NASA's STEREO-A spacecraft is monitoring Comet ISON as it approaches the sun. This image from the spacecraft's Heliospheric Imager shows, going left to right, Comet ISON, Mercury, Comet Encke and Earth late on Nov. 21, 2013. The sun sits outside of the field of view of this camera to the right. However, the sun has released a Coronal Mass Ejection, or CME, filling the right of the photo, which appears to be headed toward the comet. Credit: Karl Battams/NRL/NASA STEREO/CIOC.