Cosmic rays could leave travelers to Mars confused, forgetful and slow to react

As NASA develops plans for a manned mission to Mars, scientists said Friday that cosmic rays during an interplanetary voyage could cause subtle brain damage, leaving astronauts confused, forgetful and slow to react to the unexpected.

In a NASA-funded study of radiation-exposed mice published Friday in Science Advances, researchers at the University of California, Irvine and the University of Nevada warned that prolonged bombardment by charged particles in deep space could affect the brain cells involved in decision-making and memory, with implications for possible manned forays into deep space.

“These sorts of cognitive changes could manifest during the mission and could be a real problem,” said Cary Zeitlin at the Southwest Research Institute in San Antonio, who wasn’t involved in the study. In 2013, Dr. Zeitlin reported radiation levels between Earth and Mars detected by the Mars Science Laboratory craft during its cruise to the red planet, and found that the exposure was the equivalent of getting “a whole-body CT scan once every 5 or 6 days.”

Deep-space radiation is a unique mix of gamma rays, high-energy protons and cosmic rays from newborn black holes, and radiation from exploding stars. Earth’s bulk, atmosphere and magnetic field blocks or deflects most deep-space cosmic rays. Shielding on spacecraft also helps.

In 54 years of human spaceflight, astronauts have rarely experienced a full dose. Apollo crews, who ventured furthest from Earth’s protective shield on their journeys to the Moon, reported seeing flashes of light when they closed their eyes, caused by galactic cosmic rays speeding through their retinas.

Researchers at the National Aeronautics and Space Administration have studied the potential health hazards of space radiation for decades, including the elevated risk of cancer. But it has been hard to simulate the behavioral effects of prolonged exposure to low levels of radiation that would be encountered by interplanetary travelers.

Although NASA funded the new experiment, the agency declined requests for interviews with its own radiation experts.

A NASA representative instead issued a written statement: “NASA recognizes the importance of understanding the effects of space radiation on humans during long-duration missions beyond Earth orbit, and these studies and future studies will continue to inform our understanding as we prepare for the journey to Mars.”

To test the neural effects of deep-space travel, a dozen researchers led by UC Irvine radiation oncologist Charles Limoli briefly exposed mice to charged particles in a radiation beam at the U.S. Department of Energy’s Brookhaven National Laboratory in Upton, N.Y. Six weeks later, they tested the irradiated mice and found the lab animals lacked normal curiosity, were less active, and became more easily confused, compared with a control group, the researchers said.

“Their curiosity is way down,” said Dr. Limoli. “They don’t want to explore novelties.”

The researchers found the mice had damaged neurons and synapses in areas associated with memory and decision-making, such as the hippocampus and prefrontal cortex.

“I don’t think our findings preclude future space missions,” Dr. Limoli said. “But they suggest we need to come up with some engineering solutions.”