Space Resources

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Space Resources

Future large-scale space activities will require a high degree of autonomy from Earth, with extensive reliance upon nonterrestrial sources of energyand materials. Ambitious missions require large masses of consumables, such as propellants and life-support fluids, which traditionally have been launched from Earth. But launch costs from Earth are so high that the greatest advantage would be realized by launching small masses of processing equipment rather than large masses of intrinsically cheap, abundant, and easily manufactured materials, such as oxygen, water, liquid hydrogen, structural metals, and radiation shielding . Each of the various objects in the solar system has unique potential in terms of resource extraction.

The Moon

Operations on the Moon would benefit greatly from the use of unprocessed regolith for shielding. Oxygen can be extracted from the common lunar mineral ilmenite (FeTiO₃) by reduction using hydrogen, carbon, or hydrocarbons, leaving a residue of metallic iron and the refractory rutile (TiO₂). Lunar polar ice deposits may conceivably be exploited for the manufacture of liquid water, oxygen, and hydrogen, if the difficulties of mining in permanent darkness at a temperature of 100°K (−280°F) can be mastered. On a longer timescale, lunar helium-3 , present as an embedded solar-wind gas in concentrations of up to 10⁻⁸ g/g, may be economically extractable for export to Earth as a clean fusion fuel .

Mars

Both piloted and unpiloted missions to Mars would benefit from the universal availability of the Martian atmosphere. The principal Martian gas, carbon dioxide, can be decomposed by any of several processing techniques into carbon monoxide and oxygen for use as propellants for local transportation or for the return trip to Earth. Extraction of water from the Martian atmosphere, which would enable the use of hydrogen as a propellant, seems unreasonable because of the extreme aridity of Mars. Surface snow, ground ice, permafrost, clay minerals , and hydrated salts are all plausible sources of extractable water. The residual atmospheric gases after extraction of carbon dioxide principally would be nitrogen (which makes up 2.7% of the atmosphere) and argon (1.6%). Nitrogen is useful not only as a fire retardant in artificial air but also as a feedstock for the manufacture of ammonia,hydrazine , nitrogen tetroxide, and nutrients such as amino acids and organic bases.

Near-Earth Asteroids

The near-Earth asteroids (NEAs) and the Martian moons Phobos and Deimos present a rich diversity of compositions, many of them rich in volatile materials . A substantial fraction of these bodies are energetically more accessible than Earth's Moon, in that the velocity increment needed to fly from low Earth orbit and soft-land on the surfaces of nearly 20 percent of the NEAs is smaller than that needed to land on the Moon. The L-4 Lagrangian point on the orbit of Mars has captured a swarm of small asteroids, of which four are currently known.

The asteroid belt consists of bodies that seem to be well represented among the NEAs. The resources of interest in them would be the same as those in NEAs. Most extraction facilities placed on NEAs would visit the heart of the asteroid belt on each orbit around the Sun, making transfer from an NEA "gas station" to most belt asteroids easy. In a fully recycling economy, fueled by solar power, the resources in the asteroid belt would be sufficient enough to support a population of about 10 quadrillion people from now until the Sun dies of old age.

Gas Giants

Beyond the asteroid belt lie the orbits of the four gas giant planets: Jupiter, Saturn, Uranus, and Neptune. The total number of known gas giant satellites is close to ninety and is expanding rapidly because of advances in detection technology. We may reasonably expect several hundred satellites larger than a few kilometers in diameter to be known in a few years.

Jupiter's system consists of several very close small satellites and a rudimentary ring system; four world-sized Galilean satellites named Io, Europa, Ganymede, and Callisto; and swarms of small distant satellites, with some, like the inner satellites, orbiting in the prograde direction, but with the outermost satellite family in retrograde orbits. These may well be transient moons, captured in the recent past from heliocentric orbits (orbits around the Sun) and destined to escape again. Jupiter is also accompanied by two vast clouds of asteroids, centered on the leading and trailing Lagrange points on Jupiter's orbit. These bodies, which are spectroscopically identified as supercarbonaceous , are the presumed immediate source of the outermost captured satellites of Jupiter. The innermost small satellites are embedded in the inner magnetic field of Jupiter, subject to intense charged-particle radiation bombardment from Jupiter's radiation belts. The radiation environment improves with increasing distance from the planet, but the Galilean satellites (especially Io) present a daunting technical challenge to planned landing missions. All of the Galilean satellites except Io have abundant surface ice of varying degrees of purity, suitable for manufacture of propellants for return to Earth.

Saturn's system seems similar to Jupiter's, except that Saturn's extensive ring system suppresses its radiation belts. The largest Saturnian satellite, Titan, has a massive atmosphere of nitrogen, methane, and photochemical products that both invites detailed scrutiny and offers potential propellant for escape. Numerous small, distant satellites in both prograde and retrograde orbits have been discovered recently. Finding asteroids on Saturn's Lagrangian points is difficult and has not yet been accomplished.

Uranus and Neptune, with far lower escape velocities than Jupiter and Saturn, are readily accessible to entry probes. With a nuclear propulsion system, escape from their atmospheres is clearly possible. Both planets presumably contain about fifty parts per million of helium-3 gas in their atmospheres, making the extraction and retrieval of vast amounts of fusion fuel conceivable. There is enough helium-3 in the atmosphere of Uranus alone to power Earth with a population of 10 billion people at European or North American levels of energy use for at least 10¹⁵ years. The satellite system of Uranus contains several midsized moons and many small, distant satellites, most of which have been very recently discovered. Neptune's system, with the large retrograde satellite Triton, several irregular ring arcs, and a midsized distant satellite, Nereid, is dynamically interesting, suggesting violent events in its past that may have disrupted any system of small satellites that may once have been present.

The Centaurs

The Centaurs, which cross the orbits of the gas giant planets, are analogous to the NEAs in the inner solar system. These presumably cometary bodies, which reach several hundred kilometers in size, are vulnerable to severe perturbations by these planets. Indeed, numerical analysis of the orbit of the Centaur Chiron suggests that it could cross Earth's path someday, possessing a kinetic energy about 1 million times larger than the impact energy of the asteroid that is theorized to have ended Earth's Cretaceous era (and killed off the dinosaurs). The principal resource interest of such bodies lies in their possession of abundant propellant, which could be used for self-deflection in the frightening event that such a body should be found on a path that threatens Earth.

The Kuiper Belt

Bodies in the Kuiper belt, which lies beyond the orbit of Neptune, follow orbits that are moderately eccentric and moderately inclined with respect to the ecliptic . These bodies appear to be basically cometary in composition, although recent evidence suggests that there are two populations that are compositionally distinct. The largest-known body in the population is Pluto. Theory suggests that these bodies are about 60 percent ices by mass, with total extractable volatiles possibly reaching 70 percent.

The Oort Cloud

The Oort cloud, even more remote from human eyes and reach, consists of about 1 trillion bodies of kilometer size and larger, following orbits that are essentially random in three dimensions and lie almost exclusively outside the orbits of the planets. Typical distances from the Sun are 10,000astronomical units , and typical orbital periods are on the order of 1 million years. The few Oort cloud bodies that penetrate the inner solar system are called long-period comets. The severe lack of solar energy for propulsion and processing use, and the large mean distances between nearest neighbors, makes this realm unattractive as a potential resource.

Programmatically, initial space resource use will be confined to the Moon, Mars, and NEAs. Asteroidal and lunar resources have clear application to support of large-scale space activities such as the construction of solar power satellites and lunar power stations. The transition from NEAs to the asteroid belt seems an obvious next step. Some asteroids and short-period comets in turn belong to orbital classes that offer access to the Jovian and Saturnian families. Scenarios involving helium-3 for use as fusion fuel lead to the consideration of Uranus as the next target.

see also Asteroid Mining (volume 4); Comet Capture (volume 4); Natural Resources (volume 4); Resource Utilization (volume 4).

John S. Lewis