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In the not too distant future nuclear rockets may become a reality. NASA has been given approval and has already selected a contractor to design and build a nuclear electric propulsion system called Prometheus.
The system will use electricity generated by a nuclear reactor to power an electric ion drive system for a new generation of spacecraft which one day will be capable of taking man to the outer reaches of the solar system. It sounds like science fiction but the concept of nuclear rockets is actually 50 years old, starting with Project Pluto in 1957. Between 1957 and 1972 several nuclear rocket designs were proposed, but only a few were partially tested. Cost, engineering difficulties and public opinion led to the abandonment of most designs.
Today, the idea of using nuclear rockets is again front and centre. Many experts agree that nuclear rockets may be the only way to carry large payloads to Mars and the outer planets. So far, space exploration has involved the use of chemical rockets to propel most spacecraft through space. One of the problems with chemical rocket systems is weight. Chemical fuels are heavy and add weight to the launch vehicle which puts limitations on the size of the payload that can be launched into space.
In 1998, NASA launched Deep Space 1 and Deep Space 2 which successfully demonstrated that an electrostatic ion engine could propel a spacecraft. Deep Space 1 did a flyby of the Braille asteroid and followed Comet Borelly, sending back data and photographs. Deep Space 2 successfully reached Mars but the surface probes failed to function. In both cases, the electrostatic ion engines performed better than expected.
The engine works by bombarding a gas with a beam of electrons. This knocks electrons off the atoms of the gas, creating a positively charged ion. High voltage metal grids at the back of the engine chamber accelerate the positive ions toward the grid. As they pass the grid, they reach speeds of over 30 km/s and are focused into an ion beam before being exhausted out the back of the engine. Finally, a neutralizer collects excess electrons and injects them into the ion beam to prevent a build-up of negative charge on the spacecraft. Smaller but similar ion propulsion systems are used on satellites and space probes to make course corrections. Both Deep Space 1 and 2 used solar panels to convert sunlight into the electricity needed for the ion engine.
Prometheus 1 will use a nuclear reactor to generate the electricity required for the engine. The mission objectives will be to explore three of Jupiter’s moons: Callisto, Ganymede, and Europa, and to demonstrate that nuclear electric propulsion flight system technologies would enable a range of revolutionary planetary and solar system missions.
While an electric ion engine has smaller thrust than a conventional chemical engine, it can operate for a significantly longer period of time. This technology will allow spacecraft to travel greater distances at greater speeds, carry larger payloads, and also allow spacecraft to be more manoeuvrable. There are advantages to using a nuclear reactor to generate the electricity needed. Nuclear reactors are capable of generating far more electricity than solar panels and the excess electricity generated can, on future missions, be used to maintain life support, computer, and communication systems as well as power large centrifuges to create artificial gravity for manned missions to Mars and beyond.
Resources:
Steven R. Oleson and Frederick W. Elliott, Electric Propulsion Technology Developed for Prometheus,
www.grc.nasa.gov/WWW/RT/2005/RP/RPP-oleson.html