Swarm is ESA’s first constellation of Earth observation satellites. The three identical satellites are designed to measure precisely the magnetic signals that stem from Earth’s core, mantle, crust and oceans, as well as its ionosphere and magnetosphere. Carrying a host of sophisticated instruments, the constellation is key to measuring and separating the different sources of magnetism and to making models in unprecedented detail and accuracy. The fact that Swarm is a constellation also means that, for the first time, mantle conductivity can be mapped in 3D from space. The satellites also offer a new way of studying the effect that solar particles have close to Earth.