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Powering Up

whether robot airplanes fly on mars or not, for human voyages a key consideration will be the power plant that gets the explorers there. One possibility that was considered at the Mars meeting was nuclear power. The nuclear-powered rocket has been a mainstay of science fiction for generations, and in the 1960s the Atomic Energy Commission actually designed and tested several uranium-235-powered prototypes to serve as upper stages in space. By ejecting superheated hydrogen gas, they provided thrust as strong as chemical engines of similar size, but could get that same thrust while expending only half as much fuel mass. This efficiency would have been a tremendous advantage for the heavy manned Mars vehicles then under consideration.

Indeed, space nuclear power was the theme of several of the breakout sessions at the conference. The session I attended met in a windowless classroom in the basement, and was led off by Roger Lenard, an engineering manager from the Department of Defense’s Sandia Labs in Albuquerque, N.M. Lenard has advocated the use of nuclear power in space since his stint as director of the Timberwind project, a nuclear-powered anti-missile system designed as part of the “Star Wars” defense system. But Timberwind collapsed a decade ago under bad publicity over potential environmental impacts-firing it in combat threatened to contaminate large areas of the Pacific Ocean, including New Zealand.

Despite such concerns, nuclear power has remained popular in the imaginations of a core group of specialized engineers, and enthusiasm for extraterrestrial use of nuclear energy reached critical mass at the conference. Space nuclear advocates such as Lenard have been waiting for decades to get a mission approved, and they believe Mars is their best chance. By targeting a planet too far away to be considered anybody’s backyard, they hope to sidestep the “not in my backyard” attitude of anti-nuclear activism.

Lenard criticized what he saw as NASA’s tendency to avoid the subject of nuclear power because of its controversy. And even within NASA Mars mission studies that include nuclear power, he found what he considered to be unrealistic design constraints. In 1989, Lenard worked with NASA as a nuclear power specialist on a Mars design reference mission. That project included a single bimodal nuclear reactor, which first provided propulsion for beginning the Earth-to-Mars leg, and then provided electric power during the trip and on the surface. But bimodality is a bad idea, Lenard argued. “The design demands of a good propulsion system are antithetical to the design demands of a good power system,” he said.

Propulsion systems require a temperature as high as possible-3000 K-and operate for only a few hours at a time. “And you don’t worry a great deal about retaining all fission products,” he added, since they fade safely into the already-radioactive background of space. In contrast, providing electrical power (both in flight and on the surface) requires only modest temperatures and modest efficiency, but the system must operate over long periods.

In Lenard’s analysis, other Mars enthusiasts haven’t thought these issues through. “They presume that one development program for a reactor that does two or three things is automatically cheaper than two or three separate programs,” he told the packed room. “But we reject that. When the requirements are so mutually exclusive, this becomes a program with enormous risk, with long lead times and high cost.”

Still, excepting the wild card of political acceptability, nuclear technology is the top contender to power a Mars mission. Without nuclear propulsion, early manned missions to Mars would have to rely on chemical engines, making the vehicles larger and perhaps doubling the freight bill for getting a ship into orbit. More exotic non-nuclear systems capitalizing upon such concepts as “inertialess propulsion” and anti-gravity remain wildly imaginary and decades in the future at best.

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Tagged: Communications

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