Using the same plentiful sunlight that makes solar-electric and solar-sail propulsion possible, solar-thermal propulsion (see Chapter 12) offers a much higher thrust than either of them. This higher thrust is at the expense of efficiency, though it is still twice as efficient as its chemical propulsion brethren. Using sunlight to superheat propellant, thus achieving very high exhaust velocities, solar-thermal propulsion would appear to be a good candidate for moving large cargo in near-Earth space. The system-level benefits of the technology are somewhat different from those offered by solar-electric propulsion. The overall mission-level propellant requirements, hence the total mass to be launched by rocket, would be less than an all-chemical system. They would not be as low as that provided by SEP, however the thrust ofa solar-thermal system would be significantly higher, making it better for moving large masses around fairly quickly.
Solar-thermal propulsion might be most useful when we achieve a level of interplanetary exploration beyond simply visiting the Moon or Mars. When humans are routinely moving around in the Earth—Moon system and beginning to mine the plentiful resources of the asteroids, we will see solar-thermal propulsion begin to play a key role. It can be used to quickly and efficiently carry the minerals and ore from asteroid mines back to the Earth for processing. Such processing facilities might be in Earth orbit or on the Moon. These solar-thermal propulsion systems will be high thrust, allowing them to move large payloads, with high efficiency, thus lowering their propulsion mass and cost. Since cruise time will not be as long as that required for deep-space missions to Mars and beyond, the continuous, low thrust of a SEP system will not be as attractive as solar-thermal propulsion for these applications. Solar-thermal propulsion offers an attractive balance of quick trip times and lower mass—essential factors for a future space-based industry delivering real products from space for paying customers.
The technologies required to field a solar-thermal propulsion system are closely aligned with those that will be required for some in-space manufacturing applications. For example, large, inflatable solar concentrators used to heat the propellant might be adapted to use on the Moon or asteroids for heating rock as part of the smelting process. Just as our ancestors learned to obtain copper from ore by heating it beyond its melting point, so will we learn to emulate the process in deep space as the foundation of a non-terrestrial industrial base. These "simple" manufacturing processes will have to be relearned in this radically different environment.
The fuel for the solar-thermal propulsion system will initially be launched from Earth with the propulsion hardware. At first, this will also be true for the oxidizer required for the smelting operation. However, economics will almost certainly drive us to derive them from the resources of space. Lunar regolith contains oxygen (as well as many elements and minerals, many of which will be very useful for other manufactured goods)—required not only for our human crews to breathe but also for smelting. We can extract the oxygen to enable our factory workers to use it by adding hydrogen and heating it—using the solar concentrators developed for the solar-thermal propulsion system. On the question of where the hydrogen comes from, recall that hydrogen is an excellent propellant for solar-thermal propulsion systems and is abundant in the form of water ice in comets. Hydrogen can be obtained from water by passing a current through it in a process called electrolysis. (Which also releases the other elemental component of water: oxygen?)
Data from the Lunar Prospector mission suggests that water ice may be present on the Moon near its poles, protected from the heat of the Sun by being in the perpetual shadows created by lunar craters. The water or ice need only be resupplied infrequently as the smelting process does not consume the hydrogen, but merely uses it as a catalyst. A closed system can recycle most of the oxygen and hydrogen many times, producing excess oxygen from the regolith in each cycle. There are undoubtedly more efficient or creative methods for extracting the minerals needed for an industrial society from lunar or asteroid resources, and the expertise of creative chemical and industrial engineers will most certainly be essential.
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