This isn't the prettiest image we've ever seen — but that doesn't make it any less profound. It's a glimpse of the Earth, the PanSTARRS comet, and a developing coronal mass ejection from the Sun as seen from STEREO-B's Heliospheric Imager.
Image by NASA/GSFC/STEREO.
What's just as amazing as seeing the Earth from a distance of 100 million miles (160 million km, or 1.1 AU) is watching the Sun in such an active state; STEREO is a mere 28 million miles away from it (0.3 AU).
NASA's Earth Observatory elaborates:
The HI instrument observes the area just off the limb of the Sun (left) in order to detect the faintly visible light of the solar wind and coronal mass ejections (CMEs) as they blow out into the solar system. STEREO-B was on the far side of the Sun from the Earth, at about a 150 to 160 degree angle from the Earth-Sun line. Puffs of solar wind and from a CME are visible in a video made from the same images in March.
In the image and video, PanSTARRS and its tail are the bright, wide mass in the center of the image. A coronal mass ejection is developing on the Sun’s limb (left), and Earth is to the right of the comet. The white dots in the background are stars. The vertical lines across the image are artifacts of the imager; the light of the comet, the Earth, and some planets and stars is too bright for an instrument trying to observe the faint light of the Sun’s atmosphere.
The vertical black line that appears in the middle of the shot is an artifact of the imaging process.