Comet C/2011 L4 PanSTARRS captured in the low resolution beacon images for the STEREO Behind H1 imager.
Still waiting to download the high resolution images, but this expanded set of beacon images you can distinguish between the image artefact spike and the dust tail (the spike is the straightish line, the broad fan of the dust tail sweeps off to the right, compare to yesterdays set).
Also in the image is Mercury (up close to the Sun) and Earth (the bright dot centre right).
Here's another animated GIF. There is a better one at the STEREO site.