The High-Resolution Imaging Science Experiment (HiRISE) camera on board NASAs Mars Reconnaissance Orbiter (MRO) has become the first instrument orbiting Mars to beam back images of comet Siding Springs nucleus and coma. And by default, it has also become the first ever mission to photograph a long-period comets pristine nucleus on its first foray into the inner solar system.

Interestingly, through analysis of these first HiRISE observations, astronomers have determined that the icy nucleus at the comets core is much smaller than originally thought.

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Telescopic observers had modeled the size of the nucleus as about half a mile, or one kilometer, wide, writes a NASA Jet Propulsion Laboratory news release. 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.

The two observations showcased here are the best two from the HiRISE campaign. The top thumbnails represent images with the full dynamic range, including the nucleus and coma. Comets are composed of a central icy lump of material the nucleus and when the comet becomes heated by solar energy, ices sublime, blasting vapor and dust into space. During this process, the coma forms and the solar wind will sweep some of the vapor and dust into a tail.

The larger bottom images are overexposed photos where the coma has been brightened so astronomers can fully appreciate its extent. The nucleus in these images cannot be seen. The time between left and right images is approximately 9 minutes.

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Three days before closest approach, the MRO slewed to track the incoming comets location. However, its predicted position was slightly out, so mission scientists made some adjustments so the comet would be in HiRISEs field of view when the comet came within 85,000 miles of the Red Planets surface.

This is the first time that a fresh comet from the Oort Cloud a hypothetical region surrounding the sun approximately a light-year away containing a countless number of icy bodies that could become comets if they drop toward the sun has been observed up-close. The comets weve visited in the past with probes are short-period comets that have well-known orbits (like the famous Comet Halley). It is shear luck that Siding Spring, which was only discovered in January 2013, should be so accommodating and fly so close to a planet we just so happen to have an armada of robotic cameras ready to start observing.

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NASA Mars Orbiter Beams Back Images of Comet's Tiny Nucleus