STEREO stands for the Solar TErrestrial RElations Observatory. It was built by NASA and launched on 25th October 2007.
STEREO has two satellites, also know as probes, which work together to look at the Sun. They are are in an orbit around the Sun (heliocentric), in fact almost the same orbit as the Earth, but one probe is ahead of the Earth (STEREO-A) and one is behind the Earth (STEREO-B). Two big eyes looking at the Sun! The two STEREO probes have been slowly separating over most of the mission, so that by January 2008 they were 44 degrees apart.
They will continue to separate as the mission proceeds. This means that we can observe the Sun and CMEs from 2 different perspectives in space. From these observations, it is possible to make 3-D images and movies.
Below is a movie which shows the orbits of the STEREO probes. The green dot is the Earth, the red orbit, marked A, is STEREO-A (Ahead) and the blue orbit, marked B, is STEREO-B (Behind). The yellow dot is the Sun. The orbits of Mercury, Venus and Mars are also shown.
Each STEREO probe has 4 instrument packages on board:
SECCHI (Sun Earth Connection Coronal and Heliospheric Imager)
PLASTIC (PLAsma and Supra-Thermal Ion Composition)
SWAVES (STEREO WAVES)
IMPACT (In-situ Measurements of Particles and CME Transients)
SECCHI has four instruments: an extreme ultraviolet imager (EUVI), two white-light coronagraphs and a heliospheric imager (HI). These instruments study the 3-D evolution of coronal mass ejections from birth at the Sun's surface, through the corona and interplanetary medium, and to their eventual possible impact at Earth. The EUVI instruments are similar to the SOHO/EIT instrument. SoHO also has a coronagraph, called LASCO. The UK (RAL) leads the HI instrument.
Movie of a CME erupting taken with the STEREO coronagraph
Movie of the hot coronal loops from the SECCHI EUVI Telescope