Aside from its obvious use as a human life-support consumable, water is an extremely useful substance for spaceflight. Water can shield a spacecraft from radiation; when used with rechargeable fuel cells, it is a medium for energy storage. Separated into hydrogen and oxygen, water is the most powerful chemical rocket propellant known. Water can be transported easily through space in liquid or solid form, then as needed, separated into its component parts. As both a material and as a form of stored energy, water enables a wide variety of spaceflight activities. Acquiring enough water for space applications imposes significant mission cost when launched as dumb mass from Earth.
Among those who write about a coming revolution in launch costs to LEO, there appears to be some confusion regarding the utility and cost of mining water on the Moon. Some agree with the concept “in principle” but argue that launching water into orbit from the Earth costs less than building an infrastructure to harvest and launch water from the Moon. The argument against harvesting lunar water is now standard fare for many when discussing fuel for a large spacecraft (such as one designed to transport humans to Mars) and often is used in reference to SpaceX and their assumption about the advent of cheap access to LEO in the future.
Why do others then call for the difficult task of developing the water resources of the Moon, where they believe it is possible to launch hundreds of tons of water into cislunar space, to be processed into whatever form desired? The nub of this question relates to our basic motivation for spaceflight. Why do we do anything in space? Do we intend to operate in the current mode of custom-built, one-off missions or should we instead develop a robust, continuing space-based transportation system, one that can be used to accomplish a wide variety of missions and activities?
Our current mode of operation in space is to custom design a spacecraft and launch it—fully configured and fueled—on whatever rocket we can obtain. For 60 years, this template has provided us the means to acquire new information about the Earth, the Solar System and the universe beyond via a wide variety of robotic and human missions. However, this mode comes with drawbacks. Obviously, the mass and volume of a launched spacecraft is limited to the capacity of the biggest launch vehicle available. One way around this limitation is to launch pieces of the craft with more than one rocket, then assemble the pieces of the spacecraft on orbit.
But there is a more pressing issue to be addressed. As long as we continue launching everything we need from the Earth, no matter how inexpensive the costs of launch become, we will be mass- and power-limited in our spacecraft and thus, capability-limited. If we could acquire the materials we need directly from a source in space, we would be become capability-unlimited. Some might argue that this is “pie-in-the-sky”—that all human endeavors are fundamentally capability-limited. True enough, but this limitation is in place largely because we have never attempted to tap the unlimited wealth of material and energy resources of space.
Although launch costs can and should come down over time, we cannot count on their continued decline and should be skeptical about some marketing claims. There is still the fundamental problem of launching hundreds of tons of mass along extremely precise trajectories, at extremely precise velocities for specific lengths of time to achieve orbit (and subsequent rendezvous with other assets already in place). In part, the expense of launch is a personalized moving target—one man’s outrageously “expensive” might be another man’s incredibly “cheap.” When it comes to human interplanetary missions (as it inevitably does), we require hundreds of tons of material, all delivered to some marshalling area in space (e.g., LEO, GEO, Lagranians, or some other departure point). The question at hand is: Should these provisions be delivered from the Earth (the deepest gravity well in the inner solar system) or can they be more readily acquired at another locality?
It is true enough that mining the Moon and processing the water into usable form requires a significant capability on the lunar surface, and that means money. Estimates for the cost to establish a resource base on the Moon vary, but they could range from $40 to $90 billion, depending on the approach. One could certainly launch a lot of water into space for that amount of money; just using published commercial rates, a SpaceX Falcon 9 could put 13 tons of water into LEO for about $60 million. For that price, we could launch 650 F9s, putting 8,500 tons into space for the low-end cost of a lunar facility. The proposed Falcon Heavy (if it works and costs as advertised) would emplace even more mass (about 22,000 tons for the same amount of money).
But then what? If your goal is to complete a simple, one-off mission, fine—you did it cheaply. Oh, wait—you want to go somewhere again? No problem, you have enough mass left over to go multiple times. Eventually, you need to pony up another $40 billion. If your goal is to land on Mars and return, you will eat into your mass allowance very quickly—it takes a lot of energy to get into and then out of the deep gravity well of Mars. You will also require a lot of massive infrastructure in space to change the water into the various forms that you require. You will need to launch all that mass from the Earth as well.
The more ambitious your space aspirations become, the more it makes sense to look carefully at using space resources. To me, this gets to the real debate behind the debate. Arguments over whether to use lunar resources or not really break down into one’s long-term desires for space—permanence versus transience, space-based versus Earth-based, pioneering and residence versus junketing and “just visiting,” opening up space for all versus exclusivity and restricted access.
In learning to use the resources of the Moon, put the accent on learning. Building a lunar water processing facility is the first step towards developing a critical skill that any spacefaring species must master—that of using what we find, where we find it, and turning it into a form that we need. It doesn’t matter that other planets and asteroids have different environments and feedstocks. The skills needed to master space resource utilization are common to all localities: handling of massive amounts of granular materials, thermal processing of the feedstock, extraction and refinement of the product, and storage and delivery. We choose the Moon as our first target for resource exploitation because it is close, it is accessible and it has water in a form (ice) most readily and easily convertible into a variety of useful products.
The mission of operating a resource base on the Moon is to develop the skills and technologies needed to acquire and use off-planet resources to support spaceflight. To claim that it is “more expensive” to do this than it is to launch tons of water from Earth is to completely miss the point of attempting it—we are learning how to “cut the umbilical cord” with Mother Earth and gain the ability to live on our own in space. Having the ability to acquire provisions in space makes routine human permanence in space possible.
Arthur C. Clarke once wrote that all revolutionary ideas or concepts go through three phases of critical reaction. The first reactions are “It’s impossible—don’t waste my time. The second phase consists of “Well, it may be possible, but it’s not worth doing.” The final phase is “I said it was a good idea all along.” Using the resources of the Moon is a revolutionary idea—a good idea that can change spaceflight forever.