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’ 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 …