Visualizations by
Tom Bridgman
Released on August 22, 2023
The Solar Terrestrial Relations Observatory (STEREO) mission was launched on October 25, 2006, with the purpose of tracing the flow of energy and matter from the Sun to Earth. The STEREO mission began with two spacecraft: STEREO-A and STEREO-B. Each was launched into Sun-orbiting trajectories - STEREO-A moving ahead of Earth, and STEREO-B moving behind Earth (STEREO's Routes to Solar Orbits).
In mid-August 2023, the still-operational STEREO-A (STEREO-B went offline in October 2014) will pass Earth for the first time since its launch 17 years ago. Like race cars driving different speeds around a circular track, STEREO-A is traveling slightly faster than Earth around the Sun. After launch, STEREO-A pulled ahead of Earth and extended its lead a little bit more with each orbit. Now, STEREO-A’s lead is so great that it is catching up to Earth from behind and is about to “lap” Earth, having completed 18 circuits around the Sun while Earth completed just 17.
The STEREO mission has accomplished a number of scientific feats, including the following: