When astronauts leave Earth's surface for the weightless environment of space, their bodies undergo severe physiological changes. Our bodies' systems depend upon gravity to function normally, and a microgravity environment can disrupt many basic processes. Even in the first few hours of exposure, neurovestibular reflex pathways in our brains begin the process of adaptive deconditioning. Over days to months, flight studies in humans and other animals have seen continuous degeneration of bones and ligaments and extensive atrophy of skeletal and cardiovascular muscles. As these changes progress, it becomes more and more difficult for space-adapted organisms to return to the intense gravitational environment of Earth's surface.

To this day, countermeasures to microgravity degradation have been limited to various forms of exercise, pharmacology, and fluid redistribution. While these have managed to allow astronauts to return safely to Earth even after missions lasting months, they have only been able to slow the process of deconditioning, and studies suggest that astronauts' bones may never fully recover from their time in space.

Clearly, a more direct solution is needed.

Rotating spacecraft (as seen in science fiction films like 2001: A Space Odyssey) may provide the answer to long-duration space flight. As they spin, centripetal acceleration inward (and apparent force outward) takes the place of gravitation, and the effects of unloading disappear! That being said, a rotational environment does not quite simulate an actual gravity field: there are a number of secondary effects, all of which are intensified for small radii—gravity gradients across the spacecraft, weight changes with angular movement, and most importantly, Coriolis forces that confuse the vestibular systems within our inner ears. Due to these difficulties and the tremendous costs of building such a large rotating spacecraft, the concept has yet to make the leap from science fiction to practical health benefit.

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But what of life and work on other planets? Though artificial gravity may enable us to cross vast regions of space, it can no longer help us on the surface of Mars or other new worlds we might explore. Humans living on the surface of planets or moons with different gravity levels will not want to spin themselves day and night to keep up a comfortable environment of Earth-like g-loading. No one yet knows whether mammals can adapt to reduced-gravity environments.

Though studies in centrifuges on Earth have looked at the effects of hypergravity (above Earth's gravity) on mammals, only limited spaceflight experiments have ventured into the important domain between zero- and 1-g. As seen in the above diagram, most physiological parameters undergo huge swings in the first days to weeks of exposure and then eventually approach an adaptive "set point." The unique (and important) exception is bone loss—humans appear to lose bone mass continuously in microgravity, even with countermeasures applied.

Looking at these two set points, we must ask ourselves: where is the set point for Martian gravity? Is it closer to that of Earth, or the zero-gravity environment of space? As for bone loss, will it continue indefinitely, or does partial gravity put a halt to this dangerous deterioration? No solid research currently exists to answer these questions, but they must be answered, for if humans are to travel to Mars, and someday live there, we must know whether they can adapt, and if they do, whether they can ever return again safely to the Earth.