Resource Utilization

The purpose of resource utilization (also known as in situ resource utilization [ISRU]) is to reduce the mass, and thus the cost, of space missions. On Earth, explorers rarely took all the food and supplies they would need for their entire journey. Instead, they relied on the resources around them, hunting for or gathering food, chopping down trees for lumber, and so forth. By carrying with them only what they needed to get from one stop on their journey to another, they minimized the size of their expeditions and made them more adaptable to changes.

The same principle is applicable to exploration of the solar system. Carrying all the food, propellant, air, and other supplies needed for a human mission to the Moon or Mars would make a spacecraft very large and heavy. Given the high cost to launch payloads —up to $10,000 per pound—reducing the mass of a spacecraft can greatly lower the cost of a mission. The savings can be significant even for smaller robotic missions, such as proposals to land spacecraft on Mars, gather rock samples, and return them to Earth. The use of ISRU could make the difference between an affordable mission and one that is prohibitive.

The Moon

The Moon appears at first to have few resources to offer because of its barren surface and lack of an atmosphere. However, studies of lunar samples returned by the Apollo missions revealed that lunar rocks are rich in oxygen. Up to 45 percent of the mass of lunar rocks consists of oxygen locked up chemically in minerals . When the rocks are heated and mixed with other materials, the oxygen can be released and used as a propellant, or for breathing. The by-products of these reactions are metals such as iron and aluminum, which in powdered form could also be used as rocket propellant. Although there is no hydrogen contained in lunar rocks, a small amount of hydrogen has been deposited on the surface from the solar wind . This hydrogen could be harvested and used for propellants or combined with oxygen to make water.

There may be deposits of water ice on the Moon. Scientists theorized for years that ice could exist in the floors of craters near the lunar poles that are in permanent shadow. The ice would come from comets that collided with the Moon over the last several billion years. The existence of water ice in those craters was largely confirmed by the National Aeronautics and Space Administration's (NASA) Lunar Prospector mission in 1998, which found traces of hydrogen, and thus most likely ice, in the shadowed regions at either pole. If ice does exist there, it could be harvested and used for drinking water or broken down into hydrogen and oxygen. NASA and private companies have proposed sending rovers into those craters to confirm that ice is present there and to determine how difficult it would be to harvest it.

Mars

Mars offers even more opportunities for ISRU. The planet has a thin atmosphere composed mostly of carbon dioxide, from which oxygen can be extracted. Many scientists believe that there may be extractable deposits of water ice below the surface of Mars. Even if there are no such deposits, there are small traces of water vapor in the atmosphere.

These attributes make Mars ideal for the use of ISRU. One of the first proposals to employ ISRU on Mars was developed by Robert Zubrin, an aerospace engineer who coauthored The Case for Mars (1996). In the early 1990s Zubrin showed how liquid hydrogen, carried to Mars on a spacecraft, could be combined with the Martian atmosphere to form methane and oxygen, which could then be used as rocket propellant. This process, known as a Sabatier reaction, dates back to the nineteenth century and has been used extensively in the chemical industry. Zubrin and his coworkers showed that a Sabatier reactor could be built easily and cheaply and generate on Mars the propellants needed to return a spacecraft to Earth.

One of the disadvantages of the Sabatier reaction is that it requires a feedstock of liquid hydrogen on the spacecraft that must be kept at temperatures near absolute zero. This may prove difficult on long missions to Mars, so alternatives that do not require liquid hydrogen have been studied. One concept proposed by researchers at the University of Washington uses zirconia crystals and electricity to convert carbon dioxide into carbon monoxide and oxygen, which can then be used as rocket propellant. Carbon monoxide, when combined with oxygen, is not as powerful as methane in rocket engines, but it can be made on Mars without the need for an initial supply of hydrogen.

Water is another key resource that may be found on Mars. Images of some portions of the planet suggest that there may be groundwater sources a short distance beneath the surface. A future mission could bring drilling equipment to reach these water sources and pump it to the surface. Even if subterranean water deposits are not found, there may be ways to extract small amounts of water from the atmosphere. Engineers have proposed passing Martian atmosphere through zeolite crystals. The crystals would absorb water vapor but allow carbon dioxide and other gases to pass through. The water could be extracted from the zeolite later.

Moons, Comets, and Asteroids

The concept of ISRU can be extended to other bodies in the solar system. Comets and many asteroids are rich in water, carbon dioxide, and methane, which could be used by future missions as propellant for the trip to their next destination or home. Water ice may also exist on Phobos and Deimos, the two moons of Mars, allowing them to become refueling stations for missions to that planet. The moons of the outer planets in the solar system are also rich with various kinds of ices. Through resource utilization, it will be possible for future space explorers to "live off the land" as they travel throughout the solar system.

see also Asteroid Mining (volume 4); Living on Other Worlds (volume 4); Lunar Bases (volume 4); Lunar Outposts (volume 4); Mars Bases (volume 4); Mars Direct (volume 4); Mars Missions(volume 4); Natural Resources (volume 4); Power, Methods of Generating (volume 4); Settlements (volume 4); Space Industries (volume 4); Space Resources (volume 4).

Jeff Foust

Bibliography

Schrunk, David, Burton Sharpe, Bonnie Cooper, and Madhu Thangavelu. The Moon: Resources, Future Development and Colonization. New York: John Wiley & Sons, 1999.

Zubrin, Robert, with Richard Wagner. The Case for Mars: The Plan to Settle the Red Planet and Why We Must. New York: Free Press, 1996.

Internet Resources

Grover, M. R., E. H. Odell, S. L. Smith-Brito, R. W. Warwick, and A. P. Bruckner."Ares Explore: A Study of Human Mars Exploration Alternatives Using in Situ Propellant Production and Current Technology." University of Washington.<http://www.aa.washington.edu/research/ISRU/ARES/ares.htm>.

Joosten, B. Kent, and Lisa A. Sharpe. "Enabling Lunar Exploration through Early Resource Utilization." NASA Human Spaceflight.<http://spaceflight.nasa.gov/mars/reference/lunar/lunar1.html>.