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Extraterrestrial Civil Engineering Part 4: When 9.8 is 3.7

written by: John Moehring • edited by: Lamar Stonecypher • updated: 6/29/2011

Common place terrestrial materials handling techniques become uncommon once the effects of gravity are no longer Earth normal. Zero gravity, low gravity, and high gravity fields associated with the infrastructure of extraterrestrial life will present unique and probably frustrating challenges.

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    Construction of the infrastructure required for extraterrestrial life will become more challenging with changes from the currently taken-for-granted gravitational pull of our home planet. The Earth’s gravitational field exerts a relatively constant force at the surface. The source of this field is still not completely understood, but the strength of the field is well characterized and depends on the mass of an object. Weight, not mass, is determined in the accelerating force of a gravity field. On Earth, the accelerating force due to gravity is 9.8 m/s, which makes a 1kg mass weigh 9.8 Newtons. So on Mars, where acceleration due to gravity is 3.7 m/s as opposed to Earth’s 9.8 m/s, the same 1kg object will weigh 3.7 Newtons. Mass is an elemental material property independent of gravity and also gives rise to the important concept of inertia. When an object with mass is at rest or in motion, it wants to stay that way until acted on by an external force. The amount of the objects “desire" to stay in motion is a measure of its inertia.

    To illustrate this difference between mass and weight, consider a motionless block on a nearly frictionless and level surface, like an air table or smooth water ice. Pushing on the block will obviously set it in motion. The resistance felt when giving the block a push is an indication of the block’s inertial mass. This resistance will feel the same whether the same push is performed on a block located on earth, in space, on the moon, or on Jupiter, even though the gravitational force goes from Earth- normal, to zero, to 2.5 times Earth- normal at each location. Now change the procedure to lifting the block straight up from the surface. The weight of the block will feel different in each location, from Earth weight to zero weight to 2.5 times Earth weight. Note that in the zero gravity condition, zero weight does not mean effortless. The block will still take force to overcome its inertial mass, which is still present in a gravity field but not as noticeable.

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    Effects On Material Properties And Construction

    Experiments in low gravity fluid mechanics show that fluids behave quite differently than what common terrestrial experience demonstrates. For example, water drops readily combine, or coalesce, on Earth . The lower the gravitational force, the less likely it is that coalescence will occur. In very low to zero gravity conditions it actually takes great effort to combine drops together, which makes spraying water out of a hose quite an event. On the other extreme, high gravity makes the formation of individual drops more difficult, to the point where water will behave more like viscous goop. A simple wheelbarrow becomes almost useless in each case, dumping will no longer work in zero gravity and the apparent levered advantage for heavy loads all but disappears in higher gravity. Tilt ups will take on a whole new meaning in each case as well, from tilt up, up, and away to tilt ain’t no way.

    Of course, by the time serious extraterrestrial colonization construction efforts are underway civil engineers will have worked out the required adaptations to alternative gravitational fields. But working through these different scenarios now is certainly an endeavor worth undertaking for those future generations of engineers.

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