Yesterday, the much-anticipated comet ISON made its closest pass by Mars. Despite the government shutdown, all NASA spacecraft are still operating normally, and Mars Reconnaissance Orbiter, Curiosity, and Opportunity have all attempted imaging over the last several days. Mars Reconnaissance Orbiter's HiRISE camera is the first to achieve a positive detection of the somewhat-fainter-than-expected comet in its photos:
The caption posted at the HiRISE website, written by Alan Delamere and Alfred McEwen, explains further:
These images show a 256 x 256 pixel patch of sky at the range to the comet of 8 million miles and when the solar phase angle is 47 degrees. Three more observations of ISON are planned for 1 and 2 October as the comet moves through closest approach to Mars at 7 million miles, but with less illumination as seen from Mars.
Based on preliminary analysis of the data, the comet appears to be at the low end of the range of brightness predictions for the observation. As a result, the image isn't visually pleasing but low coma activity is best for constraining the size of the nucleus. This image has a scale of approximately 8 miles (13.3 km) per pixel, larger than the comet, but the size of the nucleus can be estimated based on the typical brightness of other comet nuclei. The comet, like Mars, is currently 241 million kilometers from the Sun. As the comet gets closer to the sun, its brightness will increase to Earth-based observers and the comet may also become intrinsically brighter as the stronger sunlight volatilizes the comet's ices.
The images attempted by Opportunity and Curiosity are released in their raw form on the mission websites, but if the comet is visible in those images, it is lost in the compression artifacts. (The usual gang of image processors at unmannedspaceflight.com has been working to bring out any signal of the comet from the raw images, so far without success.)