As humans have logged more and more time in space over the last half-century, researchers have been able to collect more data on the impact of prolonged spaceflight on the human body. This data is mostly anecdotal, with astronauts recounting their experiences for aeromedics (yes, that’s a real thing) upon their return to Earth. Other effects were predicted as results of basic understandings of physics and physiology well before humans entered orbit.
The most significant impact on the human body in space comes from the lack of gravity. Since our legs no longer have to support our weight while in space, there is significant muscle atrophy as well as decalcification of the legs, pelvis, and spine. Some researchers have estimated this effect to be up to 12 times that of osteoporosis. The excess calcium has also been known to cause kidney stones. Another known issue of experiencing microgravity is disorientation. The system that our brain relies upon to tell up from down is based upon fluid chambers located in our inner ears. Without gravity, the fluid floats in a neutral position. Many astronauts have reported suddenly feeling as if they were upside-down. Others say that they lose a perception of where their arms and legs are in relation to their body. Remarkably, these effects are known to dissipate after a few days. Apparently, the brain quickly shifts to relying only on sight orientation after the ear fluids are no longer reliable. One more effect of microgravity is the redistribution of body fluid. Gravity on Earth tends to concentrate blood and other fluids towards our lower extremities. Without that force, the fluids move towards the head, resulting in a swollen face and nausea.
Another phenomenon has to do with the lack of a normal day and night while traveling in space. While on Earth, our sleep cycles tend to coordinate with the sun, which is often referred to as our “body clocks”. In space, your position relative to the sun can be somewhat arbitrary, meaning your body clock cannot set itself. This has been known to decrease sleep totals in space as well as decreased energy and productivity while awake. Some researchers have experimented with artificial day/night lighting aboard spacecraft to increase sleep and productivity.
Luckily, neither of the preceding issues have serious long-term effects. The astronaut’s body will almost certainly recalibrate upon return to earth. The one permanent issue, however, is increased exposure to radiation. Without the protection provided by the Earth’s atmosphere, astronauts are exposed to radiation levels at least 10 times that of a human at the face of the Earth. Radiation has been known to damage the immune systems of astronauts as well as increase the likelihood of cancer and cataracts. These effects are not immediate, and unfortunately will show up many years later. Astronauts usually see these long-term effects as a small price to pay for the opportunity of a lifetime, but some have come to regret their time spent in space when these effects surface later in life.
