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Fish may be key to sustaining people on a long space mission. A well-stocked aquarium could provide enough extra nutrients to feed the astronauts鈥� crops, while also offering a source of protein.

The ideal spaceship is a closed system, where those on board can grow everything they need to survive and all waste is processed for reuse. On the International Space Station, for example, urine is collected and processed so that it is safe to drink.

For growing crops, one approach would be to use human waste as a plant feed. However, it turns out this alone is not enough, as plants need more calcium, magnesium, phosphorus and sulphur than it contains. Carrying extra nutrients on board would take up space and they could run out if the journey took longer than expected.

A team from the Krasnoyarsk Scientific Centre in Russia discovered that travelling with crucian carp could complete the loop. They found that the left-over parts of a carp dinner, such as the bones, scales and internal organs, would be enough to make a nutritionally complete plant feed.

To make the mix, space travellers would first have to create hydrogen peroxide from water and oxygen. This would be placed in an alternating current alongside human waste and the leftover fish parts. Around three hours later, the hydrogen peroxide would decompose into water and oxygen.

The results would then be mixed with nitric acid, which could also be synthesised from urine, to create the plant food.

To test the recipe, the team fed this mix to wheat plants on Earth. They found that after 180 days, the crop had grown to a similar size as would be expected with regular plant feed.

鈥淟iving and working in聽space聽will require sophisticated recycling of limited resources such as bio waste鈥� says Dorit Donoviel at the Baylor College of Medicine in Texas.

In practical terms, the idea of taking fish into space is not out of the question. 鈥淕etting fish into space and tasking astronauts with caring for them is incredibly feasible; in fact, it鈥檚 already being done鈥� says Anicca Harriot, a biologist at the University of Maryland.

Fish in space

In the past, zebra fish have flown on the International Space Station, in experiments looking at muscle biology. However, zebra fish are much smaller than the crucian carp used in the study.

Additionally, making sure the water used in the aquarium is regularly cleaned聽re-circulated, won鈥檛 be easy in on a long space flight. 鈥淭here will be major challenges with setting up the flow in the tanks聽due to the near-zero gravity environment鈥� says Ivaylo Nedyalkov, at the University of New Hampshire.

The team have not yet calculated how many fish would need to be taken on a space trip, nor how often they should be eaten. It鈥檚 also not clear if taking them on board would ultimately be worth the hassle.

Looking after fish, processing their waste and feeding it to plants might take more effort than carrying additional plant food, says Harriot.

On Earth, attempts have been made to create closed-loop systems using a technique called aquaponics. In aquaponics, fish are farmed in tanks while plants are fed their waste as fertiliser.