It could do this with plants, but it's not their priority.
With plants, the basic equation is
6CO2 + 12 H2O -> C6H12O6 + 6O2 + 6 H2O
Where the oxygen comes from water and the oxygen from the CO2 ends up in more H2O.
Indirectly that H2O from the CO2 might eventually end up as O2 if it gets taken up for photosynthesis later on, but this is not splitting it directly.
See Plants Don't Convert CO2 into O2 and Page on lamission.edu (see page 5 there for a useful diagram of how it works).
(Thanks to Daniel Spector for correcting me here)
You could split the CO2 completely to C and O2, by using laser light: Making oxygen before life.
And there are other technological ways of splitting CO2 to get one of the oxygen atoms from it, see Richard Gorman's answer to Why can't the ISS split astronaut's exhaled CO2 and recycle the oxygen?
Anyway, I'll continue here with the photosynthesis approach.
To make oxygen from CO2 via photosynthesis they would need to dedicate an entire module to growing green algae. Or if they set aside three modules to the task, they could grow most of their food in them, and as a biproduct, also create all the oxygen they need from plants. In theory anyway. Experiments on the ground by the Russians showed that this is possible. The main question would be whether the same methods work in orbit. For instance dwarf wheat, which they used in the ground experiments had disappointing yields in zero g.
That could just be a matter of choosing the right species for zero g. Or for that matter, (just my own idea, not seen it in the literature) - why not have containers for the plants around the exterior of the module, that spin around the center, like a tumble drier, at whatever spin rate is needed to get enough gravity for healthy growth of the plants?
The thing is that the ISS is not primarily designed for doing studies like this. If you set aside three modules for growing its own food, that's three modules that can't be used for their zero gravity experiments and spacecraft systems. Even one module would be more than they would consider acceptable for this.
The mass requirement for the water needn't be great. The obvious thing for a space station is to use aeroponics, where the roots of the plants are suspended in an atmosphere of water vapour plus nutrients. This uses almost no water at all.
The air in the ISS needs to be kept very dry to prevent microbial growth which is a problem in space stations, microbial films, for long duration multi year stations like MIR or the ISS. But that just means that the atmosphere of the growing chamber would need to be isolated from the rest of the ISS.
Indeed you could condense the water that forms inside any container with growing plants (due to plant transpiration) and use that to supply water back to the ISS, so it could help with purification of the water.
The power requirements for supplying artificial light for the algae is within the capabilities of the ISS. It was 48 kW for xenon lights for green algae for six crew - that's a total of 8 m2 of algae per crew member and 200 - 300 watts illumination per square meter. But with modern LED technology that could be reduced to a tenth or less.
For instance, just checking commercially available high efficiency grow lights available to aeroponic / hydroponic growers in 2015: this High Efficiency Green Energy Full Spectrum SMD LED Plant Grow Light for Indoor Gardening, Aeroponic and Hydroponics - uses 20 watts of power to illuminate 0.2 square meters. It is recommended for crops that require bright sunlight such as lettuces here: Top 10 Best LED Grow Lights: The Heavy Power List. That would be 800 watts for 8 square meters, or 2.4 kilowatts for the light needed for algae for a crew of six. Or if they grow all their own food, three times that, 7.2 kW.
The current total power supply for the ISS is 120 kW International Space Station (ISS) power system
So in principle they could generate all their oxygen, it would seem. The main difficulty there would seem to be space rather than power supply.
Or you could use natural sunlight and go all the way and have an "artificial greenhouse" in space. If you can do it in the vacuum conditions of the Moon or on Mars, you can surely also do it as a space module, and it is a natural place to test the technology first. You'd probably have it at a lower pressure than the ISS as plants can make do with much lower atmospheric pressure than humans which could simplify the technology a bit. For instance in this NASA challenge they suggest a quarter of of Earth normal for a greenhouse for Mars or in Space. Deployable Greenhouse
If this was their priority, then either they could get all their oxygen this way - or else - they would turn up some issue with this approach. If we have a future space station focused on human factors and preparation for long term interplanetary flight and missions on the Moon and around other planets without resupply from Earth, this would surely be a priority.
For more on this, see, Robert Walker's answer to Why can't plants be used in the ISS to take in carbon dioxide and give out oxygen so that oxygen content inside the ISS remains almost constant instead of taking air from earth to fill in the tanks of the ISS?
And for the methods currently used to create oxygen on the ISS: Robert Frost's answer to How is oxygen generated on the ISS?
See also my Science20 blog post: Lettuces Now, What Next - Could Astronauts Get All Their Oxygen And Food From Algae Or Plants?
With plants, the basic equation is
6CO2 + 12 H2O -> C6H12O6 + 6O2 + 6 H2O
Where the oxygen comes from water and the oxygen from the CO2 ends up in more H2O.
Indirectly that H2O from the CO2 might eventually end up as O2 if it gets taken up for photosynthesis later on, but this is not splitting it directly.
See Plants Don't Convert CO2 into O2 and Page on lamission.edu (see page 5 there for a useful diagram of how it works).
(Thanks to Daniel Spector for correcting me here)
You could split the CO2 completely to C and O2, by using laser light: Making oxygen before life.
And there are other technological ways of splitting CO2 to get one of the oxygen atoms from it, see Richard Gorman's answer to Why can't the ISS split astronaut's exhaled CO2 and recycle the oxygen?
Anyway, I'll continue here with the photosynthesis approach.
To make oxygen from CO2 via photosynthesis they would need to dedicate an entire module to growing green algae. Or if they set aside three modules to the task, they could grow most of their food in them, and as a biproduct, also create all the oxygen they need from plants. In theory anyway. Experiments on the ground by the Russians showed that this is possible. The main question would be whether the same methods work in orbit. For instance dwarf wheat, which they used in the ground experiments had disappointing yields in zero g.
That could just be a matter of choosing the right species for zero g. Or for that matter, (just my own idea, not seen it in the literature) - why not have containers for the plants around the exterior of the module, that spin around the center, like a tumble drier, at whatever spin rate is needed to get enough gravity for healthy growth of the plants?
The thing is that the ISS is not primarily designed for doing studies like this. If you set aside three modules for growing its own food, that's three modules that can't be used for their zero gravity experiments and spacecraft systems. Even one module would be more than they would consider acceptable for this.
The mass requirement for the water needn't be great. The obvious thing for a space station is to use aeroponics, where the roots of the plants are suspended in an atmosphere of water vapour plus nutrients. This uses almost no water at all.
The air in the ISS needs to be kept very dry to prevent microbial growth which is a problem in space stations, microbial films, for long duration multi year stations like MIR or the ISS. But that just means that the atmosphere of the growing chamber would need to be isolated from the rest of the ISS.
Indeed you could condense the water that forms inside any container with growing plants (due to plant transpiration) and use that to supply water back to the ISS, so it could help with purification of the water.
The power requirements for supplying artificial light for the algae is within the capabilities of the ISS. It was 48 kW for xenon lights for green algae for six crew - that's a total of 8 m2 of algae per crew member and 200 - 300 watts illumination per square meter. But with modern LED technology that could be reduced to a tenth or less.
For instance, just checking commercially available high efficiency grow lights available to aeroponic / hydroponic growers in 2015: this High Efficiency Green Energy Full Spectrum SMD LED Plant Grow Light for Indoor Gardening, Aeroponic and Hydroponics - uses 20 watts of power to illuminate 0.2 square meters. It is recommended for crops that require bright sunlight such as lettuces here: Top 10 Best LED Grow Lights: The Heavy Power List. That would be 800 watts for 8 square meters, or 2.4 kilowatts for the light needed for algae for a crew of six. Or if they grow all their own food, three times that, 7.2 kW.
The current total power supply for the ISS is 120 kW International Space Station (ISS) power system
So in principle they could generate all their oxygen, it would seem. The main difficulty there would seem to be space rather than power supply.
Or you could use natural sunlight and go all the way and have an "artificial greenhouse" in space. If you can do it in the vacuum conditions of the Moon or on Mars, you can surely also do it as a space module, and it is a natural place to test the technology first. You'd probably have it at a lower pressure than the ISS as plants can make do with much lower atmospheric pressure than humans which could simplify the technology a bit. For instance in this NASA challenge they suggest a quarter of of Earth normal for a greenhouse for Mars or in Space. Deployable Greenhouse
If this was their priority, then either they could get all their oxygen this way - or else - they would turn up some issue with this approach. If we have a future space station focused on human factors and preparation for long term interplanetary flight and missions on the Moon and around other planets without resupply from Earth, this would surely be a priority.
For more on this, see, Robert Walker's answer to Why can't plants be used in the ISS to take in carbon dioxide and give out oxygen so that oxygen content inside the ISS remains almost constant instead of taking air from earth to fill in the tanks of the ISS?
And for the methods currently used to create oxygen on the ISS: Robert Frost's answer to How is oxygen generated on the ISS?
See also my Science20 blog post: Lettuces Now, What Next - Could Astronauts Get All Their Oxygen And Food From Algae Or Plants?
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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