Later this year, the U.S.Air Force will launch the Solar Mass Ejection Imager (SMEI). SMEI is an instrument that will detect and measure coronal mass ejections (CMEs) which can cause large geomagnetic storms, a major component of space weather. CMEs are very large structures containing plasma and magnetic fields that are expelled from the Sun into the heliosphere at speeds of several hundred to over 1000 km s−1. CMEs often drive interplanetary shock waves which, upon arrival at Earth, can cause geomagnetic disturbances. There is currently no reliable way to accurately predict arrival of these disturbances at Earth or to study them in the inner heliosphere. SMEI is designed to fill this gap by detecting and tracking CMEs in interplanetary space before they reach Earth. SMEI data will be complementary to many other satellite missions and national programs, such as the current National Space Weather and International Solar Terrestrial Programs.