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A possible future for NASA’s forays into deep space can be found at the Department of Energy’s Oak Ridge National Laboratory in Tennessee, at the bottom of what looks like an indoor swimming pool.

There, bathed in the electric-blue light of the nuclear High Flux Isotope Reactor, aluminum tubes packed with small, silvery cylinders of the radioactive element neptunium-237 are being bombarded with neutrons. It is modern-day alchemy; the neutrons are transmuting the neptunium into something that, at least to NASA’s mission planners, is more precious than gold: plutonium-238 (Pu-238), one of the rarest and most fleeting materials in the universe. Once made, the Pu-238 will glow red-hot for years on end as it gradually decays into uranium. Pu-238 cannot be used to make atomic bombs, nor is it particularly useful for fueling nuclear reactors, which are widely considered too controversial and expensive for practical use in space missions. Instead, Pu-238’s steady supply of heat makes it an ideal power source for long-haul interplanetary voyages where conditions may be too dim and cold for solar power and chemical batteries.

But NASA’s supply is running out, and the Department of Energy’s efforts to make more at Oak Ridge are proceeding too sluggishly for comfort. Alarmed members of Congress have repeatedly demanded that NASA produce studies detailing just how much plutonium it needs, how it plans to acquire the plutonium, and what‘s at stake if the stockpile runs out, but to date, those demands have not passed into law. The latest push came in late July, when Senator Rob Portman and Representative Steve Stivers, both from Ohio, each introduced their own version of the Efficient Space Exploration Act, which mandates such reports. Both bills remain in committee and have not been brought to a formal vote.

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