# Discuss: why don't they just spin the ship?

It is speculation time. My roommate and I were watching a story this morning on CNN by Sanjay Gupta about how astronauts lose bone mass while in space. One of the limitations with space travel is that because of the absence of gravity, your bones steadily deteriorate. Load bearing exercise is required for bone maintenance.

To compensate for this, the people at NASA have all manner of contrivances to let the astronauts do load bearing exercise. Read the story. There is even a vertically oriented treadmill.

Anyway, this story stimulated a discussion between my roommate and me about how much of a hassle it is that we don't have magical gadgets like artificial gravity to put on space ships. We were talking about what current technologies exist to provide artificial gravity and thus prevent bone loss (in addition to making work in space much easier).

The one that sprung to mind immediately is the 2001 Space Odyssey solution (and probably a lot of other science fiction). In 2001 Space Odyssey, there is a satellite that is spinning to provide artificial gravity by centrifugal force. It seems to be a rather simple and elegant solution to the problem.

So here is my speculative question: why don't they do this already? What complications prevent NASA from usinh a spinning satellite to make artificial gravity?

What we were thinking is that it is a couple of things. My first argument was that the satellite would have to spin too fast to make something like Earth gravity. It would just fall apart. You can calculate the number of rotations per minute required for 1 g using a centrifuge calculator (on the right, rcf = g for this discussion). For example, for satellite 5 m in radius the rotation speed would have to be about 13 times per minute. The bigger the satellite, the slower it can rotate to get your 1 g of gravity. 13 revolutions per minute seems a bit fast for a large structure, but it shouldn't be insurmountable to make something that will hold together.

Then we got to thinking about some other practical issues. First, for a space station that is not too big, the gravity you would feel on your head and the gravity you would feel on your feet might be different because of the height of your body. For small satellites that just wouldn't work. Everyone would get sick. Second, whenever you are loading a centrifuge you have to worry about balancing it. Again unless the space station was quite large, wouldn't balancing become an issue? (Forgive us, biologists, for thinking of these things in relatively simplistic terms.) Finally, because of conversation of angular momentum, in order to spin the space ship you need to spin something else in the opposite direction. Maybe you could get it going with air pressure like they steer the shuttle now. But if you were trying to use a motor wouldn't the motor have to spin in counter-rotation?

The biggest obstacle that we could think of is that the space shuttle would just have to be too big. The space station in 2001 was quite large, and maybe it is just prohibitively expensive to make one large enough for this strategy to be effective.

My roommate had another idea that you could create a sleeping quarters where all the astronauts could sleep in artificial gravity. Remember the amusement park ride where you would get sandwich against the wall in a spinning cylinder. Something like that. This way at least the astronauts could have artificial gravity for part of the day.

Final speculation: why don't they just use a pill? The molecular biology of osteoclasts (the cell type that digests bone in the remodelling process) isn't entirely understood, but we could get there. Why not just have a pill that stops bone resorption during the flight? You would have to stop by the time you got home because bone remodelling is necessary for strong bones (there are some diseases where bones become brittle due to absence of remodelling...like osteopetrosis), but it could work for a while.

Anyway, I am very curious to hear what people have to say about this -- particularly the physicists and engineers. Why don't they use a spinning space station to create artificial gravity?

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There actually are drugs that does what you describe (more or less); they're called bisphosphonates, and they inhibit bone resorption by blocking osteoclast activity and stimulate bone growth by stimulating osteoblast activity. They're used to prevent and treat osteoporosis and fractures in at-risk patients.

However, I'm not sure that bisphosphonates can compensate for the loss of gravity. You'd have to do clinical trials IN SPACE!!! to demonstrate that it worked, and considering the cost, I don't think the drug companies really have the incentive to try to get their product an FDA-approved indication for "osteoporosis due to space travel"...

A couple other problems:
1) If the space shuttle has windows, the astronauts would most likely get pretty "sea sick" from the spinning.

2) The Coriolis force (as well as the varying centrifugal force you mentioned) would also cause some issues with being able to perform as if in a true gravitational field.

NASA has done studies on rotational gravity. You need to get down to around 1 rpm to reliably avoid motion sickness, though 2-3 rpm may be possible for a subset of people. This study is more than 30 years old, but I don't think the findings have changed much:

1 rpm requires a radius of rotation of 900 meters per G, which is enormous, and has a rotational velocity of 94 meters per second, which is a noticeable chunk of delta-V. 3 rpm is still 100 meters per G.

There are also problems with the fact that objects not in contact with the outer surface don't behave (from the observer's point of view) as though they're falling. That would include someone who jumps free. The illusion of gravity is only maintained by contact with that surface.

Hamster wheel? swimming pod? track with hand holds ringing around the outside of the ship? No idea!

In high density seawater, gravity & buoyancy compete:
Manatees: big lungs, big dense bones, shallow dive, no fat,
Whales: small lungs, porous bones, deep dive, much fat.

I'm surprised NASA hasn't figured out a good solution yet, isn't that one of the main objectives of the whole space program, humans in space?

Full-contact, space Muay Thai combat is the obvious approach for maintaining muscle mass and bone density.

By Spaulding (not verified) on 17 Sep 2008 #permalink

Why not forget about humans for long space journeys and rely on sophisticated robots for space exploration?

By Frank.mcauliff… (not verified) on 17 Sep 2008 #permalink

One method you missed is increasing the radius with a tether. Attatch two pods (or one pod and a counterweight) with a long rope and spin them around each other.

StephanieZ
If you jumped up on a spinning station, you might not fall straight down, but the wall would smack into you. If you adjusted the "floor" to the right angle you could maintain the illusion of gravity pretty well. Though it would be pretty interesting in that the gravity on one side of the room would stronger than the other.

Doh, I forgot the effects of momentum. If you jumped up, you would continue moving forward (tangent to rotation) and since the station would be curved upward, you would eventually hit ground again. And since the floor would be spinning underneath you, you could end up not to far off from where you jumped! /sigh, I wish it was the future already, this stuff would be much more fun to play with in practice.

I think stephanie is right. If the ship is shaped like a large hamster wheel, and you run fast enough in the opposite direction as the wheel is turning, then you are essentially standing still with the wheel turning just like a hamster. And now there is no gravity any more. You can just lift your feet, and the wheel will continue right under you while you float forever. Other people still on the wheel will perceive you as being in orbit, but inside-out. Very cool. And strange.

At 900m radius and 1rpm, too bad nobody can run that fast (approx 15 m/s = 33 mph). But you could throw a ball easily that fast and put it into inside-out-orbit.

-kevin

Most people will get their sea legs -- erm, space legs -- in a day or two. Just feed the fish a few times, keep the salt air in your hair, and everything will be fine.

Really.

There is also the structural aspect of the ISS and any STS missions. The structures are designed to exist in a low stress environment. Meaning if you were to spin them they would be ripped apart by the forces (ISS). Also due to the current design none of the plumbing would function properly. All of the pumps would malfunction under the stress and pretty much all operation would come to a halt.

And in another practical issue. Both space vehicles utilize space very well. The space travelers can use all six sides of the rooms they are in for activities or storage. When you all of a sudden introduce a mock gravitational force you loose the utility of 5 of the 6 sides.

Finally the fuel used to spin up and down would be better consumed for delta v adjustments and station keeping maneuvers. Although you could use a reaction wheel system but for the large mass of the shuttle and ISS it would be impractical. Note: ISS already has reaction wheels but are used for proper orientation of the solar panels and reliability is an issue.

By Justin Treptow (not verified) on 17 Sep 2008 #permalink

I believe the pill solution would work best ( you wouldn't have to test them in space, just in conditions promoting a comparable amount of bone loss).
Until the advent of human genetic engineering of course. That would allow you to tackle those nasty conditions that human being just weren't 'designed' for. High radioactivty, zero-g, long voyages and size restraints for example.

By Whodunnit (not verified) on 17 Sep 2008 #permalink

Kevin: Yes, you could get up to speed and cancel out the rotation... but then you'd keep travelling in a straight line, because without contact with the floor there's nothing to push you inwards, and so you'd end up touching the floor again almost straight away.

Along the lines of what Drekab said: you take two identical capsules and connect them with a cable. Point the capsules in opposite directions and fire up their rocket engines to get them to revolve around the center of the cable. Once the desired centrifugal force has been reached, the engines are turned off. At your destination, you turn the capsules around and use the engines to stop the rotation again. It's pretty simple, you "just" need a cable that is strong enough.

Of course, there's even a better solution: accelerate at a rate of 1 g for the first half of the journey, and turn the capsule around to decelerate at 1 g for the rest. This gives you apparent gravity and a shorter trip. Unfortunately, we can't bring enough fuel to sustain 1 g for the required period of time with rocket engines, so this idea will have to wait until we come up with some other propulsion system.

Just my 2 pees.

- I'd guess that perhaps 0.4g upwards would be fine, which reduces the engineering problem a bit.

- For any long mission, you are going to need a big ship in any case. Only the living quaters need spin, of course.

- Assuming that the living quarters are only a small percentage of the total mass of the ship, the whole spin up/down problem is largely avoided - electric motors are fine.

- A significant issue is that the outermost part of the ship will have lest radiation shielding by definition.

- Justin - we have special devices for making storage use of surfaces orientated normal to local gravity - they are called shelves.

By Andrew Dodds (not verified) on 18 Sep 2008 #permalink

Sounds like a combo approach may be best - astronauts (or space tourists for that matter) must exercise in certain ways, take supplements, and then you could have the basic living quarters only spinning (sleeping, eating, perhaps a meeting room), not the whole vehicle.

Not a total solution, but OK for runs out to the moon. Transport to Mars would be another matter. You would either have to solve the problem completely, which no one knows how to do yet, or possibly we could build a network of stations so that travelers could do Mars in a series of shorter trips between outposts, instead of in one big trip.

Beyond Mars, we would have to deal with timescales, speeds and other big problems, so that's probably off the agenda for now.

Drekab, I'm really sorry I couldn't find any good diagrams. I've seen someone sketch out what the motion of, say, a thrown object would look like to a person standing on the outer surface, and it's just bizzare. The reason I mentioned the observer is that I think the stresses of being in a situation where nothing moved the way you expected would be much greater in a spinning environment than they would be in free fall.

I'm not sure about this, but... If you take a pill that increases osteoblast activity while suppressing osteoclas activity in a zero-G environment, bone remodeling will be much more haphazard and the structural integrity of the bone will change. You might get bone spurs and the bone will not be as strong per gram when the astronaut returned to a one-G environment. You will need some kind of stress along the body's long axis to maintain normal bone configuration. This would not be a problem for short time flights, but it could cause problems on long flights.

By natural cynic (not verified) on 18 Sep 2008 #permalink

Stephanie Z said:

I think the stresses of being in a situation where nothing moved the way you expected would be much greater in a spinning environment than they would be in free fall.

I personally doubt that. Things may move in unexpected ways at first, but they still move in predictable ways. Over time, experience will make you get used to the motions. Probably even to the point that you'll have to re-adjust when you get back on Earth.

The reason I think this is because of experiences people had in the early days of Virtual Reality. People who would wear a VR helmet would get disoriented at first because whenever they turned their head, the virtual world moved around them with a noticeable lag. However, people soon adapted to this and stopped noticing. When they took the VR helmet off, they again got disoriented and had to get used to the world moving instantly when they turned their head.

I'm sure there are many more examples like this. People have an amazing ability to adapt.

A couple other reasons not yet listed:

1) For communication, antennas must remain pointed towards Earth or a communication satellite. Any telescopes or other observational equipment must also remain stationary.

2) Many of the experiments are meant to be done in low (or no) gravity. If they could be done at higher g's, they could just be done on the ground.

3) The shuttle is mainly used to, errr, shuttle equipment and satellites to space. You cannot launch equipment from the cargo bay if it is rotating. For one thing, the cargo arm would not work (it cannot provide the large forces necessary in a rotating shuttle/cargo system). It would also be very dangerous: rotation would introduce many chances of collision between the shuttle and cargo, but even low speed collisions would be catastrophic.

For those reasons and others, you then must have at least part of your shuttle/space station non-rotating. Then, if you want rotation for your crew, you are talking about having both a rotating and non-rotating part. It is enormously difficult to do that: how can you maintain a seal against the vacuum where those two pieces connect? It already takes careful engineering to make doors with seals that can withstand the vacuum, but those only need to work when the door is closed and not moving. Even if one could design such a seal at a reasonable cost (not to mention all the other engineering that would need to go into a rotating section), the potential for catastrophic failure introduced by having such a seal probably far outweighs the relatively mild physiological side effects to zero g living. The bone loss is only temporary and isn't in any way life threatening, correct?

So the question is, is it worth it to spend many billions of dollars in engineering and construction and introduce many new dangers in order to reduce a mild, non-dangerous, non-permanent physiological effect? At this point, I would say being able to study bone loss is actually more useful. Exploring the use of exercise equipment or medicine are cheaper and safer alternatives.

Just thinking basics here, regarding the engineering of a contra-rotational craft. One axis, two counter-rotating pods and a central shaft (on axis I assume) No problem there, just make the pods independent of the axis. By that, I mean that the pods have their own internal, and discrete, life support/systems/communications/power, but seperated from the machinery of actually rotating. In this case the vacuum problem become redundant, unless we are discussing structural rotation in a vacuum itself.
 Just a thought, maybe it could be electrical, like a solenoid, so no need for actual contact?

If the discussion is actually that we need to provide rotation (around a stationary axis)in vacuum, then we don't really have a problem. It has to be mechanical, I would say, electronically controlled without a doubt, but without the need for lubricants per se. We're not thinking of creating a space-bound version of the internal combusion engine.

As for rotational gravity, my physics is a bit sketchy on the subjest, and I'd like to see any projections, but if the orbitting rings/pods are far enough away from the central axis, there should be a pretty consistent gravity. Enough for the space-faring not to feel heavy-footed and light-headed.... ?

Another idea is to have a large number of counter-rotating rings/pods, not just two?

Either way, I'd like to think that if it can be built, we will build it.

Of course, the whole argument is redundant if there is a way to create artificial gravity without spinning. Don't get me wrong, I'd like to see it happen, but probably not in my lifetime.

Apologies: the edit should have been at the end of the second paragraph.

B