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While scientists long appreciated the power and importance of coronal mass ejections (CMEs), they didn’t fully understand how they form, what their structure and sizes are, or how they evolve as they blast off the Sun and propagate through space.

Launched in 2006, the STEREO mission is addressing these unknowns and providing advance warnings for space weather forecasting. STEREO is the third mission in NASA’s Solar Terrestrial Probes program, answering fundamental questions about the nature of space and the flow of matter and energy throughout the solar system.

The mission consists of two nearly identical space-based observatories: STEREO-A and STEREO-B, both built by APL. One spacecraft is placed ahead of Earth’s orbit while the other is placed behind, allowing the probes to capture 3D images of the Sun and interplanetary space to study the origin, propagation and evolution of CMEs. This configuration also enabled investigations of the 3D structure of the magnetic fields in the corona as well as the first studies of the entire Sun, including its far side.

Communications with STEREO-B were officially lost in September 2014. STEREO-A remains operational.
 

Each STEREO spacecraft is equipped with four instrument suites, which include an extreme ultraviolet light imager of the solar disk, two white-light coronagraphs to see details around the Sun’s bright surface, two heliospheric imagers of solar-originating structures flowing through interplanetary space, multiple instruments that measure the 3D distribution of plasma in the solar wind and the characteristics of other energetic particles coming from the Sun to better under the origins of CMEs, and a radio burst tracker that follows radio disturbances from the Sun.
 

STEREO captured more than 13 million images of the Sun, providing detailed, 3D views of at least 4,500 CMEs and other solar eruptive events. STEREO-A continues to provide early warnings of solar active regions responsible for eruptive events before those regions come into Earth’s view.