WHO NEEDS IT?

Nobody but the space buffs and the military, most people will probably answer.

Untrue. Yes, for putting a spacecraft in orbit around Neptune, nuclear power propulsion would cut the flight time of chemical fuels (20 years) in half; and other feats like a manned mission to Mars and back (in under 2.5 years with a 30 days' stay) are not thinkable with chemical fuels alone. But that is not exactly urgent.

As for the military, they need nuclear power in space for the same reasons as civilians: communications and air traffic control.

Only expert archive keepers know how many communication satellites for telephony and TV are now in geosynchronous orbit (traveling as fast as the earth revolves, so that they always remain above the same terrestrial point) Such orbits are possible only at a certain height (36,000 km or 22,500 miles) above the equator, and the low-power transmitters serving the US, for example, are crowding the place ever more densely: we country bumpkins can no longer use a mere 3 ft dish to get CNN's slanted news from one satellite without interference from its neighbor. Higher power would allow the satellites to be spread out further.

There is a number of other applications, including the internal powering of space stations, efficient orbit delivery from low earth orbits, and long-term orbital maintenance and attitude control of other satellites; but even if one were to settle for no more than com-munications with broader coverage and higher resolution, and for high power availability for on-board processing, nuclear power would pay off.

The first phase of the program, deciding on the system, came to an end in August of last year. The choice: a thermoelectric reactor, uranium fueled and liquid-lithium cooled. "Thermoelectric" works like a thermocouple: a voltage is developed across the junction of two dissimilar metals when they are heated. However, here the place of metals will be taken by an improved silicon-germanium semiconductor system. Phase Two calls for a ground test of a 100 kW reactor by December 1992. By mid-1991 a 4 to 6 kW conver-sion module with integrated heat and electric components is to go critical, and nuclear electric power sources should be orbiting by the mid- to late 90s, though nuclear-electric propulsion of spacecraft is unlikely to be ready before the 21^st century.

It is my charitable hope that Ralph Nader and Jane Fonda will live long enough to enjoy nuclear power in the skies: abundant and safe on earth as it is in the heavens.

[More V.C. Truscello and H.S. Davis, "Nuclear-electric power in space, IEEE Spectrum, Dec. 1984; personal interview with Jack F. Mondt, Deputy Manager of the SP-1OO Project, Jet Propulsion Lab., Pasadena, Calif.]