Twin satellites of the Solar Terrestrial Relations Observatory (STEREO) mission, launched by NASA on 25 October, promise to provide scientists with three-dimensional views of the Sun and solar wind. The satellites also will help with exploring the origin, evolution, and effects of coronal mass ejections (CMEs).CMEs can hurtle billions of tons of solar plasma into interplanetary space, which can disrupt satellite systems, radio communications, and power grids. CMEs also can pose hazards to space-faring astronauts. STEREO'S satellites will monitor CMEs in three dimensions, with one spacecraft flying behind the Earth at a slightly wider orbit and another flying ahead at a slightly narrower orbit, allowing for depth perception. Instrument packages mounted on each satellite also will observe properties of solar particles and track radio disturbances that move from the Sun to the Earth. Data will be used to help predict adverse space weather events.