In this paper, we reflect on our involvement in one of the first major research projects in the emerging area of geoengineering (the deliberate intervention in the planetary climate). The project, Stratospheric Particle Injection for Climate Engineering (SPICE), proposed an outdoor experiment that attracted substantial public scrutiny despite a strong consensus that the experiment posed no direct environmental risk. A programme of stakeholder engagement took place that sought a deep understanding of the views about the proposed experiment. The lessons from this experiment build on insights from public engagement with the biosciences and biotechnology. In particular, we see the importance of questions of context and purpose for scientific research. This has important implications for the governance of geoengineering research. Efforts to detach areas of research from public scrutiny by using thresholds, whether these are drawn at a particular level of environmental effect or at the doors of a laboratory, will encounter problems of public credibility. Geoengineering is unavoidably entangled in a political discussion that scientists should seek to understand and engage with.