Images of Comet Siding Spring taken by Nasa's Mars Reconnaissance Orbiter on 19 October, 2014 during the Mars flyby show that the comet is probably less than half a kilometre wide.
Telescopic estimates had put the chunk of ice and dust at a kilometre wide.
The composite picture shows two versions of each of two of the best High Resolution Imaging Science Experiment (HiRISE) images of the comet.
At the top are images with the full dynamic range, showing the nucleus and bright coma near the nucleus.
Shown at the bottom are versions where the fainter outer coma is brightened, saturating the inner region. The images at left and right were taken about nine minutes apart.
Taken from a distance of about 138,000 kilometres and with a scale of about 138 metres per pixel, the images captured using the HiRISE camera have produced the first resolved imaging of the nucleus of a long-period comet from the Oort Cloud at the outer fringe of the solar system.
These closest-approach images were made possible due to very precise pointing and slewing of the Mars Reconnaissance Orbiter by engineers at Lockheed Martin in Denver, based on comet position calculations by engineers at Nasa's Jet Propulsion Laboratory, Pasadena, California.
HiRISE had acquired three images of the comet 12 days before the closest approach, when it was barely detectable. These early images indicated the comet was not exactly at its predicted location.
The new viewing angle on the comet helped update its predicted location and timing at closest approach, allowing for the HiRISE images which would otherwise have not been possible, says a Nasa release.