# Minimum size for useful artificial gravity experiment in orbit?

As I understand it, loss of calcium and bone density during prolonged weightlessness is a major problem when contemplating long-duration spaceflights (like a mission to Mars, for example).

One long-proposed solution is a habitat that spins to provide artificial gravity, but I understand that this concept has never actually been tested.

So suppose I want to build an experimental space habitat for 3 astronauts which is spun to provide artificial gravity. Assume it is a cylinder, sealed at both ends, and it spins around it’s long axis. How large a circumference would the habitat need?

Would the biggest Russian booster available be big and powerful enough to lift such a beast, or would assembly in orbit be necessary?

Finally, since the problem of loss of bone density was known from Soviet long duration flights on Salyut and Mir, why wasn’t ISS designed to spin? Too ambitious for the funds available?

Spinning an object in space generates forces that might lead to failure of the joints etc. Plus it makes docking a bit of a pig.

There’s no definite answer to the required size. We don’t know how much gravity is needed to prevent calcium loss, and we don’t have a very good idea of what rotation speed is acceptable for long-term exposure. In any case, you can’t launch a spinning cylinder like that - the maximum internal diameter would be at most 4 meters or so, so everyone’s heads will be at the cylinder’s axis. Not only would their heads get no gravity, but they would bump into each other. A better method may be to tether two modules and spin them around each other.

As for the ISS and Mir, one of their main purposes is microgravity experiments. You can’t do that in a spinning space station. Other tasks such as earth and astronomical observations are also made difficult by spinning the station. You could separate the station into a zero-G lab and a spinning habitation module but that would be extremely complex and costly, and the vibration from the movement would not be very good for the microgravity experiments.

Also, with enough excercise calcium loss can be minimized to an acceptable level.

Rats, space station, SMALL centrifuge at various G settings. Leave em in it for 12 months and then look for demineralization. No need to go with humans on the first try here.

Yeah, but I’ll volunteer! It does raise the question, though, if rats would be a good animal model in this instance. I don’t know enough about rat physiology to say for sure. We could always send monkeys. Wait, no that’d be a bad idea, too. They’d get sucked into the temporal vortex and take over the Earth, leaving Marky Mark as the sole example of humanity. Bad idea all around.

Seriously, though, Squink’s got the simpliest idea. There has been talk of buying the external shuttle tanks from NASA and instead of dumping them into the ocean, putting it into orbit and hooking it up to other external tanks and using them to construct a habitat in space. Two of the tanks hooked together would probably provide ample room for a human study once they were converted.

Rats and chickens have been used in vitamin D, bone deposition, research like this for decades. I picked rats because chickens are horrible, disgusting, smelly creatures; not the sort of thing you want to share a closed environment with. Of course, you’re right on target with that monkeys getting sucked into a temporal vortex scenario

Do we know this for sure? Have we experimented with long-duration spaceflight and high exercise regimens to prove this?

If not, it seems to me that any planning for long duration spaceflight is a waste of time. Until we can say with a high degree of certainty, backed by actual experience in LEO, that we know how to keep people in space for 12 months or more and land them on a planet still able to walk unassisted, a mission to Mars is a no-go. There are plenty of other problems to overcome as well, but it seems to me that this is the guaranteed mission-killer if not overcome.

Thanks to all for the good answers on why ISS doesn’t spin.

Here’s an article about somebody testing out a centrifuge with a 22 foot diameter. From the description, it sounds pretty horrible, so you’d probably need something considerably larger than 22 feet across in order to eliminate dizziness.

I meant that for typical flight durations of the ISS crew, NASA doesn’t consider calcium loss to be a serious problem. If longer flights become necessary in the future, it may become a serious problem. But even then, there are two possible solutions - spin the spacecraft to generate artificial gravity, or make the spacecraft faster to shorten the flight time. I suspect the latter would be preferable in most cases. There are numerous other problems associated with very long flights such as radiation exposure, food and water supplies and availability of help in case of mechanical or medical problems.

You’ve just come full circle back to the reason for my OP. You mention spinning the spacecraft to generate artificial gravity as a possible solution. However, it seems that this idea, widely taken for granted in science fiction, has never been actually tested in orbit, to see if indeed it works in reducing calcium loss.

JRootabega linked to an interesting article that shows that experiments with moving around in a spinning centrifuge are being done here on Earth. I guess that any really long duration spaceflight is still far enough off that no one has been willing to devote the resources to testing the concept in orbit, even on a mouse scale.

Wouldn’t it be better to have two pods on a long, strong cable of the same mass? Spin the whole thing not very fast and you’ve got gravity towards the outside end of the habitable pod. This way you can have a nuke on the other end.

Of course, if the cables(s) get severed, the pods go off in whatever direction they happened to be heading at that moment. Then again, there’s very small chance of an interplanetary rescue anyway. ARG! THEN AGAIN! impacts of that magnitude will kill ANY spacecraft! arg we should go and hide in caves.

I’d heard that the spinning motion can cause motion sickness unless it is less than something like 3 revolutions per minute. In order to build something spinning that slowly and still simulating near-earth gravity would require a very large radius.

An experiment like this was tried on Gemini 11 in 1966. The spacecraft was tethered to an Agena upperstage target vehicle then rotated to achieve artificial gravity.

http://www.hq.nasa.gov/office/pao/History/SP-4203/ch15-3.htm

It will reduce calcium loss; physiologically, the body should respond to centrifugal-artificial gravity in every way the same as it does to ‘real’ gravity; both forces are the same thing - acceleration.
It’s just the pesky coriolis stuff that’s going to be a problem.