Scientific Research

Propulsion Systems

All rockets in use in the early twenty-first century are propelled by some form of chemical rocket engine. Rockets with sufficient power to place a satellite in orbit use at least two stages. However, one long-term goal has been the reusable "single-stage to orbit" engine design. This would provide quick turnaround, much like a conventional aircraft, and greatly reduce the cost of getting to orbit because of reduced processing and flight preparation. An interim step may be a two-stage vehicle with boosters that fly back and land the spaceport for refurbishment after each launch.

Once a spacecraft is in orbit, other forms of propulsion are necessary. Several exotic propulsion systems have been proposed and investigated over the years. Orion was a project to design and construct a propulsion system using small atomic bombs. While this sounds impractical, many scientists think that such a propulsion system would have allowed humans to get to the Moon more quickly at a much lower cost than the Saturn V launch system. A variation of this type of propulsion is the nuclear thermal rocket. This system uses a nuclear reactor to heat a gas, which is then expelled through a nozzle, providing thrust.

The crew of a rocket ship powered by a nuclear rocket engine would need to be shielded from the reactor. One proposed solution is to place the engine at a large distance away from the crew quarters, connecting the two compartments by a long truss. In this design, distance substitutes for heavy shielding.^* Many scientists believe that if humans are to move beyond Earth orbit, some version of a nuclear rocket engine will be necessary.

Between 2002 and 2007, NASA plans to develop an improved radio-isotope power system for use in robotic planetary exploration and targets the first use of this power system for a Mars mission in 2009. During the period between 2003 and 2013, significant funding will be dedicated to the development of a nuclear-electric-propulsion system to enable a new class of planetary missions with multiple targets, to reduce spacecraft travel time, and to decrease mission cost.

Nuclear electric propulsion systems only use the nuclear reactor to generate electricity. The rocket engine itself is electrically powered. There are three classes of electric rocket engines: electrothermal, electrostatic, and electromagnetic. In electrothermal propulsion, the gas is raised to a high temperature and expelled through a rocket nozzle. Electrostatic propulsion systems first convert the gas to a plasma (highly ionized material) and then use electric fields to accelerate the gas to high velocity. Electromagnetic propulsion uses magnetic fields to accelerate a plasma.

Other propulsion systems include various configurations of solar sails,ion propulsion systems, and laser propulsion. Several systems involve the use of stationary high-powered infrared pulsed lasers. In one interesting system, the laser is fired at a parabolic reflector on the back of the spacecraft. This reflector focuses the laser energy, explosively heating air behind the craft and propelling it forward. In space, the reflector would be jettisoned and the laser would fire pulses at a block of propellant (ice would work) heating it to vapor.

Space Power Generation

Spacecraft currently use solar power, hydrogen fuel cells, or radioisotope thermoelectric generators to generate electrical power and rechargeable batteries to store electrical energy. The International Space Station uses solar panels and rechargeable batteries. Solar power is converted to electrical power in large panels containing photovoltaic cells. These cells convert light directly into electricity using a semiconductor such as silicon or gallium arsenide. Solar panels are relatively low cost and simple. However, they are fragile, take up a lot of space, and become less effective as a spacecraft travels away from the Sun. For future missions that penetrate deeper into the solar system, and beyond, alternative power sources will be essential.

Fuel cells combine hydrogen and oxygen to make water. When hydrogen combines with oxygen, energy is released. A fuel cell converts this energy directly into electricity. Fuel cells are relatively compact and produce usable by-products, but they are complicated and expensive to produce.

Radioisotope thermoelectric generators (RTGs) convert the heat produced by the natural decay of radioactive materials to electrical power by solid-state thermoelectric converters. RTGs are lightweight, compact, robust, reliable, and relatively inexpensive. These devices allow spacecraft to operate at large distances from the Sun or where solar power systems would be impractical. They remain unmatched for power output, reliability, and durability.

Resource Utilization

If a human colony is to be established on Earth's Moon, Mars, or elsewhere in the solar system, some means of transporting large amounts of materials to the colony site must be developed. It would be prohibitively expensive and impractical to transport materials from Earth in sufficient quantity to build a base on the Moon or Mars. However, this is not necessary, since both Earth's Moon and Mars have an abundance of raw materials that could be used for construction.

The Moon may have a substantial amount of water locked in permafrost in the bottom of deep craters near its poles where sunlight never reaches or in clays. Although it would be expensive to mine this water, it would be far cheaper than transporting water from Earth. The Moon also has surface rocks rich in light materials such as aluminum and silicon dioxide. It would require large amounts of electrical power to produce pure aluminum or glass from Moon rocks, but solar energy is abundant because of the lack of atmosphere.

The Moon may even have sufficient quantities of helium-3 to make a lunar settlement economically self-supporting. The helium-3 would be extracted from lunar soil, packaged as a compressed gas or liquid, and returned to Earth for use in fusion reactors. Due to the lower gravity, launching a rocket from the surface of the Moon for return to Earth is far less costly than launching a rocket from Earth.

Mars also has significant resources available. The red color of Martian soil is due to the presence of large quantities of iron oxide. Other minerals and elements are also present. In addition, Mars is thought to have vast quantities of subsurface water. Asteroids have long been recognized as accessible, mineral-rich bodies in the solar system and are a ready target for resource mining.

SEE ALSO ASTEROID MINING (VOLUME 4); ION PROPULSION (VOLUME 4); LIGHTSAILS (VOLUME 4); LUNAR BASES (VOLUME 4); LUNAR OUTPOSTS (VOLUME 4); MARS BASES (VOLUME 4); MARS MISSIONS (VOLUME 4); POWER, METHODS OF GENERATING (VOLUME 4); SOLAR POWER SYSTEMS (VOLUME 4).

Elliot Richmond

Bibliography

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National Commission on Space. Pioneering the Space Frontier, The Report of the National Commission on Space. New York: Bantam Books, 1986.

Office of Technology Assessment. Exploring the Moon and Mars: Choices for the Nation, OTA-ISC-502, Washington, DC: U.S. Government Printing Office, 1991.

O'Neill, Gerard K. The High Frontier: Human Colonies in Space. New York: WilliamMorrow and Co., 1977.

Space Science Board. Life Beyond the Earth's Environment: The Biology of Living Organisms in Space. Washington, DC: National Academy of Sciences, 1979.

Space Science in the Twenty First Century: Imperatives for the Decades 1995 to 2015—Life Sciences. Washington, DC: National Academy Press, 1988.

Wilhelms, Don E. To a Rocky Moon: A Geologist's History of Lunar Exploration. Tucson: University of Arizona Press, 1993.

Internet Resources

Astronomy Resources from STScl. Space Telescope Science Institute.<http://www.stsci.edu/resources/>.

NASA Human Spaceflight.<http://spaceflight.nasa.gov/history/>.

Space Science. National Aeronautics and Space Administration.<http://spacescience.nasa.gov/>.