The long history of exploration and mining of mineral resources on Earth provides the foundation for the extension of the human footprint to space. Resources in space share some features with the better-known resources on Earth but also present some unique challenges. Foremost among these are the energy cost of escaping Earth’s gravity and the need to develop in situ resources that can be processed robotically, to the extent possible, because of the supplies (air, food, and water) necessary to support long-term human space travel and habitation. Despite popular imagination, it is unlikely that space resources can or will be brought back to Earth, due to the high energy cost of overcoming gravity.
The most likely commodity for in situ resource utilization (ISRU) is water, which has multiple and indispensable uses as propellant and life support for agriculture and respiration. The need for propellant to be available in space reflects the fact that fuel constitutes ~80% of the mass of a rocket that can travel beyond Earth orbit. Thus, water will be the first and most important space resource to be developed. Water is known to be present on the moon, Mars, and in C-type asteroids. It is relatively straightforward to separate water into hydrogen and oxygen by electrolysis, and solar energy provides abundant and uninterrupted power in space. Water combined with carbon in asteroids or carbon dioxide in the Martian atmosphere provides the basis for agriculture and the production of methane as a practical and easily handled propellant.
In addition to structural materials for buildings or spacecraft, the other resource needed for long-term space exploration or colonization is bulk material for radiation shielding. This could be any of the loose regolith materials known to exist on the lunar and Martian surfaces and on some asteroids. Such regolith could be used as is, or manufactured into blocks or other structural materials. All of these space resources need to be assessed and characterized to provide the equivalent of a bankable feasibility study in order to prove the business case for their future development.