Limits on the amount of radioactive Plutonium 238 available for nuclear power sources, and on the amount of radiation the human body can withstand beyond Earth's protective magnetic field, are among the stumbling blocks to exploration beyond low Earth orbit. Together, they will hamper both human and robotic exploration.

The Mars Science Laboratory rover, the most sophisticated robot ever sent to the red planet, carries 4.8 kg (10.6 lb.) of Pu-238 dioxide to fuel its radioisotope thermoelectric generators (RTG). That leaves only a little more than 30 kg. available for future U.S. exploration missions that would be too big or distant to rely on solar power. And for a two-year human trip to Mars and back, the question of surface power is moot because the crew would take on a lethal lifetime dose of space radiation long before they get home.

“The farther and farther we move away from Earth and away from the Sun, the more and more radiation we'll be exposed to,” says Dr. Kris Lehnhardt, an emergency-room attending physician who teaches aerospace medicine at George Washington University. “As a result, the question has to be can humans safely live and work in deep space if they are exposed to large amounts of intergalactic cosmic radiation, and as of this current moment, that is a question that we do not seem to have the answer to.”

Lehnhardt was a panelist at a Washington symposium on “relaunching” U.S. deep-space exploration, sponsored by the American Institute of Aeronautics and Astronautics (AIAA). He quickly put his finger on the big obstacle between humans and the surface of Mars—the high-energy atomic particles whizzing around in open space beyond the protective magnetic fields of the Earth and the Sun, and the Earth's sheltering atmosphere. While there has been some progress in developing lightweight shielding against the types of radiation reaching low Earth orbit from solar coronol mass ejections (CMES), a crew on a six-month transit to and from Mars would receive a lethal dose of protons and heavy ions from the galactic cosmic radiation believed to originate when stars explode into supernovae. The problem is not a short, powerful blast of radiation that is lethal to humans in days to weeks, but the effects of long-term exposure to radiation in the form of fatal cancers.

“What is an acceptable lifetime cancer risk for an astronaut?” asks Lehnhardt. “If they choose to go far, are they at an increased risk of cancer, and if they choose to go far and they are irradiated by a large amount of radiation, is that going to be lethal?” The answers, he says, will come with “experience, and research,” beginning on the International Space Station (ISS).

Radiation of a different type threatens to limit robotic exploration, but in this case it is a question of too little rather than too much. The same solar energy that endangers human explorers during CME powers the ISS and other spacecraft that are close enough to the Sun to convert it to energy with photovoltaic cells. The more distant a celestial body is from the Sun, the more likely a robotic probe exploring it will derive its electricity from the heat generated by the radioactive decay of Pu-238 pellets (see photo). But those pellets are in short supply, and right now there isn't anywhere to get more of them.

“There is probably something less than 30 kg left in the United States, and probably a lot less than that in the rest of the world,” says Ralph L. McNutt, Jr., a physicist at the Johns Hopkins University Applied Physics Laboratory with RTG experience dating back to the Voyager probes now reaching the outer limits of the Sun's influence.

Pu-238 is produced when Neptunium 237—a byproduct of nuclear weapons production—is irradiated. While there is plenty of the Neptunium isotope left over from the Cold War, no one has made Pu-238 since 1988, McNutt says. The supply is so short that NASA has bought the material from Russia in the past, but the Russians aren't selling it any longer because they may have their own need for deep-space power.

McNutt told the AIAA symposium the situation isn't getting better fast, even though Congress has allocated some funds for NASA to restart production. NASA has been forced to “go it alone,” without funding help from the Energy Department or other agencies, as it is behind the pace to get things started again by 5-7 years. Once production restarts, the U.S. space agency believes it can produce “a couple of kilos in a year,” but that isn't going to cover the demand laid out in the National Research Council's decadal survey of exploration priorities: Uranus, the methane lakes of Titan and the water geysers of Enceladus. The shortfall would be compounded if human exploration begins drawing on the supply for long-duration missions beyond solar-power range.

“It is the sine qua non for doing solar system exploration,” McNutt says of Pu-238, a scarcer resource than money, even in these cash-strapped times.