orbit of the Earth, would make it difficult or impossible to rely heavily on solar power in the enterprise. The craft would have to bring along such fuel as they needed. Even though they could be designed to use nuclear fuel, this is a major practical objection.
In short it is difficult to imagine anywhere in the solar system that is likely to be more rewarding for extracting 3He than the circumsolar plasmas and the solar wind near to the Sun.  The surface of Mercury is in some ways more promising than the moon, but it presents problems of its own. 

Crafty?
The simplest procedure for collecting solar 3He would be to launch the gas collection craft on a trajectory that would drop it almost directly at the sun.  Ideally it should move in a very narrow ellipse.  If the craft is launched from near Earth orbit, the ellipse would have an aspect ratio of roughly 150:1 between its major and minor axes.  The craft would be unmanned and automated.  Though not a trivial thing to do, this would not be prohibitive, partly because it would become a routine activity and would not involve a wide range of eventualities.  Down to the sun, dodging Venus and side-stepping Mercury, then back again. 
Of course, this idea is hopelessly simplistic.  Engineering the actual trajectories is a complex matter.  In practice the routes would look nothing like minimal ellipses.  The matching of velocities, the momenta of the payloads and the nature and availability of fuel and reaction mass would all affect the choice of scheduling and navigation strategies.  However, such points should be deferred to detailed design time.  They are at worst not particularly challenging, let alone prohibitive. 
Each craft would have communications and remote control equipment for routine human monitoring and certain classes of emergency.  However, most of the routine control would be automated and managed by on-board equipment.  All sun-skimming craft would be uncrewed, at least until thrill-seeking billionaire tourists began to queue up for solar-skimming trips.  
All the rival options for collecting solar wind from near the sun would depend on similar sun-skimming craft, but there are many variations on designs for working systems.  The number and types of accumulation and despatch craft would accordingly vary too.  For example, a particularly attractive idea is to park the major refining and storage depot in the shadow of Mercury at its L2 point.  Mercury has a very eccentric orbit, so this strategy would require continuous adjustment of the position of the depot, but that should not be challenging. 
Periodically shuttle craft would deliver necessary supplies and return to Earth with accumulated payload.  There would be a need for servicing craft of various types, some to deal with the sun-skimming craft and others to deal with the repository/refining L2 craft and units in Earth orbit.  Those are matters of detail that at this stage would be too hypothetical to be worth much discussion.  Such points can only be settled in practice by engineering the orbits and procedures for best economy and practicality.    
Without committing to any detailed ideas of design, let us consider the types of craft required, if only to clarify the terminology.  
The one invariant is that there necessarily would be sun-skimming craft.  Those are the ones that would actually collect the gases from the solar wind and solar atmosphere where the solar wind emerges.  They would have to be able to navigate between collection/servicing points and the chromosphere.  They would collect crude material in large quantities.  They also might undertake the initial, crude stages of refinement.  Probably they would not have impressive acceleration capacity, but could adjust their trajectory to rendezvous with servicing craft and to compensate for loss of speed in skimming the sun and picking up payload.  
Next, there would be servicing/transfer craft.  They might have the most powerful acceleration capability because they would have to rendezvous with sun-skimming craft at the slowest part of their orbits, and with repository/refining craft in planetary orbit.  They probably would do little more than shuttling payload, but at least some of them would have to maintain equipment on the sun-skimming craft.  
An extreme form of specialisation would be to have the sun-skimming craft in fixed, circular solar orbit, and a smaller number of servicing/transfer craft that visit the sun-skimmers to take off their payload.  
Repository/refining craft would be stationed in stable orbits near planets.  Typical stations would be Mercury's L2 point, or leading or trailing Earth in its circumsolar orbit.  Repository/refining craft would accumulate large consignments of gas and refine them so that as little fuel as possible would be necessary to deliver the product to Earth.  Both 3He and tritium are rare gases, even in the solar wind.  
Finally there might be specialist tugboat shuttles for delivery to Earth and to launch equipment and supplies into space from Earth.  
I have not even begun to imagine anything like a detailed design for the various craft. For one thing, too many alternative technologies might be used individually or in combination.   However some of the following principles seem to me to be basic. 
Firstly the sun-skimming craft must have an effective collection surface measuring as many square kilometres as possible. That surface, while necessarily being very large, also needs to have a very small mass per unit area and a small moment of turning.  The device must be capable of collecting helium ions from the solar wind and probably tritium ions as well.  Possibly the deuterium collected as an impurity could pay its own way as incidental cargo too.  
Intercepting helium should be no problem, but retaining it might be a more serious challenge.  One approach to collecting the helium would be to use a double panel of very thin glass or polymer film, the front of which would be permeable to helium, whereas the back would be impermeable. In between would be structures calculated to trap as much helium as possible and reflect as much light as possible.  Positively charged baffles might work. I don't know much about baffle technology, but in certain configurations in near-vacuum, a helical baffle with a sawtooth profile seems to hold promise for biasing the migration of ballistic particles inwards. Other designs might have an open front guarded by charged baffles and strong electric or magnetic fields. Possibly aerogel structures might have special advantages for some such purposes.  There is a good deal of scope for brainstorming and experiment.
The front would be turned towards the Sun or nearly, and as a result it would become pretty hot during the closest approach. It also would have to withstand the impact of plasma at speeds of hundreds of km/s, making it still hotter.  The actual speeds would depend on the tightness of the skimming and the location of the aphelion.  In general, the more distant the aphelion, the higher the speed round the sun at perihelion, and high speed at perihelion would in general be an advantage during skimming.  
Fortunately the plasma at perihelion would largely amount to a pretty fair vacuum, so there need not be any challenging pressure problem.  The rear of the panel would be turned towards space and kept as cold as possible. Several types of effective collection system could exploit those temperature differences.  Trapped gases of the desired types would be pumped into cryogenic receptacles in the shadow of the collector.  Most of the cooling of the collector would rely on sun-ward thermal shielding and exposure to cold regions of interstellar space.  
Strong electric and magnetic fields would be easy to maintain in space and could be used to guide, trap, and sort solar wind ions for collection and propulsion.  It should be possible to sort out heavier and lighter particles with various charges so as to achieve significant refinement and increased yield even during the collection phase.  Possibly tethered conductive cables or nets could be useful for such functions.  They certainly should be able to extract electrical energy and traction from the intense solar wind near the sun.  
Waste gases could be used as reaction mass for ion drives, both for course adjustment for the skimming craft, and for depot and commuting craft.  Granted, hydrogen is very inefficient material for ion drive reaction mass, but solar power would be available in generous amounts, and the ion drives would be used mainly for course corrections.  
Solar power should be more than adequate for fractionating the collected gases and for cooling and enriching them during transport back to the depot station.  Just how to harness the solar power in each case would be an engineering detail.  Photoelectric or thermoelectric power might be best where practical, but possibly the thermal gradient between the hot and cold sides of the craft would also be useful for driving pumps and the like.  Sunlight concentrated by reflection also might be useful for some functions.  Solar wind itself could generate powerful currents in collector cables used for traction.  
All these are matters of detail and of engineering ingenuity.  The point is that all the resources for propulsion, storage and versatile forms of power generation are in plentiful supply in the environment between chromosphere and Earth orbit.  Further out, say near Jupiter, not to mention the Kuiper belt, power supply is not so easy and this is important because the profitable lifetime of a soundly designed craft would have to be of the order of decades or preferably centuries.  That is not an easy thing to achieve when one has to supply fuel as well as parts, particularly moving parts.  Even so, in respect of longevity, the problems of designing equipment for extracting plasma materials from near the sun would compare favourably with any conceivable moon-mining equipment.  It would be an impressive lunar crawler, launcher, or digger that could operate without extensive maintenance for even one year.
During its career a functional spacecraft might make about two or three trips round the sun per year from Earth orbit, or perhaps ten from Mercury orbit.  If each craft could collect a tonne of 3He and tritium per year, a fleet of one hundred craft could keep the entire human population supplied with clean fuel indefinitely -- some billions of years at least.  What is more, there is no reason to limit ourselves to one hundred craft.  In well-chosen flight paths neither a million craft, nor yet a billion, would be enough to crowd the sun.  If we learn how to burn 3He easily enough, we could power human activities on every planet in the solar system, and even on space colonies.  We could collect enough fuel to power craft to extract materials from the gas giants' atmospheres or from Kuiper belt bodies.  
Given that a sun-skimming craft were designed to last for centuries, its attached collectors could be progressively extended year by year, till they measured many tens of square km.  Perhaps the best configuration would be a long, narrow rectangle formed from a number of units attached end to end. Its working attitude would be perpendicular to the plane of the ecliptic and facing the sun.  Such a narrow configuration would permit a small moment of rotation around the long axis, permitting quick adjustment to face upstream into the solar wind and to achieve skimming control at altitudes where the solar plasma is dense enough for aerodynamic effects to become important.  
The idea of such a flimsy structure being useful for handling aerodynamic forces might seem ludicrous, but if each panel could articulate with its neighbours, the coordinated control could give a very bearable distribution of stresses. 


There are obvious attractions to the idea of stationing craft at various LaGrange points, of which the Earth's L1 point is the most seductive.  However, the idea is questionable for several reasons.  For one thing, as it approaches apogee, travelling in a highly prolate, narrow ellipse, the craft would be moving round the sun at a much lower velocity than the planet and its LaGrange points.  It would require a great deal of fuel to match orbits.  
When one thinks more carefully about LaGrange points for such purposes, their attractions tend to fade.  For most purposes parking in any cheaply controllable orbit out of harm's way would be just as good.  Examples include orbits that lead or follow the base planet by a few million km. By moving just inside or just outside the planet's orbit as required, one could adjust speed and station.  The merest trickle of power for course and attitude correction would be quite adequate.  There would be no need to retreat all the way to the L4 or L5 points.  
Instead of concentrating on the L1 and L2 points of the sun/Earth system, it would be better to rely on active orbital piloting into and out of rendezvous points where the gas repository craft meet servicing and delivery craft from Earth and collection craft from the sun or from refinery units.  Ion drives would power most of the active propulsion and navigation, but planetary gravitational fields would play crucial roles.
Craft in suitable near-Earth orbits could easily maintain station, offload the collected material, and service and replenish the various shuttle craft as required. In practice if such a procedure proves to be all profitable, there would be many solar skimming and gas collection craft, possibly even hundreds following similar trajectories.  
Since the orbital period of the collector craft between Earth orbit and the solar corona should be about four or five months, one hundred collector craft could be serviced at an average of nearly one a day.  To me that seems to be a reasonable schedule, especially because on any practical trajectory the craft would be passing the rendezvous so slowly that they should remain easily accessible for a month or more. Servicing requirements for multiple craft could be overlapped and dealt with in parallel. 
The Sun/Mercury trajectory would have some points of resemblance to the Sun/Earth trajectory, but there are important differences.  For one thing, a skimming cycle for a sun-skimming craft from Mercury would last more like one month than five.  This is important because it would mean about a five times greater rate of yield from each skimming craft, meaning about five times as great a profitability.  
Conversely, if the sun-skimming craft remained in solar orbit permanently, then only the servicing/transfer craft need shuttle to the repository/refining craft.  
Tugboat shuttle craft to lift the yield to Earth orbit from the orbit of Mercury would be cheaper and fewer than the skimmers or servicing/transfer craft.  All such craft would need would be some waste gas and electric power for ion propulsion, plus the necessary cryogenic capability.  Rich in tritium and 3He though the solar wind might be, it still is nearly all normal hydrogen and helium.  The shuttle payload would be tiny compared to the skimmers' production loads.  
This implies that there would be plenty of waste gas for reaction mass.  
Mercury's L2 point on the other hand, would be a valuable asset, so valuable that it would be worth the complications of separate specialist craft for docking with, servicing, and transfer of the yield from the skimmers.  The repository and refinement vessel would park at the L2 point and maintain station out of the sunlight.  Mercury's shadow is one of the coldest places in the solar system.  It would be excellent for refining and storing the gas collected by the skimmers.  To supply the repository and refinement vessel  with power, separate craft for solar power generation could maintain a suitably close Mercury orbit in the sun, beaming microwaves to power the cryogenic craft. 
Remember that both sunlight and solar wind in Mercury's orbit are generous.  Solar power would be plentiful, some 20 to 30 times more intense than at Earth's orbit.  
Probably servicing/transfer craft would service the skimmer craft and collect gas from them, whether in solar orbit or not.  They would remove the rudimentary cryogenic storage tanks from the skimmers and replace them with empties.  In this respect the skimmers for Mercury-based operation would differ from those in Earth-based operation.  
In either case the servicing/transfer craft also could replace collector equipment and any other components that might be damaged, worn, or due for replacement with more up-to-date units.  Nearly all the craft and depots should be unmanned and automated, with remote oversight from Earth.  Possibly manned missions would deal with special cases of non-automated maintenance, but I doubt that that would be necessary.  
Possibly there would be enough solar wind at Mercury's orbit to make it worth dispensing with sun skimmers altogether.  Craft in orbit round Mercury could collect the plasma continuously.  However, I shall assume that it is profitable to collect plasma some thousands of times faster by skimming the sun.  
Purists point out that most of the LaGrange points are at best metastable, unlike the L4 and L5 points.  Some of the other points at which I suggest stationing craft of various types are outright unstable.  All this is true, but barely relevant.  The craft of all kinds stationed at the points I describe are not under passive, but active control.  An orbit that is non-stable but easily corrected is only a problem for passively stationed craft.  The non-stable orbits in question are near-stable and only trivial active adjustment would be necessary for maintaining station and attitude.   


Solar skimmer craft, unless they were stationed in circular orbit, would approach the sun on a highly eccentric, possibly even hyperbolic, trajectory that skims the chromosphere at perihelion.  In any case the speed of the craft at perihelion should be of the order of a few hundred km/s.  This is a large fraction of the speed of the solar wind itself.  If the solar skimmer craft were in fact in more or less circular orbit, the same would apply, except that the craft would be permanently moving very fast, probably in retrograde orbit to maximise its speed relative to the solar wind.  
Passing through the plasma at such a high speed has important implications. It should improve the ability of suitably designed materials and devices to capture particles selectively by collision. It also should offer means of adjusting the trajectory by tilting the collection services on passing through the plasma. This is important because it is no good simply returning the skimmer craft to the point from which the craft was launched. One needs to aim it at where the transfer craft will be when the skimmer craft returns from the sun for unloading.  Skimmer craft can of course adjust trajectory by use of ion drives, and in practice certainly would do so, but aerodynamic course correction inside the circumsolar plasma could be cheaper, more powerful, and might offer incidental advantages in the collection of pay dirt.  
Ion thrust for just one or a few months would be cutting it fine for matching orbits with high relative velocities, so being able to begin with a roughly correct orbit would be valuable.  If the craft were in fixed circular solar orbit it would be necessary for the ion drives to overcome the drag from the solar wind.  
Of course, it might be that a simple trajectory with severe demands for high acceleration just is not practical or economic.  If so a longer trajectory would be necessary.  That would be a pity, because it would slow the rate of yield, but it would not vitiate the principle.   
The craft would never actually travel faster than the solar wind. This implies that even collector craft not in circular solar orbit could continue collecting material at all points in their orbit, in principle at least. In practice most of the collection would be around or approaching the Sun, and for a short while on the outward trajectory. Probably only about a third of the duration of any eccentric elliptical orbit would account for the bulk of material collected for direct transfer to Earth orbit.  Outside the orbit of Venus practically the major worthwhile activity probably would be housekeeping, maintenance, and compact cryogenic storage of the payload.  However, skimmers delivering to Mercury might collect material practically throughout their orbits.  
The implications of such opportunities for fuel collection include the incentive to develop craft that could explore the solar wind, and the practicality of manoeuvring within the solar corona and chromosphere.  As space exploration goes such research should be cheap and rewarding even if the whole thing comes to naught because the fusion of 3He proves unattractive.  The very concepts of such fields of design and such an industry are so attractive as to make me wish that one could be more optimistic about the practicality of using 3He for fusion power at all. 

Jon Richfield

 

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