Anyone who follows Commander Chris Hadfield on Twitter (which should be everyone) knows the effects of zero gravity on the body. This physical specimen has used social media to detail the problems with returning to Earth after a lengthy stay on the ISS: bone decalcification and partial muscular atrophy, among other things. For this and other reasons, it might seem like that NASA should invest some money in a system of artificial gravity. So, why don’t they?First, all hypothetical gravity shields aside, the notion of true anti-gravity is off the table for this article. The ability to create and directly control a gravity well would be one of the all-time most fundamental steps forward for physics, and would open the door to everything from levitation to faster than light travel. Right now, it’s pure fantasy — but space-time is a real, potentially deformable quantity, so there’s no reason to think it couldn’t happen in the future. For now, though, we’ll look only at faux gravity due to acceleration.That acceleration can be linear (the backward push into your car seat you feel when speeding up) or rotational. Since we want to be able to stay in one place (and be efficient with our use of fuel) rotational acceleration is the key: spin a portion of the ISS, and astronauts on the interior surface will be thrown out at an angle parallel to the floor. It’s a difficult thing to visualize, but in physics terms the total sum of an infinite number of vectors pointing perpendicular to the radius of a circle is parallel to that radius — or straight out. Basically, they’re being constantly tossed in the direction of rotation, but since the floor is always there to catch them, the ultimate effect is to be pressed into that floor. We call this “pseudo-force” the centrifugal force. It’s not actually a force acting out, but that’s the ultimate effect.There are some problems with applying this in real life. First and foremost: there is virtually no research about the long-term effects of such a system of artificial gravity. Though it works in theory, NASA is certainly not going to subject their astronauts to long periods of rational gravity without significant research into how their bodies might react. A mission to test the effects on mice was planned a few years ago, but was ultimately defunded and cancelled before launch.Another problem is that we do know of adverse effects that arise at high rotational rates of speed. The Coriolis Effect has to do with rotating reference frames and it can seriously screw with the inner ear. The only way to maintain a strong centrifugal gravity while keeping rotational speed low is to lengthen the system as a whole — but the ISS isn’t nearly big enough. The BBC’s recently proposed Mars vehicle would expand on a tether to get a wide enough rotational diameter. The ideal for humans would be under 2 RPM, but to achieve one Earth’s gravity at 2 RPM the radius of the system would have to be over 200 meters! That means the ISS is currently about a quarter the width we’d require, though we could rotate a little faster, or settle for less than 1G, to shrink the system a bit.This motorcycle cage would work just as well if the bikes were stationary and the cage was rotating.Then there’s the fact that NASA has to be as stingy as possible with these missions. It’s not just about money; the weight of the extra fuel needed to ramp up a spinning portion of the ISS, and maintain that spin, could be used for other, more mission-critical things. And once the ISS is spinning, how do we interact with it? Assuming we don’t want to have to match its rotation to dock, we’d need a variable-speed airlock to allow humans and objects to move from the stationary to rotating portions of the station. A seemingly simple innovation leads to dozens of small, expensive little problems that need fixing.Finally, though we are making huge strides in the area of space research, keeping humans alive in space is still really difficult. In general, when multiple human lives float in the balance, NASA rightly opts for the option with the fewest possible complications. Besides — there’s not much of an advantage to keeping people in orbit for significantly longer than we already do. It’s only for much longer stays that we’d really need artificial gravity.Eventually, it will be a necessity. Today, however, it would be an arguably irresponsible pursuit of sci-fi coolness at the expense of both taxpayers’ wallets and astronauts’ safety.
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