These images were taken of comet C/2013 A1 Siding Spring by NASA’s Mars Reconnaissance Orbiter on Oct. 19, 2014, during the comet’s close flyby of Mars and the spacecraft. Comet Siding Spring is on its first trip this close to the Sun from the Oort Cloud at the outer fringe of the solar system. This is the first resolved imaging of the nucleus of a long-period comet.
The High Resolution Imaging Science Experiment (HiRISE) camera on Mars Reconnaissance Orbiter acquired images of this comet from a minimum distance of about 86,000 miles (138,000 kilometers), yielding a scale of about 150 yards (138 meters) per pixel. Telescopic observers had modeled the size of the nucleus as about half a mile, or one kilometer, wide. However, the best HiRISE images show only two to three pixels across the brightest feature, probably the nucleus, suggesting a size less than half that estimate. [More at link]