It’s hard to anticipate every health problem that might occur on a trip to Mars, which could span years. But NASA is trying. It has a list of 100 conditions or events that are most likely to happen during spaceflight, including pancreatitis, lumbar spine fracture and lost fingernails due to ill-fitting spacesuit gloves.

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). 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, 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.

If barfing to death in a spacesuit isn’t bad enough, consider galactic cosmic rays, which come from events such as exploding stars. “Imagine every element on the periodic table is highly charged and radioactive, and traveling at the speed of light,” Donoviel said. These particles consist of higher-energy protons and heavy ion nuclei. Because of their energy and speed, they can penetrate “far, fast and hard,” she said, making a more robust shield necessary. And if shields contain materials — like lead — that can interact with heavy ion nuclei and split the radiation into even more particles, they can actually make things worse. A magnetic field could potentially protect against galactic cosmic rays, but there are technical issues there, too; particles might get into the field and become trapped, negating the benefit, she said.

The rays damage and could kill cells, raising the risk of cataracts, exacerbation of underlying disease and of developing cancer over the long term. But on a shorter timeline, they can destroy or severely damage enough brain cells to reduce cognitive function. “Very, very low doses can cause memory loss and a reduction in the ability to make new memories,” Donoviel said. “Making mistakes because you’re not thinking right could be life-threatening.”

There’s no easy fix for galactic cosmic rays. NASA is working on different shielding materials. Another strategy is to develop radioprotective medications or superfoods that could boost the immune system, Donoviel said, but that’s further in the future. Right now, NASA is still trying to understand the full effects of the heavy particles on health, but will begin testing countermeasures “in the near future,” Paloski said.

Another pressing risk is permanent vision problems, known as visual impairment and intracranial pressure. These issues, which involve damage to the optic nerve, didn’t start showing up in astronauts until missions lasted six months or more, and the causes are still not totally clear, Paloski said. At first the culprit was thought to be solely the delayed effects of vascular fluid moving up into the head while a person is experiencing microgravity, but something else seems to be going on, he said. “We’re now looking at alternative hypotheses,” including whether cerebrospinal fluid (which surrounds the brain and spinal cord) or increased carbon dioxide plays a role, he said.

Among the other currently “red” risks, how to adequately stock a spacecraft with enough food and medications to keep astronauts healthy is a particularly thorny issue. You need nutritious foods and a variety of medications that will have a stable shelf life for a multiyear mission — none of which could take up too much space or weight. One solution may be the budding technology of 3-D printing, which could take the building blocks of medications or meals and create them as needed, Paloski said.

Medical care in general is tough in space. “We’re trying to figure out what’s in the black bag you need to send,” Paloski said. And the bag’s contents are limited by what could be used by a non-medical professional with minimal training or on-the-fly automated instruction, what can fit into a spacecraft and what can function in microgravity. (Even a simple blood test to, say, determine whether an infection is viral or bacterial in nature currently can’t be done in zero gravity, Donoviel said.) One imaging tool that already travels to space is increasingly sophisticated handheld ultrasound, which can provide imaging of different parts of the body. Beyond that, NASA has funded a study at the University of Washington to use a handheld ultrasound probe not only to diagnose kidney stones, but to push kidney stones to a less dangerous place in the organ and possibly even break them up.

You might notice that conditions that stem from microgravity, including reduced muscle mass and strength and reduced aerobic capacity, aren’t on that “red” list. “The countermeasure program for those changes has been improving,” with more effective exercise routines and better devices, said Jay Buckey, a physician and professor of medicine at the Geisel School of Medicine at Dartmouth and an astronaut. The Advanced Resistive Exercise Device, currently used on the International Space Station, can work all the major muscle groups.

But there are still questions: At the University of Calgary, biomedical engineer Steven Boyd is looking at whether bone that weakens during spaceflight fully regains strength after astronauts return to Earth. Loss of bone density can be reversed, but he is studying whether the struts and rods, or bone microarchitecture, which give bone its strength, are similarly recovered.

Data for all these issues is still coming in. Information gathered from a two-astronaut, yearlong mission that ended in March 2016 will provide some answers, but NASA will also rely on data on 10 to 12 more astronauts on future ISS missions to fill in research gaps and provide confidence that they’re on the right track, Paloski said. He is all too aware that, as the experience with vision problems shows, simply extrapolating the results of shorter missions may not be enough to flush out all the potential health problems of a lengthy Mars voyage. “Are there other boogeymen out there?” he wondered.