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Living in Space and Artificial Gravity

By Taylor Marvin

Via Andrew Sullivan, the Daily Mail has a recent piece that touches on the problem of sex in space:

If the human race ever travel to other star systems, the journey could take generations. … With ideas such as light-speed travel or ‘suspended animation’ still a glint in a science fiction writer’s eye, reproduction in space might be essential … But scientists are not sure it is even possible. ‘Giving birth in zero gravity is going to be hell because gravity helps you,’ said biologist Athena Andreadis of the University of Massachusetts, ‘You rely on the weight of the baby. Sex is very difficult in zero gravity, because you have no traction and you keep bumping against the walls.’ “

I’m not really concerned about this, because any future space voyage long enough for sexual difficulties to be a major issue would likely also utilize some form of artificial gravity. Prolonged periods of weightlessness have harmful effects on the human body: deprived of Earth’s 1 g of gravity, muscles atrophy, bones weaken and blood pressure drops dangerously low at the heart weakens. Contemporary space expeditions are short enough that astronauts and cosmonauts can prevent the worst of these effects through vigorous exercise, but this won’t be sufficient for longer periods of time in zero g. Because a one-way journey to Mars would likely take 11 months with conventional chemical rockets, controlling the harmful effects of weightlessness is an important consideration for longer mission (for comparison, the current record for a single spaceflight is 437 days). For more ambitions, long term goals like voyages to the outer solar system, the need for some form of artificial gravity becomes even more important.

Luckily, simulating gravity artificially is fairly simple. The easiest method is through rotation. We’re all familiar with this effect — just as water in a horizontally spun bucket is drawn outward and doesn’t fall, astronauts in a rotating spacecraft would feel a force pulling them outward, nearly identical to gravity. This type of artificial gravity is extremely simple — all it requires is a spinning spacecraft. However, there are some potential drawbacks. The simulated gravity level, or g, is a factor of the radius of the rotation and the rate of spin. To simulate an Earth-normal 1 g, you either require an extremely wide radius or a fast rate of spin. Both of these are problematic. A wide radius of rotation is difficult from a spacecraft size standpoint, and fast spin rates are, understandably, associated with nausea. Small radii also exacerbate another problem: because the perceived downward-pull force isn’t gravity but the reaction force of centripetal force, perceived gravity is stronger the farther away one is from the center of the center of rotation, meaning gravity would be stronger at an astronaut’s feet than her head — a disorientating feeling. This isn’t an issue with large, slow spins, but would be problematic for faster spin rates.

The easiest way to construct a large enough radius to allow for a slow spin is build a large spacecraft. To produce 1 g of perceived gravity at a spin rate of 1 rpm requires a radius of spin of  895 ft, implying a very large spacecraft, even if we increase the rate of spin to 2 rpm. However, this is feasible if a spacecraft’s living areas are housed in a giant ring, reducing the construction costs. This scheme is common is science fiction and proposals for future space stations.

It’s also possible to produce simulated gravity without an enormous spaceship. It would be much easier to spin a spacecraft by attaching it to a counterweight by a long tether, and then spinning the two end over end. Because the radius of spin is limited only by the length of the tether, 1 g at low spin rates are easily achieved. The counterweight could even be a discarded booster rocket, a scheme proposed by scientist Robert Zubrin in his The Case for Mars.

Simplifying things is the fact that humans don’t require a full 1 g to stay healthy in space. It’s likely that the worst of the health problems of microgravity could be prevented at Martian gravity, or 3.76 g. At nearly a third of Earth’s gravity this is much easier to achieve, requiring a smaller spacecraft and likely making Martian g the goal of future spacecraft designers. Author Kim Stanley Robinson’s novel Red Mars features good depiction of a feasible near-future Mars ship structured to produce Mars-level gravity:

From Red Mars.

From "Red Mars". The rig-like structures are rotating torus housing living quarters.

However, there’s also another, more ambitious way of producing simulated gravity: constantly accelerating faster than 9.8 meters per second. This is extremely ambitious — a spacecraft accelerating continually at 1 g could reach Mars in as little as two days, or longer if we allow time for deceleration. However, any reasonable travel times to other solar systems would likely require at least this level of acceleration, meaning that any mission aiming to reach the nearest stars in a human lifetime would have to accelerate faster than 1 g. Once you’re capable of these speeds simulating gravity is simple enough — just construct your spacecraft so the “floors” are perpendicular to the acceleration vector, and you’ll be pulled towards the floor, just like accelerating in a car pushes you back into the seats. Because a spacecraft must also accelerate once it’s reached it’s destination, a spacecraft would simply flip over halfway through it’s journey and continue to produce the thrust to simulate 1 g or more of gravity. While achieving acceleration above 9.8 m/s is far beyond our current technologies, a number of extremely exotic mid-future propulsion schemes could put it within the realm of possibility. Pulsed fusion engines could likely reach 1 g of acceleration, as could laser-accelerated light craft and even more futuristic (some would say ridiculous) anti-matter propulsion technologies.

Incredibly, humans already possess the technologies to achieve 1 g accelerations speeds. Nuclear pulse propulsion would propel a spacecraft by exploding a series of small nuclear bombs directly behind it, allowing the spacecraft to “ride” the nuclear shock wave. The technology to produce a nuclear pulse-propelled spacecraft has existed for decades, though actually building one would remain difficult. Additionally, the fallout produced by hundreds of nuclear explosions would be problematic near Earth orbit, and international treaties banning nuclear weapons in space technically make a nuclear pulse spacecraft illegal.

Ultimately, the greatest problem with linear acceleration-produced artificial gravity is the possibility of too much of it. If humans are ever capable of constructing propulsion systems capable of accelerating at 1 g, it’s likely that we could reach even higher rate of acceleration. Unfortunately, the human body cannot likely sustain more than 2 gs for extended periods of time, as anyone who’s ever been on a roller coaster can attest to. Because faster acceleration rates have the potential to dramatically cut travel times, it’s possible that future astronauts could be placed in some type of artificial hibernation while their craft accelerated, protected from higher and higher g forces by permeating their bodies with an incompressible fluid (a good depition of these scheme, and space combat at 25 g, can be found in Joe Haldeman’s classic The Forever War).

Another interesting depiction of a spacecraft capable of accelerating above 1 g is the spacecraft seen in the opening minutes of Avatar. The film states that this spacecraft is capable of reaching neighboring star Alpha Centuri in under 7 years. This implies a speed of 0.7 c (70% of the speed of light) at an acceleration and deceleration of 1.5 g, along with an over 5 year coasting cruise period. While the hibernating crew would suffer from the effects of micro gravity during the coast phase, this would likely be at least partially compensated for by a cumulative 3 years of 1.5 g.



Both the rotation and linear acceleration methods would allow for reliably producing simulated gravity and likely prevent the worst health affects of prolonged weightlessness. While it’s arguably of less concern than muscle atrophy or bone loss, they’d also facilitate space sex. Win win.


Apparently the 1999 porn film The Uranus Experiment was filmed in zero gravity produced aboard a Russian cargo plane in a dive. Unfortunately, I’ve never been able to find this cinematic treasure.

3 Comments Post a comment
  1. Tim #

    What is this, a tabloid? There was virtually no sex in this article despite what the title suggests. I want my money back.

    October 7, 2011
  2. I think with this sex no pregnancy should occur! just a thought (or) topic for research?

    October 11, 2011
  3. A cylindrically shaped house station is rotating about the axis of the cylinder to form synthetic gravity. The radius of the cylinder is 212 m. The instant of inertia of the station not having visitors is 2.24 x 109 kg?m2. Suppose 473 people young and old, with an common mass of sixty nine. kg every single, stay on this station. As they shift radially from the outer surface area of the cylinder toward the axis, the angular pace of the station improvements. What is the highest relative modify (/ ) max in the station’s angular speed because of to the radial movement of the men and women?

    I am in an over the internet physics course and cant determine this just one out. Any assist would greatly be appreciated.

    March 4, 2012

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