NOW that Galileo is safely on its way to Jupiter, it's worth reconsidering - calmly - the concern raised over its plutonium-fueled power packs. Future interplanetary missions are scheduled to carry similar units. Furthermore, the Galileo controversy has obscured a very serious safety issue.
The continuing use of nuclear-power sources - especially reactors - in earth orbit is dangerous. It should be stopped. But mixing this danger with the miniscule risk of contamination from the Galileo power packs - as some launch protesters have done - needlessly confuses matters.
The United States would not have had to ``kiss Florida goodbye'' if the shuttle had blown up on the pad, as one fear-struck headline put it. Nor would radioactive plutonium have floated in a plume killing tens of thousands of people if Atlantis had suffered a midair disaster, as one launch opponent claimed.
The radioisotope thermoelectric generators (RTGs) that supply electricity to interplanetary craft do indeed carry significant amounts of radioactive plutonium. It is a type of plutonium that decays rapidly, with half of it decaying in 88 years. It is not in the form of metal that burns in air and can disperse widely. It is a ceramic substance shaped into pellets that are protected by several layers of tough materials.
These RTGs are designed to survive an accident, including atmospheric reentry, intact. That's exactly what happened in the few RTG losses in the past. Most of these units are on the sea bottom, inside containers that should outlast the lifetime of the plutonium.
Some antilaunch advocates distorted this record by noting a 1964 incident when an RTG did burn up on reentry and disperse its plutonium. That was a Defense Department unit, not a National Aeronautics and Space Administration assembly. It was deliberately designed to disperse in this way. In retrospect, it's obvious that was an irresponsible design. It is the opposite of the design philosophy for the interplanetary RTGs.
NASA uses its RTGs only when there is no alternative. Missions to Jupiter and beyond are too far from the sun to use solar cells. Equipment such as the Viking landers on Mars or the Apollo instruments left on the moon spend too much time in darkness to use solar power either. So unless such valuable exploration is abandoned, RTGs are needed.
Earth orbit is another environment altogether. There's no overriding need for nuclear power packs there. Yet there is an ever-present danger of such packs contaminating the atmosphere and Earth's surface. The reactors that the Soviets, particularly, have used for military satellites are especially risky. Two of those Soviet reactors, out of some 30 launched so far, have reentered and spread radioactive debris on land.
The US has not used such reactors. But it is designing them and currently plans their use, particularly in the proposed Strategic Defense Initiative system. This is foolish, given what is known of the risks. There are strong movements both in the United States and the Soviet Union to ban nuclear-power sources in space. The US Congress has legislation before it that would do just that. But this makes an exception for interplanetary RTGs.
A few protesters raised a needless alarm over Galileo. Such excess tends to discredit legitimate concerns, whatever the issue. It would be unfortunate if it were to undermine the substantial concern we all should feel about nuclear-power sources in Earth orbit.