Earth's last battle will be fought in space.
Why it is hard sci-fi
A 1972 film about a botanical freighter in deep space carrying the last forests of Earth in pressurised geodesic domes. The crew are ordered to jettison and destroy the domes, and a single botanist refuses. The film commits to the working details of running an enclosed ecosystem with limited light and a small maintenance crew, three small drones called Huey, Dewey, and Louie. The drones do real work.
Science inside it
Closed-system ecology, light-budget management for plant growth, autonomous maintenance robotics, and the long-term operational realities of a multi-year deep-space cargo run. The films failure mode is the clearest one. Once the freighter is past Saturn, sunlight no longer drives photosynthesis, and the botanist has to install supplemental lighting himself.