Nuclear engine promises to slash travel times to Mars
BY JEFF FOUST
Posted: January 19, 2001
Yigal Ronen, a professor of nuclear engineering at Ben-Gurion University of the Negev, believes that americium-242m (Am-242m), a little-known isotope of an artificially produced element, could power future robotic or human spacecraft far more efficiently than chemical or other nuclear propulsion sources.
Ronen found that Am-242m could sustain nuclear fission even when formed into thin sheets less than a micron (millionth of a meter) thick. Such thin sheets would let the byproducts of the fission reaction to easily escape their fuel elements, allowing them to ionize and heat materials like hydrogen that could serve as a propellant.
"The gas will be magnetically confined so temperatures of about 250,000 degrees can be reached," explained Ronen. "With such temperatures a velocity of 80 km per second can be obtained."
Ronen estimates that such an engine on a typical robotic spacecraft would require only about 375 grams of Am-242m per day, or several kilograms for the trip from Earth to Mars. The material could be produced by irradiating another isotope, Am-241, with neutrons, a process that Ronen admits is expensive today but still feasible.
The idea of using americium to power spacecraft is still in the earliest concept stages, Ronen emphasized. "We have not done an elaborate design," he said. "Actual reactor design, refueling, heat removal, and safety provisions for manned vehicles have not yet been examined."
Another issue that would have to be addressed by a mission using an americium or other nuclear-powered engine is strong opposition by many environmental and anti-nuclear activists to any spacecraft mission that uses radioactive isotopes. In 1997 a small but vocal group of protestors staged rallies and made legal attempts to stop the launch of NASA's Cassini mission, which carries 32 kg of plutonium dioxide in heavily-shielded radioisotope thermoelectric generators (RTGs).
Ronen shrugged off such political issues. "The controversy about using radioisotopes in space is not related to a real problem, it is mainly a political issue," he said. "I don't know how to deal with a problem which is 'no problem'."
This proposal is not the only concept for propulsion systems that could shorten travel times to Mars. Another proposal, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), would use magnetic fields and electromagnetic waves to heat hydrogen, turning it into a high-velocity propellant like Ronen's americium engine. The VASIMR team, led by NASA astronaut and plasma physicist Franklin Chang-Diaz, believes such an engine could send a human mission from Earth to Mars in 90 days.
Ronen believes that, given time, americium will prove to be a key fuel for future space missions, citing the support of, among others, Nobel laureate physicist Carlo Rubbia. "Rubbia has also recognized that this is the most probable fuel that will be getting us to Mars and back," Ronen said. "I think that we are now far enough advanced to interest international space programs in taking a closer look at americium-based space vehicles."