Gravity determines the scale of many physical phenomena on and above a world's surface. Assuming that toolusing intelligences come from, and would settle, worlds with solid surfaces and gravities equal or less than that of their origin, we note that such worlds in the Solar System have surface gravities significantly lower than that of Earth. The distribution of these
surface gravities clumps around factors of about 2.5 and favors worlds with about one-sixth Earth gravity, conventionally considered too small to retain atmospheres. Titan, however, shows that low surface gravity does not, in itself, preclude the presence a substantial atmosphere. Where small worlds have low exobase temperatures,and protection from stellar winds, substantial atmospheres may be retained. Significantly lower surface gravity affects a number of physical phenomena that affect environment and technological evolution--most significantly, heavier than air flight is easier and
races evolved under such conditions should reach space at an earlier level of development. It is proposed that low gravity worlds in other planetary systems could be the sites of native or transplanted interstellar settlement, and, perhaps, be the majority of such sites. The most important thing a habitable planet has to do is to retain a biologically compatible atmosphere. A planet on which life evolves to intelligence must retain its atmosphere for billions of years. A planet which has been terraformed may only need to retain an atmosphere for a few thousand years (it can, of course, be replenished), a much less stringent condition. There are two main ways that an atmosphere leaks away from a planet. They are thermal evaporation and "pick-up" by the solar Surface Gravity and Interstellar Settlement G. D. Nordley 3 wind. Gravity is important in the former mechanism and not so
much in the latter. In thermal evaporation, atoms that exceed escape velocity at the top of the atmosphere can generally be considered to have escaped (Titan, as we shall see, is a little different). In solar wind pick-up, magnetic fields generated by passing solar wind ions accelerate atmosphere ions to escape. Preventing muscle loss in space is simple: Just spend an hour a day in a giant human centrifuge that’s spinning at 30 rotations per minute. And don’t lose your lunch. Scientists still don’t know why the “use it or lose it” adage holds true for muscles. But astronauts start losing muscle mass within days of entering a zero-gravity environment, and the longer they stay in space, the more their muscles shrink. On Earth, elderly hospital patients can lose more than a kilogram of muscle in just three days of lying in bed.
Source(s):
http://www.wired.com/wiredscience/2009/07/humancentrifuge/

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