This animation includes images captured by STEREO Ahead on November 28 and 29. STEREO Ahead precedes Earth in its orbit. It happened to have a view on the comet almost directly perpendicular to the plane of the comet's orbit, the only spacecraft with such a perspective. As the comet approached the Sun, it was slightly in front of the Sun as seen from STEREO, so its tail was bright, its dust scattering light toward the spacecraft. As it departed, it was behind the Sun, at a significantly lower phase angle, so appeared dimmer. As time went on, the viewing geometry grew less favorable as the comet drew farther from the Sun, so it faded rapidly.
NASA / STEREO / Emily Lakdawalla
Here is the view from STEREO B.
This animation includes images captured by STEREO Behind on November 27, 28, and 29. STEREO Behind lags behind Earth in its orbit. As the comet approached the Sun, it was moving almost directly toward STEREO, so it appeared to be moving very slowly and its tail is relatively dim (compared to views from other spacecraft) because we are seeing it at a low phase angle, in reflected sunlight. After perihelion, the comet's direction of motion was at least initially almost entirely in the plane of the photo, so it appeared to be moving very fast, and the comet was also between the Sun and the camera, so its coma and tail were extremely bright due to forward scattering of light by the dust. As time went on, the viewing geometry grew less favorable as the comet drew farther from the Sun, so it faded rapidly.