Some meteorites representative of certain classes of asteroids are 25% or more water by weight. This is consistent with infrared spectra of some asteroids, indicating hydrated minerals are abundant in some varieties of carbonaceous chondrite asteroids. Since water is very valuable in space, it would be desirable to be able to process asteroids to recover this water and other volatiles. The Asteroid Redirect Mission concept has formulated a method for returning asteroids of 1,000-ton mass into the Earth-Moon system orbit using only ~10 tons of propellant. If ~25% of that returned asteroid mass were recovered as volatiles and solar power used to make those volatiles into propellant, then the overall system would generate approximately 25 times as much propellant as it uses. This could be used to make sustainable human missions to Mars or otherwise spread humanity into the solar system.
The basic concept for a processing facility for volatile-rich asteroids returned by subsequent flights of the Asteroid Redirect Mission is to “cook” the asteroid using a solar oven to “bake out” the volatiles. The concept is that an asteroid would be placed into a spherical shell that is black both on the inside and the outside, and sealed. A mirror would concentrate sunlight that would impinge on the outside of the black sphere, heating it rapidly. This thin shell would get very hot on the inside, and radiate more and more energy in the thermal infrared as it rapidly heats up. The outer surface of the asteroid would begin to also heat rapidly, and evolve the water that may be adsorbed onto the surfaces of mineral grains and also incorporated into the mineral structure of the grains.
The vapor that is driven off pressurizes the black sphere and escapes through a small hole in the back of the sphere to go into a long, cigar-shaped “cold trap.” The outer surface of the cold trap is coated black so that, in the shadow of the mirror, it receives no sunlight and radiates into the black sky. The back of the mirror (made from a thin foil or membrane) will be slightly hot from the small fraction of sunlight absorbed, so the cold trap surface will get colder as one progresses farther from down the trap away from the mirror. The volatile vapors condense onto the inner surface of the cold trap in order according to their vaporization and freezing points. In this way, nearly pure volatiles of several species (e.g., water, ammonia, CO2, CO) can be separated from the asteroid and from one another.
Because some of the reflected sunlight reaches the black sphere at a substantial angle from the surface normal, the black sphere needs to be coated such that it absorbs from significant angles off normal. One way of accomplishing this is to have the surface coated with carbon fibers sticking up from the surface to act as a light trap.