Each of these images was captured from a different perspective by one of STEREO spacecraft on Oct. 14, 2012. The image on the left, STEREO-B, shows a dark vertical line slightly to the upper left of center. Only by looking at the image on the right, captured by STEREO-A from a different direction, is this feature revealed to be a giant prominence of solar material bursting through the Sun's atmosphere. On the evening of October 25, 2006, the twin STEREO spacecraft launched into space to enter fairly simple orbits: both circle the Sun like Earth does, with STEREO-A traveling in a slightly smaller and therefore faster orbit, and STEREO-B traveling in a larger and slower orbit. Those simple orbits, by design, have resulted in interesting geometry. As one spacecraft gained an increasing lead over Earth, the other trailed further and further behind. In February of 2011, each STEREO spacecraft was situated on opposite sides of the Sun, and on Sep. 1, 2012, the two spacecraft and Earth were almost exactly equidistant, each with a direct view of a different third of the Sun.
By offering such unique viewpoints, STEREO has offered scientists the ability to see all sides of the Sun simultaneously for the first time in history, enhanced with a view from Earth's side by NASA's Solar Dynamics Observatory (SDO). In addition to giving researchers a view of active regions before they even come over the horizon, combining two views is crucial for three-dimensional observations of the giant filaments that dance off the sun's surface or the massive eruptions of solar material known as coronal mass ejections (CMEs). Examine the images, taken almost simultaneously, to see how a feature on the Sun can look dramatically different from two perspectives.
