Matthieu Komorowski wanted to be an astronaut. Still does. The French-born anesthesiologist, currently getting a PhD at Imperial College London, applied to the European Space Agency in 2008. But he knows his chances are limited. “Being basically a medical resident I didn’t get very far in the selection,” Komorowski says. “But I’ve been working on building up my skills.”

Among those skills: administering anesthesia for surgery. And as Komorowski found when he started looking at the literature on space medicine, that might be more helpful than it sounds. Of all the concerns about astronaut safety and health, traumatic injury is the one that worries people the most. It has the biggest potential impact on a mission and, worse, it’s the one people know least about.

In part that’s because it has never happened. Over decades of Apollo, Mir, Skylab, space shuttle, and International Space Station missions, astronauts have had medical concerns and problems—and, of course, there have been deadly catastrophes. But no astronaut has ever had a major injury or needed surgery in space. If humans ever again venture past low Earth orbit and outward toward, say, Mars, someone is going to get hurt. A 2002 ESA report1 put the chances of a bad medical problem on a space mission at 0.06 per person-year. As Komorowski wrote in a journal article last year, for a crew of six on a 900-day mission to Mars, that’s pretty much one major emergency all but guaranteed.

Worst case: Someone goes outside the spacecraft to fix something heavy and it gets away from them, crushing an arm or a leg. The astronaut gets exposed to vacuum, but makes it back inside the vehicle—dehydrated, partially frozen, bleeding heavily, in shock. What happens next will depend on whether the crew is in orbit around Earth, or in interplanetary space—and on what kind of gear is on board.

NASA doesn’t seem headed for Mars any time soon, but people like Elon Musk are making noises about missions as early as the end of this decade. At the International Astronomical Conference in Guadalajara last September, Musk described plans for a Mars mission that seem to now be delayed or scaled back. But he still says SpaceX is going. Speaking to the ISS Research and Development Conference in Washington DC on July 19, Musk also said: “If safety is your top goal, I would not go to Mars.”

Yes, sure, space is unsafe. Even if you manage to stave off killer radiation, you still have to worry about muscles atrophying and bones getting less dense—and more breakable—in weightlessness. Not to mention the ever-present danger, thanks to long-term isolation in a confined space, of “psychiatric decompensation.” That’s NASA-talk for catastrophic marbles-losing.

Spend a long time in space, though, and your body starts to change in all sorts of other ways, too, and they all make traumatic injuries even worse. Your total amount of circulating blood and red cell mass goes down. Your blood vessels don’t constrict and dilate as well. That suite of cardiovascular problems adds up to what on Earth would look like the result of significant blood loss—and this is before you get injured. Your hormones go kind of wonky, and your immune system and wound healing get sluggish. Your bones break more easily and heal more slowly, if at all. Meanwhile, infectious bacteria become more resistant to antibiotics, and, oh, hey, you know how you always get sick after a “long” airplane flight? Imagine if the flight lasted two years.

Thanks to a freedom-of-information request from Vice, the medical gear on board the ISS is public knowledge. The crew has access to a small but professional pharmacy, including some serious drugs and EpiPens. They have an automated emergency defibrillator, gear to administer intravenous fluids, and diagnostic equipment like blood pressure cuffs. The ISS also carries an ultrasound device, for example—the only sophisticated imaging device on board, but one that’s great at finding internal bleeding and monitoring fluid levels in eyeballs, a thing astronauts have to worry about so they don’t go blind. It might also have therapeutic uses. Oh, and they have some dental equipment, which, nope, hard pass. “When it boils down to it, there’s a few things we train to handle right away,” says Steve Swanson, who commanded the ISS for six months in 2014. “Anything besides that, we were going to be calling the ground.”