The Importance of Lunar Oxygen
About 85% of the weight of a typical spacecraft at launch is the oxygen used for rocket fuel. Since oxygen is the most abundant element in lunar soil, comprising nearly half of the lunar regolith by weight, oxygen mined from the moon can play a critical role in space industries. Besides giving us considerable leverage in our development of a space transportation system, lunar oxygen can become one of the lunar community's most important economic exports.
Lunar oxygen, condensed into liquid and stored in tanks made from lunar metals, can be shipped economically from the moon to refuel spacecraft throughout cislunar space. In fact, one of the early development goals of the Artemis Project is to provide this refueling service to passenger-carrying spacecraft in low Earth orbit. That extra fuel load gives a commercial passenger spacecraft what it needs to fly all the way to the moon, land, and return to low Earth orbit without need for another refueling. In this scenario, we would refill the oxygen tanks on the moon before the passenger craft takes off for Earth, delivering another load of oxygen for the next customer.
This is a wonderful scenario, but first we have to get the oxygen out of the moon's soil.
How to Extract Oxygen from Lunar Soil
First, Find Some Ilmenite
Moon dust is a mixture of many different minerals, and nearly all of them contain oxgyen in considerable abundance. One of the most common lunar minerals is ilmenite, a mixture of iron, titanium, and oxygen. (Ilmenite also often contains other metals such as magnesium which we'll blithely ignore here.) For this discussion, we'll concentrate on extracting oxygen from ilmenite because there's lots of the stuff available, and because the chemical processes involved are fairly straightforward.
Chemistry of the Lunar Oxygen Extraction Process
To separate ilmenite into its primary consituents, we add hydrogen and heat the mixture. This produces raw iron, rutile, and water. (Rutile is titanium dioxide, the ore commonly used for producing titanium metal on Earth. In its crystalline form, rutile is a gemstone. As a powder, it's the most common white pigment used in paint.)
The chemical reaction looks like this:
About 45% of the lunar soil and rocks is made up of oxgyen, and eventually we'd like to be able to use all of it.
That titanium is another important metal we'll want to use in a lot of lunar industries, and the oxygen attached to it is seductive. About 45% of the lunar soil and rocks is made up of oxgyen, and eventually we'd like to be able to use all of it. But our primary concern in this essay is the oxygen in that water we just extracted. With just those water molecules, we can immediately recover 10.5% of the weight of the regolith as oxgyen.
To get pure titanium and more oxygen, we'll have to use a more complex process, perhaps using a chlorine or flourine reaction; but for the initial pilot plant we have an easier approach. Water is made of just hydrogen and oxygen, so we can use simple electrolysis to separate the two gasses.
Lunar Oxygen Extraction Pilot Plant
The process starts with regolith-handling robots bringing raw moon dust to the pilot plant. There, with a system we could design to be the size of a briefcase for the first flight, the pilot plant takes over.
Once the process is going, the hydrogen we get from electrolysis of water can be recycled and used for the next load of ilmenite. But to get it started with an initial supply of hydrogen, we need only heat the raw regolith to about 600 degrees C. That will drive off hydrogen (along with a host of other interesting gasses, such as helium) that we use to reduce the first load of lunar soil.
Figure 22.214.171.124-1 Lunar Oxygen Pilot Plant
The robot dumps the moon soil into a hopper, which filters the dust into the first reaction chamber. Here we use simple solar reflectors to heat the vessel and drive off the volatile gasses. Heating the vessel provides the gas pressure we need to move the gasses to the separation unit, which pumps hydrogen into the hydrogen tank and the rest of the gasses into the next industrial process down the line.
The regolith and hydrogen are introduced into another chamber where more solar heating is used to raise the temperature above 900 deg C, where the hydrogen will reduce ilmenite into iron and rutile. Gaseous water vapor is pumped on to the electrolysis vessel. If we raise the temperature above 1,525 deg C, the iron will melt and separate out from the solids, leaving the rutile behind. The rutile will decompose at 1,640 deg C before it melts.
Electrolysis is simply the process of applying electricity to water to cause it to separate into oxygen and hydrogen. We use photovoltaic solar cells to generate the electricity. Each of these gasses will collect at an opposite pole of the electrolyis apparatus. From there the hydrogen is pumped into the hydrogen storage tank, where in joins other hydrogen extract directly from the regolith in the initial heating process. The oxygen is pumped into a storage tank where it is condensed into liquid form for use as rocket propellant or introduced into the lunar settlement's life support system.
This article barely scratches the surface. We would like to carry a small pilot plant on the first Artemis Project manned mission, perhaps even on an earlier robotic flight. To do that, we have to answer a host of technical questions and then design, develop, and manufacture the pilot plant.
Many areas of further investigation are open for your amusement:
- detail the processes inside those mysterious reaction chambers
- determine what temperatures are required in each reaction chamber
- calculate how much power is needed for pumps
- define the exact method to be used for separating gasses during the preheating process
- design a holding facility for the gasses
- design instrumentation to control and monitor the process, and to report the experimental results
- design the solar reflectors used for heating the reaction chambers
- detailed design and fabrication of a small pilot plant
- fabricate a prototype oxygen plant for experimentation on Earth
- tromp out into the desert and scoop up some basaltic fines for use in the prototype plant
- develop an economic model for the lunar oxygen business
Spinoff: Titanium Production from Ilmenite
There's an interesting side effect to developing this technology for oxygen production. Today on Earth, most titanium metal is produced from rutile laboriously mined from sands in Florida and Australia. However, if an efficient process is developed for extracting rutile from ilmenite, we might serendipitously have an economic effect on the world's titanium production. On Earth, ilmenite is about fifty times more abundant than rutile; so this research could have a positive effect on terrestrial production of titanium. Even if that happens, however, we still should be able to deliver titanium to the spaceborne community at much lower cost than Earth-based producers.
Join Us in the Grand Adventure
As you can see from the above list of things to do, there are parts of this project you can contribute to whether you're a high school student or a PhD chemist. Unlike the helium-3 question, oxygen extraction is an area of experimentation that doesn't require an elaborate laboratory with and nuclear reactors. This area is vital to the initial economic stability of the lunar community, so if you want to get into technical investigations besides designing spacecraft it would be a good place to concentrate your efforts. Please contact the Artemis Society Office of Space Flight Administrator <email@example.com> if you would like to participate in this technical work.
If you have done research in extracting oxygen and other material from the lunar regolith (or any other subject applicable to development of a permanent lunar communty), please allow us to add your papers to the Artemis Data Book. Contact the ASI web team <firstname.lastname@example.org> to arrange to have your papers presented here on the Artemis Project's World Wide Web site.