Five images of comet C/2013 A1 Siding Spring taken within a 35-minute period as it passed near Mars on Oct. 19, 2014, provide information about the size of the comet's nucleus. These observations by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter suggest that the nucleus is smaller than 1.2 miles (2 kilometers) across.