Space is not a friendly place.
In fact, space travel abounds with health risks, mostly caused by radiation and weightlessness. Space travelers risk bone and muscle loss, cataracts, kidney stones, vertigo, motion sickness, low blood pressure and cancer, to name but a few prevalent perils [sources: NASA; Roach].
Astronauts orbiting Earth on the International Space Station (ISS), around 250 miles (400 kilometers) up, experience roughly 10 percent less gravity -- or would, were they not in near-weightless free fall. The moon exerts 16.6 percent of Earth's gravity; Mars gravity stacks up to only about 37.7 percent [source: NASA].
Yes, NASA is the easiest and most guaranteed weight-loss program out there. It's also the most difficult to get into but, at a cost of only tens of millions of greenbacks spent per astronaut, it's still cheaper than Jenny Craig.
Like that weird kid sitting in the back of homeroom eating his fingernails, the human body likes to consume unneeded bits of itself. For space travelers, that poses a problem, because -- while they remain weightless -- much of their muscle and bone mass fall into the "surplus" category.
Microgravity exposure affects the body like a Charles Atlas ad in reverse. Astronauts struggle to stay ripped -- and fight bone loss -- using exercise, but research suggests that missions lasting longer than 180 days render this effort moot. The silver lining? Losses, which occur rapidly in the initial stages of reduced gravity, plateau after six months [source: Fitts].
Exercising in microgravity is about as efficient as doing underwater pushups. Weight machines must generate inertia using flywheels, and treadmills must hold astronauts down with awkward, chafing tethers that apply only 70 percent of their body weight [sources: Roach; Wall]. Astronauts already spend about 2.5 hours per day, six days a week, exercising during a six-month stay aboard the ISS. Longer missions will require getting more out of those hours, not adding to them [source: Wall].
Astronauts experience bone density loss rates comparable to, or worse than, post-menopausal women (1-2 percent per month with exercise), particularly in the large weight-bearing bones (pelvis, hips, legs). Back on Earth, recovering bone mass and volume can take much longer than the mission itself; even then, bones remain less dense and more porous, and non-load-bearing areas might never fully recover [sources: NASA; Nimon; Roach]. Space, like old age, is not for wimps.
As bones break down, they release calcium into the blood and urine, increasing the risk of kidney stones. Astronauts can kill two birds with one renal calculus using bisphosphonates, or osteoporosis drugs. Bed rest tests -- used on Earth as a stand-in for zero gravity -- have yielded positive results, and an ISS study was under way as of April 2012 [source: NASA].
Bisphosphonates have been linked to necrosis of the jawbone, however, which might influence future use [source: Merigo; Roach]. Or not. Space, after all, is a hostile place. Going there requires balancing risks and for many, the ticket's worth the price.
Radiation-induced cataracts aren't the only threat to astronauts' peepers [source: NASA]. Time spent weightless compresses eyeballs, puffs up optic nerves and distorts vision. These effects can persist long after returning home. More time spent in space, as on a Mars jaunt, increases the likelihood of permanent vision problems or even blindness. Solutions might include generating artificial gravity by spinning all or part of the spacecraft or treating the causes with drugs [source: Chang].
All of that fuss and bother, and we haven't even made planetfall.
Once on Mars, explorers would confront frigid temperatures and an unbreathable carbon dioxide atmosphere too thin to protect against radiation. Travelers might harvest water from the poles or from subsurface ice, but colonists would need to grow their own food [source: Kaufman].
Sound daunting? Don't worry. There's a good chance you'll crack up before you even get there.