By abandoning the Moon, the administration’s proposed space policy has left the space community with a huge question mark over the important issue of learning how to harvest and use space resources. Clearly if we don’t go to the Moon with people or machines, there is no way to use the abundant water, metals, and other lunar surface materials to create new capabilities in space. Supporters of the new path suggest instead that we can obtain all the materials we want from near-Earth asteroids, small, rock-like objects that co-orbit the Sun with the Earth. Indeed, some asteroid types appear to contain significant quantities of water, thus offering a possibly rich source of off-planet water.
Water is an extremely useful substance in space. By virtue of its varied utility, water enables extended human presence in space. Besides its obvious role as a sustaining substance for human life (both drinking and providing oxygen for breathing), water is also an excellent material to shield from cosmic radiation and a medium of energy storage, both by thermal storage and also through its use in rechargeable fuel cells, where hydrogen and oxygen are combined at night (producing water and electricity). Stored water is disassociated by solar generated electricity during the day and re-stored as hydrogen and oxygen. Most importantly, water can be converted into liquid hydrogen and liquid oxygen; in this form, it is the most powerful chemical rocket propellant known.
So what are the relative benefits and drawbacks of using asteroidal (not lunar) resources? The biggest advantage of asteroids is that they have extremely low surface gravity. As these objects are simply very large rocks, they don’t have much mass and hence, virtually no surface gravity. A mission to an asteroid is more akin to a rendezvous in space than it is to a planetary landing. The advantage this confers is that vehicles can come and go to a given asteroid without the requirement to expend large amounts of propellant in a landing, with total changes in velocity measured in the few meters to tens of meters per second range. In contrast, a landing on the Moon requires a propulsive burn of over 2200 meters per second, both coming and going. This deep “gravity well” penalty is much smaller than launching from Earth (11,000 meters per second), but is still substantial compared with “dimple” dimensions of asteroid gravity wells.
If the propulsive energy of access were the only (or even the main) consideration for resource exploitation, asteroids would win hands down. But there are some other issues to consider. Water is indeed present in the materials of Near-Earth asteroids, but in a chemically bound form. Water molecules fill sites in the crystal structures in rock-forming minerals, bound strongly to its encasing structure. These chemical bonds must be broken to extract the water and that takes energy. On the Moon, water occurs in bound form, but also in its native state as ice in the lunar polar regions. Ice-laden dirt can be scooped up and minimally heated to extract the water. In contrast, it takes 100 to 1000 times more energy to extract a kilogram of water from chemically bound asteroidal minerals than it does to scoop up the “free water” found in the lunar cold traps. The greater quantity of energy needed to extract water from an asteroid is annoying, but can be handled through the use of large solar arrays or even a nuclear reactor to generate copious amounts of electrical power. But both solutions bring significant mass penalties and a nuclear reactor significantly increases cost, both from the technical development it would require and from the hurdles raised by legal and environmental groups it would have to overcome.
A more critical issue is the location of the two resource bodies. The proximity of the Moon is a major boon for its utilization. The Moon is both close and accessible. In terms of closeness, it takes 3 seconds for a radio signal traveling at the speed of light to go the Moon and back. This makes the remote, telepresence operation of lunar robots from Earth feasible. Early steps in the location, surveying and harvesting of demonstration amounts of resources on the Moon can be done remotely with robots controlled from Earth. We do not have this luxury with asteroids.
Asteroids orbit the Sun (like the Earth does) and vary in distance from Earth by tens of millions of miles over the course of a year. At best, asteroids are several tens of light-seconds away and at times, tens of light-minutes. This long radio time-lag means that direct remote operation of robots on asteroids will be cumbersome, if not impossible. For well understood routine tasks, this may not be a serious issue, but space resource utilization is something we have yet to learn. It is unclear whether we will be able to harvest and process asteroid water using remote robots, but it is almost certainly possible to do so with robots on the Moon.
The other aspect of the Moon’s proximity is accessibility, the ability to access a space destination routinely and often. As the Moon orbits the Earth, we can go to and come back from the Moon pretty much at will – launch windows are almost always open. In contrast, because even near-Earth asteroids follow their own paths around the Sun, launch windows are short and come at irregular (albeit predictable) intervals. Round trips to and from asteroids are even more difficult and after multiple weeks to months of travel, loiter times are either very short (on the order of a week or so) or very long (a year or more). This wildly variable duration of access may be handled on a robotic mission, but it precludes any significant human/robot interaction during the materials processing on an asteroid.
Finally, there is the issue of surface gravity. Much of the “dirty work” of resource processing involves separating some substance from another, or extracting something embedded. Having gravity usually makes this an almost trivial step, one that we don’t think about very much – unless we don’t have it. The Moon does indeed have a significant gravity well (about 1/6 that of the Earth) and although this works against us when we want to export product, it works in our favor when we need to process materials. The extremely weak surface gravity of an asteroid is almost microgravity and makes it very difficult to separate materials there without specialized equipment, again adding mass, power, complexity and cost to the processing chain.
In short, there are many considerations to take into account when planning an architecture based on resource exploitation. The seemingly damning case against going to the Moon to harvest material resources largely revolves around its relatively high surface gravity. It takes roughly two tons of water-equivalent liquid hydrogen-liquid oxygen propellant to lift one ton of water to the L1 point, where it can be used to supply and fuel a variety of spacecraft destined for many different places. That same ton of water lifted from the Earth would take over 19 tons of propellant to deliver it. The other side of that coin is that gravity is extremely useful - if not critical - for many materials processing techniques. Gravity can only be artificially created near an asteroid at some expense and mission complexity, whereas on the Moon, it’s a feature that comes for free.
Learning how to access and use space resources is a critical skill for a space faring society – skills and knowledge that will reap rewards right here on Earth. The Moon offers us a school and a laboratory for acquiring this critical knowledge. By virtue of its proximity, accessibility and resource endowments, the Moon satisfies our early space ISRU needs and allows us to create new capabilities to routinely access cislunar space, where all of our economic and national security space assets reside. The asteroids have much to offer for material resources and we will eventually journey to and use many of them. But we have business on the Moon first. Mining the unlimited wealth of the Solar System will become inevitable once we have learned the lessons of how to do this job on our nearest neighbor.