by Mike Combs, Copyright © 2000
This article also appeared in the Summer 2000 issue of Space Front, a publication of the Space Frontier Foundation, and was presented at the 2004 International Space Development Conference.
Certainly many early space thinkers have contemplated use of the moon’s resources in proposed lunar colonization programs. But probably the first to consider use of lunar materials for the construction of a wide variety of useful structures throughout cislunar space was the Princeton University physicist Gerard K. O’Neill.
After surprising himself with calculations indicating how large an Earthlike space habitat could be built, the next question was: From what do we build these high-orbiting settlements? Lifting construction material from the Earth’s surface could immediately be ruled out. Even assuming the lift cost reductions then being promised by NASA for their proposed new Space Shuttle (can you remember those days?), sending the parts up from Earth for even one of the more modest of O’Neill’s designs would overwhelm the most extravagant budget imaginable.
But then O’Neill made a simple calculation. The amount of energy needed to lift a kilogram of material from the surface of the moon into a High Earth Orbit (HEO) is about 1/20th what is needed to lift it from Earth to that same orbit. This fact points to our moon as the source of the raw materials needed to settle orbital space.
What is there to mine on the moon? From Apollo, we know ordinary lunar soil consists of:
| 40% | Oxygen |
| 20% | Silicon |
| 12% | Aluminum |
| 4-10% | Iron |
| 6% | Titanium |
| 3-6% | Magnesium |
Oxygen (obviously useful for breathing) is also 86% the weight of both water and rocket fuel. The silicon can go into glass and solar cells. The metals are useful for structural materials. Aluminum and titanium are valued by the aerospace industry for their combination of strength and light weight. Titanium, additionally, is a good high-temperature metal.
O’Neill, through his Space Studies Institute (SSI), also sponsored research toward creating fiberglass, ceramics, and cement from lunar materials.
It seemed that over 99% of the raw material needed to build a space habitat could be derived from lunar resources, with no need to lift it out of the Earth’s much steeper gravity well. Additional research sponsored by SSI soon demonstrated that similar percentages held for Solar Power Satellites (SPS) optimized for use of lunar materials. The main point weighing against SPS had always been the enormous launch costs. Here now were two products which could greatly benefit from use of lunar resources: An environmentally benign source of constantly renewing electrical power potentially worth hundreds of billions of dollars in the global marketplace, and habitats providing a high standard of living for the workers constructing these energy collectors. Additional products could include enormous geostationary communication platforms, and large spacecraft for solar system exploration.
Later, Near-Earth-Objects (NEO’s) were also proposed as sources of raw materials for space construction. Some of these Earth approaching asteroids have round trip delta-V’s which compare very favorably with that of the moon. Some additional advantages of NEO’s are availability of a wider variety of materials (including volatiles scarce on the moon), and even shallower gravity wells.
But the moon will always retain advantages of its own. Most relate to its proximity to Earth. These include: more frequent launch windows, much shorter flight times, and a radio signal delay time of only a bit over a second (presenting the possibility of teleoperations from Earth, thus reducing initial manpower requirements).
I once entered into a debate with a fellow space advocate after he stated that the resources of Mars “make the Moon pale in comparison”. This was in reference to the fact that water, carbon, and nitrogen are generally more available on Mars than on the moon. The discovery by the Lunar Prospector Probe of water ice at the poles of the moon has weakened this argument, at least to a certain extent. And from an energy of retrieval standpoint, asteroids will always be better sources of hydrogen, carbon, and nitrogen than Mars due to the relative steepness of the Martian gravity well.
One factor which should engage our enthusiasm for the moon’s resources more highly than for those of Mars is that lunar resources are sufficiently nearby that their utilization could have returns to the terrestrial economy. Surely this is vital for getting the space enterprise off the ground, given that every investor currently in existence lives on the planet Earth. The usefulness of Martian resources for the purpose of constructing habitats for living on Mars is indisputable. However, lunar resources can be used to build habitats for living in High Earth Orbit, as well as SPS, enormous communication platforms in Geosynchronous Earth Orbit (GEO), and even roomy ships for journeying to Mars. These are all products marketable to Earth, as opposed to hypothetical future Martian investors. Mars will never be in as good a position for export as the moon due to the combination of its greater distance and steeper gravity well. Export is vital for balance of trade, and hence economic viability.
When Prospector detected water ice at the lunar poles, the usefulness of this water for lunar surface operations was obvious to everyone. What few realized, however, was that even for a water (or rocket fuel) market in Low Earth Orbit, the energy needed for supply from the lunar surface is still less than for lifting it up from Earth. The distance involved is much greater, but in terms of delta-V, the moon is a better source.
In addition to having a shallow gravity well, the moon is also airless. This opens the possibility of electromagnetically launching lunar material horizontally off the surface and into space. O’Neill began building models of a device he named the mass driver. It was essentially a stretched-out linear motor or electromagnetic catapult with recirculating “buckets”. The third model built had the same diameter as what had been proposed for the moon. Accelerations of 1,800 gravities were achieved with off-the-shelf-parts.
But Mark Prado, webmaster of the PERMANENT website, thinks SSI’s reliance on the mass driver for launching lunar ores into space may be a mistake. PERMANENT is a proposal for the use of lunar and asteroidal resources for space construction projects. As a physicist who has worked on electromagnetic launchers, Prado says he personally has a great deal of confidence in the mass driver concept. However, he fears business leaders may be leery of it as an unverified technology.
Among some space advocates, there is even greater pessimism regarding the lunar mass driver proposal. I once found myself in a debate on a couple of the space-oriented Usenet newsgroups with some folks who insisted a mass driver could never achieve lunar escape velocity (2.4 km/sec). On the one hand, I had Marshall Savage describing future space-based mass drivers hundreds of billions of kilometers long, capable of accelerating multi-ton starships to near lightspeed. On the other, I had these fellows expressing doubts over a proposed lunar mass driver 160 meters long and about the diameter of a dinner plate, capable of launching softball sized spheres of sintered soil to lunar escape speed. Personally, my incredulity border lay somewhere in between.
One of the debaters sent me a detailed technical explanation of his doubts. The issue he raised involved the switching speed of the coils, which was a function of their inductance/capacitance. Such considerations, it was said, would forever limit mass drivers to velocities below what was needed.
At first his explanation depressed me, because it made sense to me. Then it occurred the implication seemed to be that Gerard O’Neill, a physicist with Princeton’s Institute for Advanced Study (the same place Einstein worked) and inventor of the colliding beam storage ring, had negligently overlooked a principle of electricity which I learned in my first or second year of college. Not to mention the many electrical engineers and graduate students who worked on mass driver models alongside him.
I turned to SSI, and was referred to Dr. Les Snively, who worked with O’Neill on Mass Driver Model Three. He stated that the coil switching limitation cited as a concern was in fact an implicit part of their computer models. He also mentioned a strategy O’Neill had devised to overcome it: using greater numbers of single-turn coils at the later, highest speed portions of the accelerator.
It turned out that one reason for the state of pessimism regarding mass drivers was the unfulfilled promises of a coil gun developed by Bill Cowan at Sandia Labs. The goal had been an Earth-to-space launcher, but the velocities predicted by computer models turned out to be elusive. It was generally agreed that this failure “poisoned the well” for additional funding of high speed launchers based on similar (and sometimes dissimilar) technologies.
One can point to significant technical differences between the O’Neill mass driver design, and the Cowan coil gun design. Whether they add up to the difference between a workable and an unworkable approach may not be clear. I’m not aware that Cowan’s design ever used the single-turn coil solution suggested by O’Neill. Certainly the operating conditions and design goals were different (for example, launching through vacuum vs. launching through the Earth’s atmosphere).
I finally decided that people were essentially saying, “We know the lunar mass driver won’t work, because it reminds me of this other technology which was a disappointment”. If it’s truly a case of pronouncing that the apple’s gone bad because the orange is rotten, then perhaps we should not write off the lunar mass driver just yet. For myself, I no longer consider mass drivers a “demonstrated” technology, despite the demonstration of the accelerations required. But neither do I rule them out.
Even if moon-based mass drivers remain forever impossible, other means for economically lifting lunar resources into space may be feasible. A “skyhook” or space elevator for the moon is certainly a less difficult technical challenge than one for the Earth, and may even be possible with existing materials. Whereas construction of an Earth skyhook would begin in GEO, the lunar skyhook would start at the L-1 point between the Earth and the moon. If one end is extended to the lunar surface, the Earthward end would reach sufficiently far into the terrestrial gravity well that payloads released from there would end up in an elliptical Earth orbit. Like the mass driver, such a structure would enable transportation of resources off the lunar surface using electricity rather than rocketry. And the electrical costs would be trivial.
Thus it is possible that in our future, space may be filled with clean, inexhaustible power generators, vast communication platforms enabling ubiquitous wrist communicators, giant ships for exploration of the planets, and even orbiting extensions of life’s ecological range; all manufactured from the common, powdery gray dust of our nearest neighbor in space.
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