In this, the 50th year of using nuclear energy for space missions, the U.S. is preparing to restart domestic production of a plutonium isotope that fuels space vehicles — a topic that was front and center at the recent Nuclear and Emerging Technologies for Space conference, held in The Woodlands, Texas.
Despite the utility and the necessity of using radioisotopes to power missions ranging from the Mars Rovers to the Voyager 2 probe now exploring the furthest edge of our solar system, the assembled experts said the public has a poor grasp of the safeguards in place for nuclear power in space.
“It’s widely misunderstood,” says Cornell University professor and Mars Rover principal investigator Steve Squyres. “People hear nuclear and radiation, and they immediately get nervous — with good reason. The problem is the devil is in the details, and the details in this case point a certain direction about what’s safe and what’s not.”
“I am utterly convinced there is no credible risk to human health and safety from launching,” Squyres said in an interview. “I really believe that, and I’ve looked at it hard.”
Plutonium-238 is used because its rate of radioactive decay generates sufficient heat, and hence power generation, to finish missions that span years or decades. (Its rate of decay, or half-life, is 87.7 years, compared to the 24,100 years of P-239, the stuff used in nuclear power plants and weapons.) Also, P-238 only emits alpha particles as it decays, rather than the more penetrating gamma or X-rays emitted by other radioisotopes.
Ralph McNutt, space scientist at the Applied Physics Laboratory at Johns Hopkins University and project scientist for the MESSENGER Mercury space probe, described the technology’s history and current use in an opening plenary session at the conference. “Radioisotope power systems are an enabling technology that we use on space missions to be able to go out in regions of the solar systems and do things we simply could not do any other way,” he explained.
“If you’re sending astronauts to Mars, I wouldn’t want to drive a solar-powered vehicle,” Squyres said during his public talk, covering future nuclear-powered missions in space well beyond programs in the next decade’s space plans.
Squyres also spoke of the “holy grail of planetary exploration” – sending a robotic submarine to Jupiter’s moon Europa to look for life underneath the miles-thick ice covering its surface. If, as predicted, geothermal vents lie beneath the ice on the ocean floor, Europa may offer the best chance of finding life in our solar system. “How do you get down through ice? It’s best to melt using something really hot. I think I see a solution, that’s all I’m saying.”
But supplies of plutonium-238 are just about spent. The U.S. stopped producing the isotope in 1988, and subsequently bought it from Russia, which recently reneged on its contract to supply the U.S. At the conference, Wade Carroll, the Department of Energy’s deputy director for space and defense power systems, announced that the federal budget includes money to relaunch domestic production.
NASA was appropriated $3.5 million in the 2011 fiscal year and $10 million the next to relaunch domestic production of plutonium-238 at Oak Ridge National Laboratory, but before that begins, it must undergo a full National Environmental Policy Act review. If approved, the U.S. would produce up to 2 kilograms per year. It will take five or six years before new plutonium will be available.
Danger associated with plutonium-238 comes almost exclusively from inhaling it; particles can lodge in the lungs, where they emit damaging alpha particles into the body. Once in the lungs, it can spread throughout the blood and get lodged in bone and the liver, leading to cancer. On the other hand, if particles land on skin, the layer of dead skin cells block radioactive alpha particles from moving deeper and the radioisotope can be readily washed off. Even swallowing the isotope is not a significant health risk; the digestive tract does not readily absorb it.
Accidents associated with producing or maintaining the radioisotope have occurred. In 2000, a faulty glovebox at Los Alamos National Lab leaked, exposing several workers to radiation from the lab’s plutonium-238 stockpiles.
And despite precautions, scenarios exist in which plutonium-238 from spacecraft could contaminate Earth. If a nuclear-laden spacecraft performed a high-speed slingshot fly-by and a calculation mistake occurred, the craft could enter the Earth’s atmosphere, disintegrate, and spew plutonium throughout the planet.
After the Cassini-Huygens probe launched in 1997, NASA guided it in slingshot maneuvers by Venus, Earth, and Jupiter before heading out to Saturn, and if an accident would have occurred during this maneuver, it could have resulted in an estimated 2,300 cancer deaths worldwide. No current or upcoming space mission plans to use a high-speed Earth flyby.
Plutonium pellets are clad in iridium and then placed in carbon fiber shells to protect them from extreme heat in the case of an accident. Unlike in the case of a slingshot maneuver, if an accident occurred during launch, the pellets would break into chunks too large to stay airborne and be inhaled. Protestors attended the launch of the Cassini-Huygens probe in 1997, but not the more dangerous flyby. Both went off without a hitch. “The stresses that can occur in a launch accident cannot destroy these things,” says Squyres, who chairs the NASA Advisory Committee.
Later launches of the Mars Rovers and most recently, the Mars Science Laboratory Curiosity, which lands August 5, have not met with any protests. “People are starting to get that launching these things is not risky,” said Leonard Dudzinski, NASA executive for radioisotope power, at the plenary Q&A.
“Anybody involved in flying one of these things knows how much goes into worrying about safety,” said McNutt. Analysis, an environmental impact statement, and presidential approval are required anytime nuclear material gets launched into space.
“Unless we maintain the ability to make plutonium-238, we will not have these missions,” concluded McNutt. The public will have to weigh the benefits of these pioneering space missions against the costs and risks of use, including domestic production, and will get a chance as NASA submits its plans for new domestic production and the National Environmental Policy Act process unfolds.
