And the agency has highlighted 23 particular health risks of long-duration space travel that require further work to mitigate before a crewed spacecraft takes off for Mars in the 2030s (NASA’s schedule) or sooner (Elon Musk’s schedule).1 But nine of these risks are currently considered “red,” i.e., they have both a high likelihood of happening — defined as greater than a 1 percent chance of occurring, and high stakes (i.e., death, permanent disability or long-term health impact) if they do: space radiation, visual impairment, cognitive or behavioral conditions, the long-term storage of medications, inadequate food and nutrition, team performance issues, in-flight medical conditions, user issues with onboard technology and bone fracture. These are high priority and “really have to get worked out” before we go to Mars, said William Paloski, director of NASA’s Human Research Program, though he adds that space exploration is inherently risky and volunteers will have to be aware of all known risks, red or not.

Radiation is a biggie. There are two types that are worrisome. Solar particles from the sun can deliver high doses of radiation unless people are shielded by a spacecraft, other sturdy shelter or sufficient water,2 said Dorit Donoviel, deputy chief scientist of the National Space Biomedical Research Institute, a NASA-funded consortium of research institutions led by Baylor College of Medicine.

These solar particle events can also be predicted, but sometimes only about 20 minutes in advance. The problem is, if you’re on the surface of Mars in a spacesuit or land vehicle, that may not be enough time to get to shelter. And if you’re exposed to one of these events, you could experience the kind of acute symptoms people suffered near the nuclear blast zone in Hiroshima, including severe nausea. “You are dead if you vomit in a spacesuit” because the fluid and particles glom up the air supply, Donoviel said. Even if that initial hit is survivable, the radiation wipes out blood cells, leaving you vulnerable to deadly infections. Better prediction tools would help give people more warning of a solar particle event and more time to reach shelter, she said.