Nuclear Propulsion

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Nuclear Propulsion

Nuclear energy remains an attractive potential means of propulsion for future spacecraft. When compared with conventional rocket engines, a nuclear propulsion system would in theory be less massive, and could provide sustained thrust with greater energy. Many believe nuclear-powered spacecraft can and should be built, but first many technical problems and other hurdles must be overcome.

Both the U.S. and Soviet space programs were researching nuclear propulsion as far back as the early 1960s, and since then, dozens of ideas for nuclear propulsion systems—and the spacecraft they would power—have been proposed. Each system, however, is based around one of the two methods of generating nuclear energy: fission and fusion.

Fission Propulsion

Fission is the act of splitting a heavy atomic nucleus into two lighter ones, which results in a tremendous release of energy. Common fuels for fission reactions are plutonium and enriched uranium, a soft-drink sized can of which carries 50 times more energy than the space shuttle's external tank.

Fission has been used to generate electricity on Earth for six decades, often by using the heat from the reactor core to boil water and spin a turbine. But a reactor core could also be used to heat a propellant such as hydrogen into a super hot gas. The gas could then be expelled out of a nozzle, providing thrust, just like in a conventional chemical rocket. Engines of this type are called nuclear thermal rockets (NTRs), and were ground-tested by the United States in the Rover/NERVA program of the 1960s.

A related method, being studied by the National Aeronautics and Space Administration (NASA) in the early 2000s, would give an NTR the equivalent of a military jet's afterburner. In this scheme, liquid oxygen could be pumped into the exhaust nozzle. This would cool the hydrogen enough that it could combine with the oxygen and burn, providing additional thrust and leaving water vapor as a by-product.

NTRs could produce enough thrust to carry a spacecraft into orbit, but because the propellant itself would quickly run out, they are unsuitable for longer missions to Mars or beyond. An alternative approach to NTRs is to use the reactor to produce electricity, which could power various types of electrical thrusters. Such nuclear-electric propulsion systems (NEPs) would use electric fields to ionize and/or accelerate propellant gas such as hydrogen, argon, or xenon. NASA plans to put development of NEPs on a fast track beginning in 2003.

NEPs would be able to produce smaller amounts of continuous thrust over periods of weeks or months, making them extremely suitable for robotic missions to the outer planets or slow journeys between Earth orbit and the Moon. For human missions, when diminishing supplies for the crew make speed a more important factor, a combination of NTRs and NEPs could be used.

Fusion Propulsion

We have nuclear fusion to thank for life on Earth: Most solar energy comes from the four million tons of hydrogen that is converted into helium every second in the interior of the Sun. But fusion can only occur in superheated environments measuring in the millions of degrees, when matter reaches a highly ionized state called plasma. Since plasma is too hot to be contained in any known material, controlled nuclear fusion remains one of humanity's great unrealized scientific goals.

However, plasma conducts electricity very well, and it could be possible to use magnetic fields to contain and accelerate it. It might even be more feasible to use fusion in space, where it would not be necessary to shield the reactor from the environment in all directions, as it would be on Earth.

Experiments toward developing a fusion propulsion system are underway at NASA's Marshall Space Flight Center in Huntsville, Alabama. The Gas Dynamic Mirror (GDM) Fusion Propulsion system would wrap a long, thin current-carrying coil of wire around a tube containing plasma. The current would create a powerful magnetic field that would trap the plasma in the tube's center section, while each end of the tube would have special magnetic nozzles through which the plasma could escape, providing thrust.

The amount and efficiency of the energy released by fusion makes it a good candidate for interplanetary travel. As a comparison of their efficiency, if a chemical rocket were an average car, a fusion rocket would get about 3,000 kilometers (1,864 miles) per liter! Fusion also has great potential as an energy source because of the nature of the fuel and reaction—hydrogen is the most common element in the universe, and the by-products are non-radioactive (unlike fission products, which remain hazardous for many years). But until fusion becomes a reality, fission is humanity's sole option for nuclear-powered space travel—and is not without strong opposition.

Pros and Cons

Plutonium is one of the most poisonous substances known; doses of one millionth of a gram are carcinogenic, and it is difficult to contain the radioactive by-products of fission safely. These dangers have made nuclear fission controversial from the outset, and the prospect of a nuclear reactor reentering Earth's atmosphere and scattering radioactive material over a wide area makes many people nervous.

Many space probes have not carried reactors but are powered by radioisotope thermoelectric generators (RTGs), which derive electrical power from the slow decay of radioactive material. There was concern in 1997 that the Cassini probe to Saturn might meet with an accident as it flew by Earth, scattering its RTG's 33 kilograms (72 pounds) of plutonium into the atmosphere. This did not occur and Cassini continued on its route to Saturn. However, such concerns, along with the high projected cost of research and construction, have been obstacles in the way of nuclear-propelled spacecraft.

Still, nuclear propulsion could dramatically decrease travel time to the planets. A round trip to Mars could be accomplished in half the time with fusion power, which would lessen the crew's exposure to the hazards of weight-lessness and cosmic radiation . A nuclear-propelled craft could conceivably be used repeatedly for round trips to the Moon and planets, cutting down the cost of operating such a long-term transit system. Funding for the development of new nuclear propulsion will be boosted in 2003 with a view to production of an operational system within a few years.

see also Accessing Space (volume 1); Antimatter Propulsion (volume 4); External Tank (volume 3); Faster-Than-Light Travel (volume 4); Interstellar Travel (volume 4); Ion Propulsion (volume 4); Laser Propulsion (volume 4); Lightsails (volume 4); Power, Methods of Generating (volume 4).

Chad Boutin