Despite the radiation risks, NASA is forging ahead with plans for a trip to Mars. Last month, engineers evaluated how crew interacted with a new control scheme.

NASA’s biggest obstacle to sending humans to Mars may not be related to line items in budgets, but to the safety of the astronauts themselves.

Despite having sent humans into space for nearly 55 years, NASA doesn’t quite understand what risks the radiation out there poses. More importantly, the agency doesn’t know exactly how to manage those risks—and it might not be able to.

Space radiation presents the tallest hurdle to NASA’s future travel plans that extend beyond low-Earth orbit and for periods longer than a year. On the International Space Station, astronauts are bombarded with 10 times as much radiation as they experience on Earth in a given period; on a Mars, they will encounter 100 times the terrestrial dose.