This article analyses the origins and development of the relationship between the European Space Agency (ESA) and the EU space program as separate, but related processes of European integration. For over fifty years the ESA has been separate from the EU institutionally despite having nearly all of the same Member States, and its role in uniting European countries in a common outer space endeavour has been significant. The ESA embraces the motto ‘United Space in Europe’. The EU’s interest in space has only emerged in the last twenty years, but has grown rapidly. As such, space has become a case of informal (i.e., outside of EU structures) differentiated integration. While it may be easy to assume that the ESA is outside of EU structures because Member States have wanted to keep space policy intergovernmental, the author argues that fundamentally different ideas about the role and use of space between the EU and the ESA have been the main, underlying driver. A long-standing spaceflight idea – mainly, the peaceful use of space for all of humankind – is embedded in the ESA’s DNA, while this is not the case for the EU (This article is an output of the EUFLEX project, which has been funded by the Research Council of Norway (project number 287131)).