The testbed balloon was intended to be an engineers’ model: a device for testing and observing how a full-size balloon might behave in the world. Without it, the engineers have been using computer models to explore how strong the pipes and pumps would have to be and to see how a balloon floating upwards to 20 km would behave. They are still asking real-world questions but doing so with virtual tools. The Stratospheric Particle Injection for Climate Engineering (SPICE) project involves making models of various kinds – mental models, computer models and a range of laboratory experiments. Within the team there is an ecosystem scientist working with a plant model to study the effects of diffused sunlight on leaves, chemists modelling the interactions between various particles and the atmosphere, and physicists working on climate modelling. With its blend of understanding and intervening, geoengineering research invites a new set of conversations between models and the real world.