Life Aboard an Interstellar Starship: The Practical Difficulties of Intergalactic Diversity
Star Trek: The Next Generation presents a harmonious view of interstellar spaceflight: a synchronized crew of diverse cultures and races exploring the galaxy.
Let's assume, for the moment, that we can reach such a future; that technology will one day allow us to warp spacetime and travel across immense distances while ignoring the effects of time dilation; that we will invent food replicators capable of transforming blocks of matter in-to any form of sustenance we desire; that linguists and technologists will team up to create a universal translator; and that physicists will discover a way to artificially control gravity. Despite all of those incredible assumptions, life aboard an interstellar starship would still present practical difficulties.
Although evolved sensibilities and communication technology will break down cultural barriers between species, physiological differences will be harder to conquer. A significant barrier is erected of biological time. To the best of our knowledge, almost all life comes equipped with an ingrained "master clock." Located within the brain, this "clock" -- built from a bundle of neurons -- coordinates groupings of interacting molecules spread throughout the body. These molecules control circadian rhythms: physical, mental, and behavioral changes that respond to light and dark cycles. When these rhythms aren't followed, problems arise. For example, humans forced to work at night and sleep during the day experience fatigue, general malaise, and increased rates of metabolic disorders like obesity and diabetes. A functional crew will need to minimize these issues. While Star Trek understandably glosses over the complications of circadian rhythms, on a real intergalactic starship, they will need to be dealt with. Not all of the crewmembers will be from planets with 24-hour day-night cycles, so duty schedules will have to be individually tailored and coordinated to fit their biological clocks.
Air conditions present a higher problem to hurdle. On the International Space Station, the matter is simple. Earth life is adapted to a pressure of roughly 14.7 psi (the pressure at sea level) and air composed of 78% nitrogen and 21% oxygen, so those are the conditions that suffuse throughout the orbiting station. But what will the conditions be onboard a starship of diverse life forms? After all, some crewmembers might breathe fluorine, or chlorine, or nitrogen... Moreover, high pressures that would squish some life forms might be perfect for others, and lower pressures that would boil the water in human lungs might be business as usual for other species. The only sensible solution would be to set the ship's life support to a setting that would accommodate the most crewmembers. Everybody else, however, would need to don a spacesuit, though the atmosphere in their personal quarters could be tailored to their physiological needs.
A final practical difficulty for our future starship is gravity. Yes, gravity will be artificially controlled -- that's a gimme -- but again, not all species will have the same "gravitational needs," so to speak. Some may hail from smaller worlds where gravity is not as pressing. Others may originate from larger worlds where gravity is punishing. Each would have evolved to be suited to its own gravity. A life form from a larger planet living in a lower-gravity environment would see their tissues and bones waste away, while a life form from a smaller planet placed in a higher-gravity environment might be at an increased risk of fractures, or worse, have their bones (if they have bones) snapped like twigs.
Gravity probably presents the biggest conundrum of all. Common use areas would again have to be set to a level of gravity that's effectively a compromise -- suitable to the majority of the crew. Private quarters could be kept at varying levels of gravity. But despite the accommodations, it's difficult to imagine anybody serving onboard a starship for more than a few years. They'd need to return to their own planet for gravitational rehabilitation.
As the human race considers extending its reach to other worlds, brave spacefarers will be forced to exist and persist outside their most essential comfort zones. A Mars day is forty minutes longer than an Earth day, and its gravity is just one-third that of Earth's. To face, and overcome, those differences is not science fiction, it is a very real future.
Update 7/9: There's a terrific discussion in the comments about other potential issues and ways to solve them. Feel free to chime in with your thoughts!