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Artificial Gravity Art Harman describes how an indoor centrifuge could provide full gravity to the crew for several hours a day and how this would create a similar hydrostatic physiologic environment that we experience on Earth.

There are a number of conditions known to result from being exposed to insufficient gravity. These include:
  • Bone loss resulting in osteoporosis
  • Loss of muscle mass
  • Uncoordinated walking due to the loss of the downward gravity force.
  • Loss of the blood vessel's ability to maintain adequate blood pressure in the head to prevent passing out.
  • Abnormal distribution of fluid up from the lower part of the body towards the head resulting in cardiovascular changes.
  • Swelling of the optic nerve pushing on the back of the eye distorting focus.
  • Changes in the immune system.

Artificial gravity can be supplied in the form of an indoor centrifuge. It would be 15 meters in diameter and would spin at 11 rpms providing the equivalent of Earth's gravity. It would have chambers on the end tall enough for crew to stand up in. The chambers would swivel out when spun up so that the force vector would always be pointing down between the feet. The crew would spend about two hours in the morning and two hours in the evening in the centrifuge conducting "sedentary activities". Four hours is about the amount that we are upright each day on Earth. The sedentary activities are those which most of us do anyhow and so wouldn't involve any difference in normal daily activity. These activities include:

Lifting small weights
Listening to music
Playing a video game
Reading a book
Replying to e-mails
Surfing the Internet
Video conferencing
Watching a movie

When one is spun up at 11 rpm, any movement of the head in any plane will create a strong disorienting sensation. But, if one simply does not move one's head there is no perception of being spun around. This does not require that the head be strapped down only that one chooses to keep looking forward.

One thing that the centrifuge will do is to shift the body fluid downward as Earth gravity does. This is called hydrostatic pressure but should not be confused with blood pressure. Our body has several mechanisms to control the blood pressure throughout the body. But it is the pressure of fluid outside of the arteries which we are talking about here. It is simply the pressure of fluid at different heights. An example of this is that there is more water pressure at the bottom of the swimming pool than near the surface. If people stand on their feet for a long time, there can be swelling in the ankles. This is an example of hydrostatic pressure in the body.

The concept of the indoor centrifuge in the UniHab is that it will replicate the hydrostatic pressure distribution that we get on Earth throughout the period that we are upright (i.e. standing or walking). When we lay down on Earth, the difference in pressure between our back and the top side of our bodies is relatively small because the column of body fluid is relatively small. So, perhaps sleeping in the centrifuge won't provide as much benefits as one might think. To approximate the hydrostatic pressure distribution during the many hours that we sit on Earth, perhaps the crew should do more standing when they are outside of the centrifuge than they would normally do on Earth. The premise here is that the closer to natural physiologic conditions that we get, the more likely the crew will not experience disease.

We really don't know what effect that the Moon's 1/6th gravity has upon a developing fetus or child but it's probably not good. We also don't know how much of a benefit partial artificial gravity would give. The Plan for Sustainable Space Development supports artificial gravity experiments to find out. But there is a matter of timing. Exactly when do we need to know the answer?

For many years, we have done little to find out the artificial gravity prescription. There was a plan to place a centrifuge module on the International Space Station but budgetary cuts nixed that. Now that module sits unused in a Japanese parking lot. But then, crew on the ISS don't really need to know the artificial gravity prescription. Crew rotations typically last four to six months such that any impact of microgravity is acceptable.

But, do we even need to know the artificial gravity prescription before we go to the Moon? Probably not. As discussed in the Extending Crew Stay page of this Plan, we can use biomedical criteria and for those crew who are nearing these criteria, we can return them to Earth in three days. So there's no compelling need to wait to go to the Moon before artificial gravity experiments are done and, once on the Moon, we can do those experiments as a side project prior to the much longer missions to Mars.

With an indoor centrifuge providing full artificial gravity, crew should be able to remain on the Moon for at least three years and possibly much longer.

In the long term, we need to figure out the artificial gravity prescription for healthy gestation and childhood. And, since in-space pregnancy could happen in the relative near-term (e.g. in an Earth-orbiting hotel) there is some urgency to figuring this out. Also, the four couples of the initial crew should avoid getting pregnant until the animal studies are done. Perhaps the most logical solution would be for the men to undergo reversible vasectomies (see the picture above) and both the husbands and wives should have their gametes frozen on Earth as the ultimate method of ensuring that they could retain the option of having healthy children. These young couples could be fairly motivated to complete the animal studies so that they would know what it would take to have a healthy child on the Moon. There is the possibility that a fetus or child would essentially need full gravity 24/7. This is not practical in an indoor centrifuge. If this is the case then, as a practical matter, that type of space settlement involving children would be limited to large, rotating, orbiting settlements with a long enough diameter to allow for full gravity with a rotation speed that could be accommodated to.

A series of animal studies using the indoor centrifuge could be conducted whereby the artificial gravity prescription (AG Rx) for healthy gestation and childhood could be determined starting with a rapidly-producing mammal such as mice. The question that we would be trying to figure out would probably be, "How many hours a day of full artificial gravity is needed"? Determining the AG Rx for healthy gestation would probably take two or three sessions of 25 days each. So, in two and a half months, one could move on to the next animal model (e.g. marmoset). The longer studies would be to determine the AG Rx for healthy childhood. But, one could begin to guess possible childhood AG Rxs for later animal models and so begin those studies even before one has the precise answer. Ultimately the goal would be the first off-Earth birth of a human child. Click to read about the ethics of doing this. It is difficult to say, but perhaps the AG Rx for healthy gestation and childhood for a macaque monkey could be established in 5-10 years after the initial crew arrives. So, the very first crew could also be the first crew to attempt an off-Earth pregnancy. Ultimately it would be their choice having been informed by their own experiments.

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