Outer space is the most noxious of substances: devoid of air and filled with a soup of deadly particles in the form of high-energy photons and energetic bits of atomic nuclei. The lack of gravity there affects every element of your being, as even the proteins in your body can’t figure out which way is up.

Books and magazine articles about space voyages often compare the adventure to setting off to new lands across treacherous oceans. Our ancestors paddled across the South Pacific in outrigger wooden canoes made by hand with primitive tools. They set off never expecting to return. They spent days, weeks, and months on the open waters, exposed to the elements, with precious little food and water. Many died along the way, but a few made it to their destination and started a new life. No doubt these early migrations tens of thousands of years ago were perilous, but it’s not as if each drop of water burns a hole in your DNA; the sea mist doesn’t destroy your brain cells; the choppy waves don’t cause fluid to build up in your eyes and cause permanent retinal damage. When you finally get to dry land, you can walk. You don’t need a team of doctors and engineers to carry you off the boat because your legs are too weak to support you. And, chances are, you will find food and water at your destination when you arrive.

Excerpted from Spacefarers: How Humans Will Settle the Moon, Mars, and Beyond, by Christopher Wanjek. Buy on Amazon. Courtesy of Harvard University Press

In short, some things can actually live in water, and that which cannot live in water can cross it on driftwood. But space is both sterile and sterilizing. The challenges presented by every journey on Earth that has occurred in the centuries and millennia past, however arduous, pale in comparison to those of a journey in space beyond the Moon. Suggesting otherwise minimizes the sacrifices that the first generation of spacefarers will make. To be clear, space travel is technically feasible today from an engineering standpoint. We placed humans on the Moon fifty years ago, after all. We’ve sent probes clear out of the Solar System, and we have made soft landings of probes on the surfaces of Venus, Mars, Saturn’s moon Titan, the comet 67P / Churyumov–Gerasimenko, and several asteroids. But sending humans beyond the Moon is considered by many doctors to be so dangerous that it is tantamount to suicide.

How Bad Is Bad?

The expected radiation exposure for a Mars excursion for an astronaut—a federal worker—far exceeds the levels permitted for terrestrial workplace activities by the US Occupational Safety and Health Administration (OSHA). To even consider going to Mars, NASA needs a special waiver from OSHA based on a principle called ALARA (As Low As Reasonably Achievable). The waiver, which NASA has, requires the agency to carefully assess the health risks to astronauts prior to launch.

Yet radiation exposure is only one danger. The NASA Human Research Roadmap has identified thirty-four known health risks and 233 “gaps” in our knowledge of risks. For example, four known health risks are associated with radiation: radiation poisoning from solar flare; brain damage; cardiovascular damage; and regular ol’ cancer. But among the gaps are questions about hereditary, fertility, and sterility effects from space radiation. So, it’s likely that there are more health risks than we realize. Here are the 34 known risks of space travel—risks that go beyond basic mechanical dangers, such as the rocket blowing up.