Would a biological resource-system be feasible for long-term manned flights?

Question

Supposing a long-term manned flight is planned would a biological resource-system be considered as an option?

Would a biological resource-system work or is a technical solution superior?

Currently technical solutions turn urine into drinkable water again and turn CO2 (carbon dioxide) to O2 (oxygen) [and C (carbon)]. Would the same be feasible with plants or other biological solutions?

This isn't limited to oxygen and water; food could be grown as well. Would it be considered or dismissed, because testing such a system could be difficult.

Answer 1

If I understand what you are asking correctly, this is essentially the same idea as the Biosphere 2, where your spaceship air and water is managed by plants, to keep it livable. The Biosphere experiment provided a baseline for what would be required to pull that off, and actually tested such a system. The design called for 8 people to stay inside of the 204,000 square meter facility for 2 years, with the only outside environmental change to come from the form of managing the temperature and rainfall, all from within the contained system. A photo of the system can be seen below, sourced from Wikipedia.

In order to pull this off, you need to have a huge facility. Just to put this in perspective, the entire volume of the International Space Station is 837 meters squared. The biosphere is thus about 240 times larger than the ISS. Granted, there are aspects of the Biosphere which would not need to be reproduced to completely maintain systems on a space station (Variable air chambers, etc), but that would only reduce the size maybe by a factor of two. And even at that, the system wasn't able to maintain a truly independent environment, during the first test, they had to bring in oxygen and resorted to using emergency food stores.

The bottom line is, to use this would require an extensive system. A hybrid system could be put in place, where machines do some of the conversion from Carbon Dioxide to Oxygen, and food is grown, with a significant reduction in the required space, and that's the most likely solution that we could see anytime in the near future.

Answer 2

A recent experiment on the ISS showed that plant growth in space, with help from directional light, is entirely possible. The findings show that the plants grown in space were smaller than the equally aged ground control counterpart, but this can no doubt be accounted for when considering something like this.

This source (an ask-a-scientist program by the Argonne National Lab) gives us some great information on the amount of oxygen an average adult human breathes per hour, as well as the amount of oxygen an individual leaf produces. With all this taken into account, they estimated that 300-400 plants were necessary to sustain one human.

This estimate assumes 30 leaves per plant, which is reasonable.

If we generously assume that each plant takes up a cubic foot of space, then the maximum amount of space needed for a two-man crew would be 800 cubic feet.

Let's say they had sets of these plant pods in 20ft x 4ft x 1ft arrangements. You'd only need 10 of such platforms to have enough for a two-man crew. Stack five of them on one side and five on the other, and you'd have a working greenhouse capable of providing oxygen for a two-man crew.

Of course this is no small measure. These sets that we're talking about would be quite large.

In terms of possibility, I'd say it's 100% possible, even with today's technology, to achieve something like this. Perhaps not yet with harvesting water, but I can imagine technical means of doing so. In terms of considering it, I'd say there's an incredibly low chance--it's already wildly expensive to build anything of moderate size to send to space. Accounting for an 800ft³ layout of plants would make no sense.

And clearly, there are a lot of variables to account for. It simply wouldn't be foolproof at this point to send astronauts on a voyage of any distance without a controlled amount of oxygen.