Nuclear planes are "not good for anybody," says Theodore Rockwell, founder of Radiation, Science & Health, Inc., a non-profit organization in Chevy Chase, Md., that advocates for objective review of low-level radiation science policies. During World War II, Rockwell worked a program at the U.S. Department of Energy's Oak Ridge National Laboratory, where he helped develop new materials and designs for radiation shielding. Although a supporter of nuclear power, he draws the line at using it to fuel aircraft.

In addition to the potential passenger dangers, he says that pilots could only fly a limited number of flights to prevent overexposure to nuclear radiation, noting that after a few flights, they would have been exposed to far more radiation than nuclear power plant workers. "Power has to be predicable to be useful," he says, "and nuclear is by far easiest to predict on Earth."

A more promising green fossil-fuel alternative is biofuel. Richard Altman, executive director of the Commercial Aviation Alternative Fuels Initiative (CAAFI), a consortium sponsored by the FAA that promotes the move away from petroleum-based jet fuel. CAAFI favors "drop-in" fuels that work within existing infrastructures—that is, they are interchangeable with fossil fuels and require no alterations to aircraft or airports. These alternatives include liquids from both renewable (plants) and nonrenewable (coal and natural gas) sources that are better than an oil refinery on a greenhouse gas (GHG) life-cycle basis. They have the same safety and density as fossil fuels, Altman says, and would not alter planes’ performance, range, or capacity to carry passengers.

*Note (12/5/08): The weight given for the B-36 was corrected after publication of this article.