These images were taken of Comet C/2013 A1 Siding Spring by the Mars Reconnaissance Orbiter on 19 October during the comet’s close flyby of Mars. The comet is on its first trip this close to the sun from the Oort Cloud at the outer fringe of the Solar System. These are the first resolved images of the nucleus of a long-period comet.
The images of the comet were acquired at a range of about 138,000 kilometers. The scale is roughly 138 meters per pixel. Telescopic observations had suggested that the size of the nucleus was about 1 km wide. However, the best Mars Reconnaissance Orbiter images are two to three pixels across the brightest feature, suggesting that the size of the nucleus is less than half the earlier estimate.
This composite image shows two versions of two of the best images take by the HiRISE camera on the Mars Reconnaissance Orbiter. At the top are images with the full dynamic range, showing the nucleus and bright coma near the nucleus. At the bottom are versions where the fainter outer coma is brightened, saturating the inner region. The left and right images were taken about nine minutes apart.
Image Credit: NASA