Gravity in SciFi: part 1
Editor note: This article came about with me researching artificial gravity for a little sci-fi novel I’m writing. What I found in the research was there are two types of articles on artificial gravity: ones so superficial they end up saying nothing and ones so bogged down with the math that they equally say nothing. In this article I plan to keep the math down to a minimum (hopefully zero) and still say something useful.
Gravity. It’s a very heavy subject to delve into (sorry, couldn’t resist the pun). The human body needs gravity. On the international Space Station the astronauts spend almost a third of their waking hours exercising and do other activities just to combat the effects of micro-gravity on their bodies. Yet when they return to Earth they still have all forms of issues with their bodies from eye problems to reduced bone mass.
In sci-fi writers deal with this need for gravity in two ways; rotational artificial gravity and what has been called ‘magic’ artificial gravity. Part one of this two parter will delve into rotational artificial gravity. Part 2 will be the sci-fi staple of ‘magic’ artificial gravity.
When I was a kid they had this ride at the playgrounds that was round, had bars on the edge that you would grab on to and could be spun very fast (or as fast as the rusted center section allowed). H youow this thing worked is you would do is have a buddy spin you around and around really, really fast till you got really, really dizzy. Afterwards you would get off, stagger around a bit like an idiot and then threw up. It was great fun. (I have noticed that these devices are no longer in playgrounds. I guess parents got tired of cleaning barf off the kids.) If you have ever been on one of these devices then you have seen the basics of rotational gravity.
As you are being spun around on this playground device you could feel yourself being pulled outward (I have no idea what the real name of this thing is). The reason you feel that pull is that a body in motion wants to move in a straight line. If you were to suddenly let go of the device (which I did more than once and always seemed to end up in and ant bed) you would fly off in a straight line converting all the energy of the rotation into forward straight line motion. But as long as you continue holding onto the bars you are counter acting the energy of Newton wanting to throw you out in a straight line with your energy of holding onto the spinning device with your hands. Therefore you feel a pull, like a type of gravity, pulling you outward.
Rotational gravity in a space ship would work the same way. A space ship would have a large cylinder (or torus) that would spin. You, the intrepid space traveler, would be standing on the inner edge of the rotating cylinder. The spinning cylinder would want to throw you off in a straight line but the solid floor would keep you from doing that. Therefore, you would constantly be being pushed against the floor.
Sounds great! You make a round thing, spin it up and, poof, gravity. Unfortunately, there is one big catch in that the rotating cylinder needs to be big, really big. The reason being is two fold.
Remember the part about the playground ride making you dizzy. Well, on a small scale the human body can feel the difference of you wanting to go in a straight line and continuing to spin. The results are a disorienting and dizzy feeling. But if the rotating body is large enough then then spin can be slowed and the effect is no so noticeable. This is due to that the larger the rotating device is the slower it needs to spin to produce the same gravitational effect.
Also, if the rotational area is too small the feeling of gravity between your feet and your head can be sensed. This would seriously mess with your inner ear (your bodies place of balance) which would be disorienting and would more than likely trip you up if you tried to walk. Again, the larger the rotating device the smaller the difference between the feel of the artificial gravity from your feet to your head.
So what is the proper size?
Let’s look at couple of examples of rotational artificial gravity in classic sci-fi.
In the movie ‘2001: a space odyssey’ we see Bowman running around the Discovery main habit area. The circle looks to be about three to four stories tall. We also learn that this section is spinning to create artificial gravity. So, would a rotating cylinder of this size work? Unfortunately no. At this size, Bowman’s feet and Bowman’s head would be spinning at a significantly different rate and feeling a different pull of gravity. His inner ear would be telling him one thing and his feet would be telling him another. More than likely, he would end up flipping head over heels trying to just walk, much less run. Also, to maintain around 1 gravity the habit would have to be spinning at 1 rotation per second! It’s obvious you would feel such a fast spin and quite, probably, puke your brains out.
2001 may have got it wrong, how about someone who got it right. ‘Babylon 5’, “Five miles long in neutral space …all alone in the night.” One of my favorite openings monologues to any show. That and “If not for the courage of the fearless crew the Minnow would be lost.” You know, if they were out for only a three hour tour, why were the Howells caring around so much crap? And how the hell did Ginger keep that party dress looking so good for so long? I just don’t get it. Oh the many imponderables of ‘Gilligan’s Island”.
OK, back to the subject, Babylon 5.
Babylon 5 was really big. If you have ever had to wait for a train to by the average maximum length of a train is around 100 cars or one mile. (Trivia: the reason for this max length of 100 cars is that sidings for trains are only a little over a mile long.) Therefore, Babylon 5’s full length would be the length of a 500 car train! Long wait. Now nailing down the size of a CG created mythical TV show space station can be a bit challenging but, from what I could find the height of the habitat cylinder on Babylon 5 is around 130 stories tall. To maintain 1g the cylinder it would need to be spinning at about 1.5 revolutions/minute or 1.5 mph. Such slow rotation and large size would make the artificial gravity on Babylon 5 almost imperceptible from Earth gravity. But is it necessary to make the spinning section so big? Short answer, no.
A three story rotating ring is too small and a 130 story tall rotating ring may be fine for a space station but would be prohibitively large for a space craft. So if you were going to build a space craft with rotational artificial gravity, what is the minimum size you could get away with? After all, getting something into space is expensive.
Well, from what I have found, the minimum size is around 20 stories tall. Any smaller than that and the bad effects I just mentioned come into play. That is still really big. Go downtown and find you a 20 story building and that is the size that the ring would have to be. That would give a circumference of about 1300 feet. If the width of the habitual area was 20 feet wide that would give you over 25000 square feet of usable area. That’s quite a bit.
Sounds doable, in sci-fi anyway. Still, there are a few issues that would still need to be addressed.
Turning. There are two things that would keep you from turning such a large rotating cylinder. First is that something spinning that large is going to resist a change of direction. Spin a wheel on a stick then try to move the spinning wheel, it will resist. This is how gyroscopes work. When the device changes direction the gyroscope resist and registers the change. Therefore a space ship would need the power to turn against the resistance and would probably be rather slow in doing so due to the amount of power it would require. Also you would want the turn to be slow, from a human point of view anyway. The spin of the ring is throwing you in one direction, if you turned too fast you would find yourself being thrown in the opposite direction of your turn. So if you spun the ship around too fast you would throw everyone against one of the walls. Not nice.
Another issue is that the spin would have to be real smooth. If there was any jerkiness in the motion the commutative effect was be nausea. Again, clean up puke. In reality, this more than likely wouldn’t be an issue as the centrifugal force of spinning such as large object would keep the speed constant.
So there you have it. All the problems solved for artificial gravity now NASA, get to it.