STEREO is a U.S. mission to observe the Sun with twin satellites launched in 2006. With support from CNES, several French laboratories helped to build STEREO’s instruments, which are still operating.

The STEREO mission (Solar TErrestrial RElations Observatory) comprises twin satellites orbiting the Sun, one ahead of the Earth (STEREO-A) and the other trailing behind it (STEREO-B). Thus offering the ability to view from 2 different vantage points in space, STEREO has acquired the first 3D images of coronal mass ejections (CMEs) and how they interact with the interplanetary medium.

The twin satellites are carrying the same 4 instruments and were launched on 25 October 2006 on the same Delta II vehicle from Cape Canaveral, Florida. After launch, they gradually moved out of Earth orbit until in 2011 they were 180° apart, making it possible to view both sides of the Sun simultaneously for the first time. They are now observing the side of the Sun not visible from Earth. A CNES antenna near Toulouse has been receiving data continuously from the 2 satellites since 2006, operating as part of the U.S. ground station network.

French research laboratories worked on some of STEREO-A and STEREO-B’s instruments with support from CNES. LESIA (Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique) was the Principal Investigator for SWAVES, while the IAS (Institut d'Astrophysique Spatial) and the Institut d’Optique Graduate School (IOGS) contributed to SECCHI. IRAP (Institut de Recherche en Astrophysique et Planétologie.) developed the detectors for the IMPACT instrument. The IAS is also involved in analysing data from the mission through a doctoral thesis co-funded by CNES.

STEREO is the third mission of the international Solar Terrestrial Probes programme (STP).