Alfred T. Kamajian

Researchers at Ben-Gurion University of the Negev in Israel are suggesting a new way to rocket through space at record speeds: propel your craft using the products of fission reactions in thin metallic films of americium-242m (Am-242m). The approach, described in the December issue of Nuclear Instruments and Methods in Physics Research A, might one day cut the travel time from Earth to Mars down from eight to 10 months to a mere two weeks.

In their paper, Yigal Ronen and his colleagues demonstrate that Am-242m can sustain nuclear fission reactions within films less than a thousandth of a millimeter thick. Not only do such thin films make for relatively light nuclear reactorsa necessary condition for efficient interplanetary cruisingbut they also allow the high-energy, high-temperature fission products to escape, freeing them for use in propulsion. More common nuclear materials don't afford fission products such liberty: uranium-235 and plutonium-239 both require large fuel rods, which absorb the reaction fragments.

At a recent meeting on nuclear reactors for space vehicles, Nobel laureate Carlo Rubbia raised the idea of using fission products to heat a gas for propulsion. Ronen in his paper says that Am-242m could be used in this way, or to fuel a special generator producing electricity. Alternatively, the fission products might themselves serve as the propellant. "There are still many hurtles to overcome before americium-242m can be used in space," Ronen notes. "There is the problem of producing the fuel in large enough quantities from plutonium-241 and americium-241, which requires several steps and is expensive. But the material is already available in fairly small amounts. In addition, actual reactor design, refueling, heat removal and safety provisions for manned vehicles have not yet been examined. However, I am sure that americium-242m will eventually be implemented for space travel, as it is the only proven material whose fission products can be made available for high-speed propulsion."