by K. Eric Drexler
From L5 News, February 1983
Artist rendition copyright © 1982 by Kim Poor. All rights reserved.
Advocates of space development must decide where to turn for space resources, the key to making space a true frontier. We discuss both the Moon and the asteroids, but one source will surely be used first. Discussion of the choice between them has been unfocused and inconclusive. Although a clear-cut answer may be too much to expect, and research on both paths should doubtless receive our support, a debate could sharpen the issues and help us set priorities. To this end, Dr. Pournelle has asked that I fire an opening volley in support of the asteroidal side of the question.
Many people assume that the Moon is the “logical next step” in space development: it is, after all, the closest source of raw materials in space. But why go to the Moon for resources?
Because the Moon is closer? Transportation costs matter more than distance, and distance in space does not determine transportation costs; dropping mass into the Sun, for example, costs more than throwing it to the stars. Using delta-V as a rough cost measure, two known asteroids are more accessible than the surface of the Moon. One of them was discovered within the last year. The Spacewatch project promises to find more such asteroids, and some should he even easier to reach.
Because it is cheaper to return mass from the Moon? Most say that a lunar mining system would cost tens of billions of dollars, requiring hundreds of tons delivered to low Earth orbit. The schemes usually proposed for getting off the Moon require either rockets or a catapult and mass-catcher system. The former has a relatively high operating cost; the latter requires a relatively large initial investment. In contrast, low-thrust systems such as ion engines, deployable solar sails, or Lightsails can reach asteroids. These systems appear to require only tens of tons in low Earth orbit, making initial costs on the order of one billion dollars seem likely.
In times of limited budget, proposals with low initial investments seem far more likely to receive support from the government, and far easier to contemplate developing privately. But what about the cost of the materials returned? Returning materials from selected asteroids involves a lower delta-V than is required to leave the Moon. A wide range of efficient, low-thrust propulsion systems can do the job. Thus, the cost per kilogram of material returned promises to be lower as well. The combination of lower front-end and operating costs makes a powerful argument.
Because travel times are shorter? Who cares? In a one or two decade program, the one to two year travel times typical of asteroid missions matter little — except to a crew, but is a crew essential? Automated spacecraft have flown as far as Saturn, with excellent reliability. Plans exist for returning small samples from Mars with robots. The Soviets returned samples from the Moon years ago, without sending cosmonauts. Returning larger “samples” from asteroids to meet practical needs requires larger equipment to fill larger containers, but need not involve much more complexity. Why send people to dig dirt? They will find work enough in space without such dull employment.
Because we’ve been there? We know what the Moon holds, because we have been there. We know what the asteroids hold, because they have been here. See Meteor Crater, or a meteorite collection.
Because it has more valuable resources? Unfortunately, the Moon is a slagheap of dry volcanic rock; comparable materials on Earth are ignored by the mining industry (except perhaps for use as gravel). To extract useful metals from lunar rock promises to be difficult. The most valuable materials on the Moon — traces of nickel-iron and possible deposits of ice at the poles — fell there from asteroids and comets. Why not go to the source? Meteorite compositions and reflection spectroscopy indicate that asteroids contain water, organic materials, and usable steel in abundance. This steel holds nickel, cobalt, copper, platinum metals, and gold. Together, these metals have a terrestrial market value of over $1,000 per ton of ore — and ore is the correct word, because the mining industry would find it valuable.
Because the Moon is like a planet? Advocates of space development surely recognize the benefits, denied to the planetbound, of zero gravity and continuous sunlight. After reaching free space, why burn fuel to drop down another gravity well and sit in the dark for two weeks at a time? In order to face a choice between living at 1/6 g or building a massive centrifuge, on massive bearings? In order to be forced to go back into space anyway, to process materials in zero gravity?
If industrial processes or human habitations need gravity in free space, platforms slung on cables spinning in space can provide any desired level of pseudogravity. Building them and taking an elevator from the hub will cost less than building at the bottom of a gravity well and shuttling up and down with rockets. Besides, they offer a choice of gravity from place to place, and thus from moment to moment.
Watching Our Thinking
Why has the Moon been so popular, if asteroidal resources used in free space offer so many advantages? In part, because the Moon, too, has its advantages — particularly if one insists on sending crews to dig dirt, as has commonly been assumed. The choice between these sources of materials is not simple; it is a systems problem involving questions of cost, of value, and of the approach chosen to do the job. This complexity explains why the choice is not simple, but to explain why the Moon so often seems like the simple and obvious choice is another matter, a matter of psychology.
Research psychologists have studied how the human mind tends to draw informal conclusions, examining where intuitive rules succeed, and where they too often mislead; an excellent book on this is Human Inference, by Nisbet and Ross. Knowledge of these universal quirks of the mind can be a great help in avoiding pitfalls in one’s own thinking.
The Moon is favored by the most basic biases discussed in this book: those of “availability” and “vividness.” In short, the Moon is a big obvious thing in the sky, seen by all: asteroids are invisible to the naked eye, and thus attract less attention. Further, the idea of traveling to the Moon and getting materials there is vivid — we have seen photos and movies of it actually being done, which makes the idea of going back both simple and obvious. These factors, together with the feedback effect of discussion generating interest, and thus more discussion, would lead to a bias toward the Moon regardless of the merits of the case.
Nisbet and Ross emphasize human reliance on heuristics, rules of thumb that work well enough to become established in human thinking despite their lack of precision. Several heuristics that work well on Earth tend to distort thought about space mining in general, and about the asteroids in particular. One is “travel costs generally depend on distance.” The choice between the asteroids and the Moon violates this heuristic, because the asteroids, although more distant, require a lower delta-V. Another heuristic is “it costs about as much to get from B to A as to get from A to B.” Perhaps more than anything else, this heuristic underlies public incredulity regarding the whole idea of mining materials in space; after all “space travel is expensive.” The best counter to this is to point out that, while it may take a rocket to reach space (A to B), ordinary rocks fall from space every day — the return trip (B to A) is downhill.
Even after years of thinking about space development, the application of these last two points had not fully penetrated my own mind. The cost of carrying materials to the asteroids depends on the delta-V required to match orbits with them. Therefore, since the delta-V’s for reversing the process are the same, the cost of returning materials must be similar, right? Not necessarily. The cost of returning materials need be little more than the cost of nudging them onto an orbit that intersects Earth’s; with proper packaging and guidance of the cargo, atmospheric drag can then take care of matching the orbits. Indeed, drag can land cargo in a salt flat or place it within a small push of being in either a high or low circular orbit (though not in between). Unfortunately for lunar miners, Earth’s atmosphere is no help in escaping the Moon.
One could go on listing heuristics that are misleading in space, or that rest on the limits of outdated technology. “To get something done, someone must be there to do it,” for example, or “to build something, you need ground to build on.” Many of them, if accepted uncritically, lead to a bias toward the physically-near, planet-like (and thus somewhat Earth-like) Moon. The flaws in these heuristics in no way prove the superiority of the asteroids; they merely show that clear thinking on this subject requires caution, and that the conventional wisdom is not to be trusted until it has been carefully examined.
An Asteroid Scenario
To make the idea of asteroid mining more vivid, a scenario may help. How might the process begin, and where could it lead?
First, we drop our eyes from the splendor of the full Moon, shake our heads to clear them, and look seriously at the choice of asteroidal resources vs. lunar resources. In part for scientific reasons, we support asteroid missions and the Spacewatch asteroid-search telescope. Recognizing the high thrust-to-weight ratio of metal films in sunlight (and their lack of fuel consumption), we better define Lightsail construction procedures and configurations (see the May 1979 L-5 News). Agreement grows that asteroids have valuable, accessible resources. Space industry draws investment; L-5 membership skyrockets. Probes survey newly found asteroids to select the best targets for initial use. Government and industry decide that a fraction of the price of the Shuttle is little to pay to open the Solar System; Lightsail development begins in earnest.
Lightsail production begins in orbit, and sails depart for nearby asteroids. They deliver devices that sweep loose regolith into bags, then they return the bags to LEO. Engineers use the mass as radiation shielding for habitable modules of space stations, and for hardening military satellites. The Soviets protest dirt in orbit as “an anti-satellite weapon.” Water from carbonaceous chondrites is electrolyzed, producing cheap fuel in orbit for hydrogen/oxygen rockets; radiation sheilding and fast, inexpensive rockets lead to a construction base in geosynchronous orbit.
Total sail capacity grows steadily as orbital factories continue production. Selected asteroidal steels are purified, removing over $1,000 of platinum metals per ton, then foamed in zero gravity for sale on Earth. Steel structures become common in orbit. The orbital industrial complex expands. Mass production of sails lowers the transport cost from certain asteroids to less than $1/ kg; use of asteroidal nickel in sail reflectors further lowers the cost to $0.10/kg. Nickel and cobalt, then steel, follow platinum to markets on Earth. Steam turbine power satellites with steel radiators become economical. Space industry rivals genetic engineering and electronics as a growth sector in the Western economy.
An expedition at last departs to build industrial facilities at the two most accessible asteroids, to pre-process metals and organic materials for shipment and easy capture through atmospheric braking. With cheap steel and water, space stations become large enough to hold parks and gardens, then grow still larger. People stay longer. They bring their families.
With cheap fuel, the cost of reaching the lunar surface drops dramatically. A Moon base is built using asteroidal steel and propellants, for scientific and sentimental reasons; advocates of Moon mining have an uphill battle against the conventional wisdom about space development. In time, however, the Moon becomes a source of aluminum and titanium for use in space industry, since it proves to be richer in these materials than are the more accessible asteroids.
This scenario assumes use of Lightsails; asteroidal resources would remain attractiveeven using ion engines, deployable solar sails, or chemical rockets burning LOX and LH2 from electrolyzed asteroidal water.
Twenty years ago, in a fit of political hysteria, the U.S. took a path that bypassed building a shuttle and space station, building instead a giant missile to fulfill an ancient dream. In 1969 it reached its goal, but at a great price to true space development: the “Moon shots” dominated the news about space, and made spaceflight seem like an expensive stunt. The space program collapsed afterwards.
The L-5 Society was founded on the idea of free space as a site for industry and settlement; before its founding, popular ideas about space had mostly remained in a lunar planetary rut. Today, it is said that a lunar base is the logical next step. There is even talk of lunar colonies, far from the terrestrial markets that could pay for them. Let us turn our eyes from the “romance” of the Moon long enough, at least, to consider sailing on sunlight to mine steel, water, gold, and platinum from flying mountains.
Authors Note: Since this was written I have received a press release from the World Space Foundation, announcing that Eleanor Helin has discovered yet another asteroid (1982 XB) that is more accessible than the lunar surface.
K. Eric Drexler has served on the L-5 Society Board of Directors for a number of years, and is a past Secretary of the Society. He has long been an active advocate for asteroidal resource retrieval and utilization. (Photo by M.A. Leonard.)