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...