Cornell University astronomer James Bell thought he had a great proposal for a planetary mission for NASA’s New Frontiers program. His probe would explore the cloud of asteroids that follows Jupiter in its orbit around the Sun. But while the science was compelling, he says, powering the spacecraft was problematic. In NASA’s 2009 solicitation of proposals, the use of radioisotope power systems was specifically excluded. Bell and his team had no choice but to equip his would-be probe with massive solar arrays sufficient to generate power where sunlight is 1/25th its brightness on Earth. That drawback, he says, doomed the proposal.
Bell’s experience illustrates a quandary facing upcoming missions to the outer solar system, and to places like the lunar poles and some craters, where sunlight is faint or never shines: The plutonium-238 that has fueled past missions in those environments is running out. There is only enough left to...
