THE CREW AND THEIR HEALTH
- establishing humanity's first, permanent foothold off Earth -

HEALTH CONSEQUENCES
In order to extend crew stay, the crew's health needs protecting. Two of the largest of these health factors are radiation and the reduced gravity on the Moon. In addition to the dirt on top of the habitat providing protection from radiation, an indoor centrifuge could play a critical role in helping the crew maintain their health.

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.
  

CENTRIFUGE DESIGN
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:

HYDROSTATIC PRESSURE DISTRIBUTION
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.

ON THE WAY TO MARS
If the ISS has shown us anything, it has shown us that extended zero gee causes a variety of health consequences. Recently, one of the astronauts developed a blood clot in a neck vein. Fortunately, with the in-orbit refueling of the Starship, transit times to Mars could be as little as three to four months. This is less time than the average that astronauts stay on the ISS. So likely, zero gee won't be a problem when traveling to Mars.

But there is another very simple solution and we are frustrated that an appropriate animal experiment hasn't been done to demonstrate it. Tether-and-spin up would likely be a straightforward solution. After trans-Mars injection is completed, the spent upper stage and crew cabin could be separated with a tether between them. A relatively small amount of propellant could be used to spin up the system to either Mars gravity level or up to a full gee. Here is a video illustrating this concept: