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

BENEFITS OF EXTENDING CREW STAY
If we could extend the length of how long the crew could remain on the Moon, this would result in several benefits. It could either cost less since there would be fewer launches necessary to maintain (or grow) the population of the base. Secondly, each time we rotate crew, there is a danger to their lives and consequently a danger to the program. The Challenger and Columbia accidents illustrates this concept.

BIOMEDICAL RETURN CRITERIA
How long can we remain on the Moon and how can we know when the crew has to return. A number of biomedical criteria could be established. The crew should be periodically monitored. If they are nearing any of the criteria, then they are sent back to Earth. Bone mineral density is a good example but there are several other indicators that would be monitored. On the Moon, we wouldn't need to know beforehand how long the crew could remain before the criteria is met. They would remain as long as possible until the criteria is met. With exercise and artificial gravity perhaps the crew could remain on the Moon for a long time.

RADIATION ISN'T THE LIMITING FACTOR
Even a modest amount (e.g. 30 cm) of lunar dirt on top of the UniHab would provide full protection against solar storms and would reduce the radiation levels of the galactic cosmic rays by about 50%. This would allow for the crew to remain on the Moon for a few years before they reached their career limits. They would then have plenty of time to maintain the telerobots to push even more dirt on top of the habitat. Therefore, it is not the radiation exposure which would limit the length that the crew could stay. Rather, it is the health effects of reduced gravity of the Moon that would likely determine how long the crew could stay.

Paper: Reducing Mission Costs by Extending Crew Stay

LENGTH OF CREW STAY
How long then could crew stay on the Moon if exposed to 1/6th gravity plus about four hours of full artificial gravity each day. We simply don't know. Those artificial gravity experiments have not been done. However, we might be able to speculate on a lower limit of the stay.

The Russian, Valeri Polyakov has the record for the single longest stay in space (on the Mir). That was for 14 months. Several others have stayed in space for over a year at a time. So, certainly the crew should be able to remain on the Moon for at least that long. If multiple crew have been able to remain in space for missions lasting at least a month, we can speculate that they could go for, perhaps, two years before their health required them to return to Earth. The crew on the Moon would engage in the same level of exercise program as the crew on the International Space Station and would be exposed to 1/6th gravity continuously. So, with lunar gravity, perhaps they could remain on the Moon for at least three years. Then, with four hours a day of full artificial gravity, perhaps they could remain for at least five years. This is speculation, of course, but it is not unreasonable speculation. But five years is the lower limit. Perhaps four hours of full, upright, artificial gravity is sufficient to stave off the effects of lunar-level gravity for many years. So, it would be fair to say that the initial crew could remain on the Moon for an indefinitely long period of time.

Extended stays of three years or more have interesting implications as to what then would be the deciding factor as to how long the crew could remain on the Moon. The answer is that it is the crew's social status that becomes the dominating factor.