Star Wars Does It Again

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In the eight years since it was founded, the Strategic Defense Initiative has poured $24 billion into various schemes for knocking down ballistic missiles, many of them dubious. But no Star Wars project seems more clearly -- or appropriately -- destined for the technological trash heap than the one that came to light last week. According to documents made public by the Federation of American Scientists for the express purpose of torpedoing the scheme, the Pentagon has for several years been secretly developing a new kind of booster rocket -- code-named Timberwind -- that would loft giant weapons into space on short notice. Its power source: an onboard nuclear reactor running at extremely high temperatures and spewing radioactive exhaust directly into the atmosphere.

The idea behind Timberwind is simple. Just pump liquid hydrogen through a small nuclear reactor heated to several thousand degrees Fahrenheit. The liquid hydrogen is instantly converted to hydrogen gas, which then blasts out of a nozzle. The resulting thrust is two to three times as great as that generated in conventional rocket engines by the explosive mixture of hydrogen and oxygen. Much larger payloads could thus be lifted into orbit.

That is the theory. In practice, it's more complicated. The reactors must be built of materials that are both lightweight and capable of withstanding extraordinary temperature changes, from several hundred degrees below zero to several thousand degrees above. To reduce the risk of fatal meltdowns, the uranium fuel must be packed in tiny particles coated with several layers of carbon alloy and carefully machined to very close tolerances. And because the fuel gives off "hot" -- meaning radioactive -- by-products, it is inevitable that the escaping gas will pick up some radioactivity on its way out.

These technological problems may be solvable. Timberwind proponents say cleanup systems could remove radioactive by-products before they are discharged into the air. Better still, the atomic engines would be handy on a manned mission to Mars. Nonetheless, the program's political problems may be insurmountable. The 1979 accident at Three Mile Island shook America's confidence in nuclear technology, and the Challenger explosion dramatically demonstrated the vulnerability of space launches. Not surprisingly, many scientists are bothered by the idea of putting these two technologies together. In 1989, antinuclear activists, protesting potential "Chernobyls in the skies," organized the first civil-disobedience demonstrations aimed at halting a U.S. space shot. Their target: NASA's Galileo spacecraft, an interplanetary scientific mission that used as its power source two | radioisotope thermoelectric generators fueled by plutonium. In October 1989, the Galileo launch went off without a hitch, despite the protests.