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4.2.1 Survey of Solar System Resources
A survey of extraterrestrial resources reveals a number of major stores of energy and raw materials within the Solar System. Ultimately the most significant of these is the Sun itself. Total solar output is 4&times;1026 W, approximately 6&times;1013 times as much as mankind produced on Earth in 1980. An extremely power-intensive (15 kW/person) world society of 10 billion people drawing its materials resources solely from the common minerals of the Earth's crust would require only a trivial fraction (4&times;10-11) of the available solar output (Goeller and Weinberg, 1976).

It is especially significant that the mass of capital equipment required to produce a unit of useful solar power in space is very low. It is anticipated that large-scale solar thermal power stations can be built for 0.1-1 metric ton equipment per megawatt (t/MW) and 1-10 t/MW for solar electric power. These estimates are calculated for 1 AU (i.e., Earth-orbital distances) from the Sun. Alternative terrestrial mass/power ratios are much larger - hydroelectric plants, 103 to 104 t/MW; projected nuclear fusion power stations, 103 t/MW; coal-fired plants, 2&times;102 t/MW (with 4000 tons of coal consumed per MW/yr); and terrestrial (ground-based) solar power, more than 103</SUP> t/MW. Thus, energy systems in space can grow at much faster rates using nonterrestrial materials than is possible on Earth. Energy payback times (time for recovery of initial energy investment) for construction of heliocentric orbital systems at 1 AU is on the order of 10 days. The intensity (I) of solar power varies inversely with the square of the radius (R) from the Sun (I/l<SUB>0</SUB> = R<SUB>0</SUB><SUP>2</SUP>/R<SUP>2</SUP>; I<SUB>0</SUB> = 1.4 kW/m<SUP>2</SUP>, R<SUB>0</SUB> = 1 AU = 1.54&times;10<SUP>11</SUP> m), so space energy systems may be operated at least out to the distance of Saturn (about 10 AU) before capital/energy efficiency ratios (measured in t/MW) approach values comparable to alternative terrestrial power systems. This is because very low mass optical reflectors can be used to concentrate the available sunlight.

Other power sources which eventually may become accessible to mankind include the kinetic energy of the solar wind (10<SUP>14</SUP> MW); differences in the orbital and rotational energies of the Sun, planets, moons, and asteroids (perhaps allowing payloads to move between these bodies) (Sheffield, 1979); and the thermodynamic energies associated with the differentiation of chemical elements in planetoids across the Solar System. Tidal dams on Earth, terrestrial rocket launches, and space probe gravitational swing-bys have utilized trivial fractions of the potential and kinetic energies of the Earth and Moon, and Mercury, Jupiter, Saturn and their moons.

An appreciation of the magnitude of accessible matter resources in the Solar System is gained by noting that terrestrial industry processed about 20 billion tons (about 10 km<SUP>3</SUP>) of nonrecoverable materials in 1972. (Annual tonnages of chemical elements used industrially are listed in table 4.1.) It can be estimated that humanity has processed slightly less than 10<SUP>12</SUP> t (about 500 km<SUP>3</SUP>) of nonrenewable materials since the start of the Industrial Revolution four centuries ago, assuming a 3% annual growth rate. For comparison, a 4.3 km-radius spherical asteroid (density 1.5 t/m<SUP>3</SUP>) also contains 10<SUP>12</SUP> t of matter. Thousands of asteroids with masses in excess of 10<SUP>12</SUP> t already are known (Gehrels, 1979). Approximate total mass of the known minor planets is 2&times;10<SUP>18</SUP> t, the moons 7&times;10<SUP>20</SUP> t, and meteoritic and cometary matter roughly 10<SUP>12</SUP> t. The planets have a total mass of 2.7&times;10<SUP>24</SUP> t.

Mankind has launched about 5000 t into LEO since 1959. Most of this was propellant for Apollo lunar missions and for satellites hurled into geosynchronous orbits or into deep space. Approximately 1000 t was hardware. Averaged over the last 10 years, humanity has ejected mass from Earth at approximately 0.05 t/hr or 400 t/yr. Waldron et al. (1979) estimate that oxygen and possibly most of the fuel (silane based) for liquid-propelled rockets can be produced from lunar soil using chemical processing plants with intrinsic capital mass of 100 t/(t/hr) of output. Thus, a LEO propellant production plant weighing about 10 t could service all current major needs if provided with lunar materials. At some point in the future, major mass fractions of space facilities may be constructed of nonterrestrial matter. Space hardware should be produced in orbit at the rate of 10 kg/hr to match the 1970s and anticipated 1980s launch rates.

The United States Space Transportation System (STS), popularly known simply as the "Shuttle," is expected to establish approximately the same mass/year launching ratio during the 1980s at a cost of about $1000/kg to LEO. Energy represents only a small fraction of this expenditure. Perfectly efficient conversion of $0.05/kW-hr electricity into LEO orbital energy (about 10 kW-hr/kg) would cost roughly $0.50/kg for materials transport to orbit, a factor of 2000 less than near-term STS lift prices. Projected bulk transport versions of the STS may lower Earth-to-LEO expenses to $100/kg; still some 200 times greater than the equivalent cost of electrical energy at present-day rates. When launch charges reach $1/kg a large Earth-to-LEO traffic becomes reasonable, since most terrestrial goods are valued at $1-2/kg (Ayres et al., 1979). However, if space industry someday is to approach cost distributions typical of terrestrial industries, then the supply of bulk or raw materials from the Moon and the asteroids must fall to a few 4/kg (Criswell, 1977a, 1977b). On an energy basis alone, this goal appears achievable using high throughput lunar-mass drivers and relatively cheap solar energy (O'Neill, 1974).

Atmospheres of the various planets and moons are valuable as sources of materials and for nonpropellant braking of spacecraft (Cruz et al., 1979). Deliveries of