The twin STEREO spacecraft confirmed their reality in February 2009 when sunspot 11012 unexpectedly erupted. The blast hurled a billion-ton cloud of gas (a coronal mass ejection, or CME) into space and sent a tsunami racing along the sun's surface. STEREO recorded the wave from two positions separated by 90 degrees, giving researchers an unprecedented view of the event.

The twin STEREO spacecraft (called "Behind" and "Ahead" denoting their relative positions in space), now almost 120 degrees apart, captured this large and dramatic prominence eruption over about a 30-hour period between Sept. 26-27, 2009. Prominences, called filaments when they are viewed against the surface of the Sun, are clouds of cooler gas suspended above the Sun’s surface by magnetic forces. This erupting prominence was large enough that both spacecraft were able to observe it for hours on end, one of the first times that has occurred.