Is the assumption of the nay-sayers that air-recycling and water-recycling degrades over time ? So, that after some months the ability to recycle is reduced in both the percentage that is handled, and in the quality of the output ? If so, then is it a fatal degredation ?
It's a good question. The short answer is, we don't know.
Nobody has yet tried to run air- and water-recycling systems on these scales. By "these scales" I mean time (long) and mass (very low).
The total mass of the spacecraft is going to be tiny -- some tens of tons. Mars Direct, for instance, assumes about 45 tons, of which about 11 tons is disposable food. That leaves thirty-some tons for everything else.
Thirty tons is really dinky. It's about the mass of an eighteen-wheeler truck carrying a moderate load. And that's got to include everything -- crew, solar panels, exercise equipment, radiation shelter. It's not very much.
Time: Mars Direct assumes six months to reach Mars, 18 months on Mars, and 8 months to come back. There has been only one attempt to run a closed system over a time scale of many months: Biosphere Two. And B2 was over 100 times bigger than the proposed Mars spacecraft. It was a huge building the size of a football field, filled with ponds, trees, pipes, machinery, and hundreds of tons of rock and soil. The damn thing had a coral reef inside, complete with wave generator. And
they still ran into serious problems with their recycling: within a couple of months, oxygen levels had dropped by nearly half, while CO2 levels fluctuated wildly.
Biosphere 2's first mission lasted one year, but they had to inject additional oxygen twice. Even so, most of the vertebrate species died, and the environment inside was seriously degraded. There's universal agreement that the air inside was getting pretty stinky. Unfortunately, by the end of the first year, the B2 science -- never good -- was getting pretty sloppy, so AFAIK there isn't a detailed analysis of what was going wrong. (Yes, B2 involved some grotesquely wasted opportunities. Don't get me started.) But things were definitely going from bad to worse.
A Mars mission habitat would be less than 1/100 of the size of Biosphere 2 and would last about three times as long. We simply do not have that technology right now. We don't even know what might go wrong. It's like a guy in 1890 speculating about airplanes.
So, no Mars mission is going to be plausible until we have such systems. And it's going to take a while to develop them, and then another while to test them. Someone will have to be locked up in a little room for a year or two. And we're not even close to being able to do that yet.
If you want a guess, I'd say we're 10-15 years away.
Doug M.