Designing With an Eye On Plutonium's Risks

Mindful of the risks to using nuclear power sources in space, planners for NASA's upcoming Cassini mission say they are working to keep those risks to a minimum.

Their approach acknowledges that "things like Mars 96 happen, so you have to be prepared to deal with that reality," says Richard Spehalski, senior program manager for the Cassini project at Caltech's Jet Propulsion Laboratory in Pasadena, Calif. "That's reflected in the design of the spacecraft, the design of the mission, and in the design of contingency plans."

The nuclear power sources used, known as radioisotope thermoelectric generators (RTGs), use plutonium 238 for heat. Unlike a nuclear reactor, whose heat comes from chain reactions, an RTG uses heat from plutonium's natural decay. Solid-state devices turn the heat into electricity. Smaller amounts in special containers are used to keep a craft's electronics warm enough to operate. The solar-powered Sojourner, a tiny rover launched earlier this month on the Mars Pathfinder mission, has several of these plutonium heaters on board.

Alternatives, such as fuel cells, batteries, and solar panels, exist for producing electricity in space. But for Cassini, mission planners say, they fail to meet power, weight, or reliability requirements. The European Space Agency has developed in the lab a lighter, more-efficient solar cell. But in a memo last August, ESA officials acknowledged that too little is known currently about the cells to make them a viable alternative.

Plutonium 238, while nothing to trifle with, is less radioactive than its nuclear-weapons cousin. The danger to humans comes mainly from inhaling tiny particles of plutonium dust, according to physicists, as well as NASA's environmental impact statement.

In the 1960s, the first RTGs were designed to break up on reentry - and did so. When studies found radiation after the breakups, designers shifted gears. From the high-temperature ceramic fuel pellets to heat and impact shields, components aim to keep the fuel pellets intact and shielded during the heat of reentry and aftershock of landing from Earth orbit. The design aims to keep the plutonium from being released into the environment. If it is, the ceramic pellets are designed to break apart in chunks too big to inhale.

Much of the concern surrounding Cassini centers on its gravity-assisted fly-by of Earth on its way to Saturn. According to NASA's environmental impact statement, if plutonium is released during an inadvertent reentry, up to 5 billion people could be affected. But the statement adds that the radiation dose a person might receive would be far lower than what they get from natural sources. Any adverse health effects, it estimates, would likely be indistinguishable from other, more-common occurrences.

Mission planners have designed the orbital fly-by to reduce the likelihood of an unplanned reentry. NASA estimates the risk at about 1 in 1 million. Some dispute the figure based on how dramatically NASA revised its risk estimates for the space shuttle after the Challenger disaster in 1986.

On missions involving nuclear materials, the White House gives final approval to proceed. That call typically is made a month before the scheduled launch, after an interagency panel submits its final safety analysis, according to Mr. Spehalski.

Such missions also fall under United Nations scrutiny. The world body adopted a set of principles on the use of nuclear power in outer space in 1992. The UN principles acknowledge the usefulness of nuclear-power sources in some space applications, outline sometimes contradictory provisions on radiation protection, and sketch out procedures in case a nuclear power source reenters the atmosphere.

If the Mars 96 debacle is any indication, bugs still need to be worked out of the system. Initial projections of where the debris would fall were wrong. Eyewitnesses saw what is now widely believed to be pieces of the craft burn up over Chile and Bolivia. The Bolivians, who have asked for US assistance in searching for plutonium-bearing debris, have complained that Russia has not been helpful. Moscow still claims the debris fell into the ocean.