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STEREO (for Solar Terrestrial Relations Observatory) is two space probes in solar orbit able to take stereo images of solar phenomena, being in different directions from the Sun. They were launched in 2006. Contact with one the two (STEREO-B for STEREO Behind) was lost in 2014 and has been out of use since, but the other (STEREO-A for STEREO Ahead) is still collecting data as of 2025. STEREO's observations also include some stars, discovering over 100 eclipsing binaries. Instruments included:
SECCHI included:
EUVI, COR1, and COR2 together were termed SCIP, for Sun centered instrument package.
STEREO-A was placed in an orbit slightly smaller then Earth's, such that over time, it slowly shifted further ahead of Earth. STEREO-B was analogously placed in an orbit slightly larger than Earth's, to fall behind. Given this continuing change, the angle between their directions from the Sun grew with time. STEREO-A, which is still active, has progressed so far ahead of Earth, that it lapped Earth, and for a while in 2023 it was roughly in a line between Earth and the Sun, located somewhat toward the Sun from Earth L1. Since that time, it is again orbiting somewhat ahead of Earth, again moving further ahead of Earth with time.
An unrelated series of coordinated stereo solar observations termed STEREO-1, STEREO-2, etc., were carried out in the 1970s, consisting of French observations from the ground, and observations by Soviet Union Mars orbiters, including Mars 3 and Mars 5.