While that collective experience is enough to have taught us how the body responds when gravity’s pull is substantially reduced, the magnetosphere still shields the I.S.S., and only the 24 astronauts who flew in the Apollo program have gone beyond it. (The moon orbits an average of more than 238,000 miles away.) Though these two dozen astronauts spent little more than a week at a time without its protection, they have died of cardiovascular disease at a rate four to five times as high as that of their counterparts who stayed in low Earth orbit or never entered orbit at all, which suggests that exposure to cosmic radiation might have damaged their arteries, veins and capillaries.

We can’t send people to Mars, or to live on the moon, until we can be reasonably confident that they’ll survive getting and residing there. But the space-based medical science needed to make that possible has been hindered by small sample sizes that aren’t representative of the general population. (All of the Apollo astronauts were white men born between 1928 and 1936.) Space tourism, though, promises to offer opportunities to study the effects of radiation and low gravity on a much broader demographic than “really well-selected superpeople,” as Dorit Donoviel, the director of the Translational Research Institute for Space Health (TRISH) at the Baylor College of Medicine, describes those who have historically qualified to leave the planet. “Old, young, pre-existing health conditions — we are starting to gather a knowledge base that in the future will be essential even for NASA,” Donoviel told me, “because we have to learn about the edge cases to really understand what is going on in our bodies to adapt to a hostile environment. You don’t learn as much from people who are healthy. It’s when people get sick that you understand how people get sick and how to prevent it.”

Epidemiologists face the same predicament on Earth: Before they can figure out how to protect the population, they must wait for harm to come to enough people to expose the causes. As less-rigorous medical screening allows more tourists to reach space, the chances increase significantly that someone will get hurt or have a health emergency there. Aerospace medicine is one of three specialties certified by the American Board of Preventive Medicine, because surgeons for a given flight tend to be stuck on the ground; they have to optimize the health of their patients and ward off potential disasters before departure. The problem is, they can’t know what those disasters will be until they occur. Which means that, as with every expedition into the unknown, at some point some intrepid or desperate souls are just going to have to blast off and see what happens.

Scientists once predicted that we couldn’t live in the absence of Earth’s gravity. Without this still-barely-understood force pulling us downward, how would we swallow? Wouldn’t our tongues loll back into our throats? Wouldn’t we choke on our own saliva? And if we survived those perils, wouldn’t escalating pressure in our skulls kill us after a week or so? But when Yuri Gagarin returned from his single, 108-minute orbit around our world in 1961, humanity’s first trip beyond the mesosphere, he proved that our internal musculature could maintain our vital functions in conditions of weightlessness. He ate and drank up there without difficulty. Technically, he hadn’t escaped Earth’s influence; to orbit is to free-fall toward the ground without ever hitting it, and he was in a condition known as microgravity. This felt, he reported, “like hanging horizontally on belts, as if in a suspended state,” a circumstance passingly familiar to anyone who has been on a roller coaster or jumped off a diving board. Gagarin said he got used to it. “There were no bad sensations,” he added.

Either Gagarin was fibbing, or he had a strong stomach. Initially, many space travelers puke, or at least feel motion-sick — space-adaptation syndrome, or S.A.S., is what such nausea, headache and vomiting are called outside our atmosphere. “It’s the same as sitting in the back of the car in childhood, reading something with your head down,” says Jan Stepanek, director of the aerospace-medicine program at the Mayo Clinic in Scottsdale, Ariz. “It’s a mismatch of what the eyes are seeing and what the inner ear is telling you.” Only in this case, that mismatched perception is a result of the organs and hairs of the vestibular system floating free without their usual gravitational signals. You acclimate eventually. In fact, researchers only learned about the prevalence of S.A.S. symptoms in the 1970s, when they heard Skylab astronauts talking about it with one another over a hot mic. Astronauts, it turns out, are not ideal subjects for medical study, because they are notoriously stoic and unforthcoming about any symptom that might ground them.