A pair of NASA spacecraft launched in 2006 October to study coronal mass ejections (CMEs) and the solar wind. One of the pair, STEREO A (Ahead), is in an orbit slightly smaller than that of the Earth and moves ahead of the Earth by just over 20° per year, while the other, STEREO B (Behind), is in a slightly larger orbit than the Earth’s and falls behind by a similar amount. Together they provide a three-dimensional view of the Sun and its activity. A set of telescopes called SECCHI on each craft consists of two white-light coronagraphs, Cor1 and Cor2, to observe the inner and outer corona out to 15 solar radii; an Extreme Ultraviolet Imager (EUVI) to observe the chromosphere and inner corona at four different wavelengths; and a Heliospheric Imager (HI) which observes CMEs out to more than 300 solar radii, i.e. beyond Earth’s distance. In addition, each craft carries an interplanetary radio burst tracker (SWAVES) to trace radio disturbances in the solar wind, and two experiments to study energetic particles from the Sun and the interplanetary magnetic field (IMPACT and PLASTIC).

http://stereo.gsfc.nasa.gov/ Official mission website.

http://www.nasa.gov/mission_pages/stereo/main/ Mission page at NASA.