Moon Miners' Manifesto

#102 February 1997

Save the Lunar Ice!

by Jim Chestek
Co-author of Doomsday Asteroids

Now that we know that there is a modest amount of ice on the Moon, there is sure to be a vigorous debate on how to use it. There will be a few, no doubt, that will want to keep it in pristine condition; sort of a "wilderness refuge," to be stared at, presumably, by twenty-second century tourists. But given its immediate value to the twenty-first century, there is little chance that will happen.

Nevertheless, the announced "find" is a sharply limited one. One published estimate is that it is about 81,000 tons of ice. This translates into only 9,000 tons of hydrogen, which is the critical element. (There is an abundant supply of oxygen bound into the minerals of the moon; it is not in short supply.)

There are two immediate and strongly competing uses for this very limited resource. The first is life support/agriculture to support colonists living on Luna. They will use it sparingly, and recycle it endlessly. Even this modest amount will be capable of providing the life support for a small colony.

The second, and more immediate use is for rocket propellants. We are accustomed to using hydrogen/oxygen rocket propellants, and they are almost the best chemical propellants possible. Used in this manner, the water (hydrogen) on the Moon will be used only once, and then is lost for good. This will be the "easier" way to use it, and the one first in demand. "Why not land our rockets with empty fuel tanks, and refuel on the Moon?" will be the argument.

This use will enable us to get people and things to the Moon sooner and cheaper than the alternatives. And there are alternatives. Land, like Apollo, with full fuel tanks. This will be more costly, but it will allow landings anywhere, not just at the South pole. More importantly, it will keep the hydrogen on the Moon, where it will be essential for closed life support systems.

Another alternative, not as quickly available, will be to use native lunar resources, such as oxygen and aluminum, as rocket propellants. We have not even started to develop rocket engines to use that propellant combination, and it may be a major engineering challenge, but there is little doubt that it could be done.

Finally, the ultimate means of lunar launch will be by magnetic catapult, aka by O'Neill fans as a mass driver. In time, one long enough to support the launch of people could (and should!) be built.

We need to recognize the very limited life of this discovered lunar ice, if used for rocket fuel. If we use it in space shuttle-like engines, we can burn it all up with 45,000 tons of oxygen, for a total of 54,000 tons of propellant. Using shuttle engines (the highest Isp that we now have) that will place only 90,000 tons of useful cargo into an Earth return trajectory. At the rate of 200 flights per year for a cargo rocket lifting ten tons, we would use all of this supply up in only 45 years.

I, as one space enthusiast, will argue that this is the wrong way to use the lunar ice. Save it to help the citizens of Luna City to get established! However, the realist in me expects to lose this debate to people of "more practical" nature, who will be on the ground first, and will use it to get back home, and the devil with possible future colonists!

Fortunately, I see one viable way to recover from this short sightedness. We can mine the asteroids for water, and take hydrogen back to the Moon. This is a round-about way of doing things, but it gives me comfort to see a means to recover from the short range thinking that would use lunar ice as rocket propellant.

My favorite scheme is to capture an water bearing asteroid (there are expected to be such) into a highly-eccentric-Earth-orbit (HEEO). Then we can catapult, with a linear electric motor, payloads to the Moon. At first, we will have to use rockets to land this stuff on the Moon. But if we start with a 100 ton rocket leaving the HEEO asteroid, we can land perhaps 55 tons on the Moon. Of this, we might reasonably expect 45 tons to be hydrogen to leave on the Moon. Two thirds of a ton of hydrogen will be combined with 5.3 tons of lunar oxygen to launch the 'tanker' back to the asteroid for another load.

This scheme for putting an asteroid into HEEO is described in more detail in a book just released by Prometheus Books, Doomsday Asteroid, of which I am co-author. Since this scheme can place millions of tons of water at our disposal in space, I can rest more easily about whatever may be done with the initial supply of water on the Moon. But lets try to save at least some of it for Luna City.

"Asteroids will be our friends
if we go halfway to meet them."

MMM

Contents of this issue of Moon Miners' Manifesto