ESA’s first planetary defence spacecraft has departed planet Earth. The Hera mission is headed to a unique target among the more than 1.3 million known asteroids in our Solar System – the only body to have had its orbit shifted by human action – to solve lingering mysteries associated with its deflection.By sharpening scientific understanding of the ‘kinetic impact’ technique of asteroid deflection, Hera aims to make Earth safer. The mission is part of a broader ambition to turn terrestrial asteroid impacts into a fully avoidable class of natural disaster.