  This chapter describes the SMILE (Solar wind Magnetosphere Ionosphere Link
Explorer) mission, currently under development as a joint project of the
European Space Agency and the Chinese Academy of Sciences. SMILE aims to study
the solar wind coupling with the terrestrial magnetosphere in a very novel and
global way, by imaging the Earth's magnetosheath and cusps in X-rays emitted
when high charge-state solar wind ions exchange charges with exospheric
neutrals. SMILE combines this with simultaneous UV imaging of the northern
aurora and in-situ plasma and magnetic field measurements of the magnetosheath
and solar wind while flying in a highly elliptical northern polar orbit. In
this chapter, after a brief introduction to the science that SMILE is targeted
to investigate, the payload and the spacecraft, and progress in their
development, are described. The mission operations in space, the ground segment
and the scientific preparations that will ensure the optimal exploitation of
SMILE measurements are also presented. The international space plasma and
planetary communities are looking forward to the step change that SMILE will
provide by making visible our invisible terrestrial magnetosphere.
