HiRISE … successfully imaged the comet at very high resolution. But the photos don't show as many pixels across the comet as predicted, because the comet turned out to be smaller than predicted. Although the comet's small size is a little disappointing for the images, it's actually quite an exciting scientific result, probably. The size of unresolved solar system objects is estimated by taking its absolute magnitude (which can be measured reasonably precisely) and assuming an albedo, or reflectiveness, of its surface. All the comets we've ever seen up close are exceedingly dark, with albedos around 5 percent, give or take a couple percent. The fact that Siding Spring turned out to be smaller than predicted means we guessed its albedo wrong. Its surface is more reflective than we predicted. Which is wonderful, because it's consistent with the hypothesis that comets are usually dark because the Sun has baked away brighter ices, and that they start out their cometary lives much brighter. So we may have just learned something about the nature of bodies that are too far away from the Sun for us to see. (I am hedging all of my language here because I would like to see a bright comet nucleus albedo confirmed by more quantitative analysis and, ideally, evidence from another one of the spacecraft.)

I find the HiRISE images to be a little bit difficult to interpret. One important piece of information is that the phase angle of the observation is about 110 degrees, so we're seeing the nucleus of the comet in a crescent phase. That is, most of the nucleus of the comet that is in the field of view is actually the night side. The bright blob is not the nucleus; the nucleus is about a quarter of the bright blob, and about three quarters dark pixels next to the bright blob. The sunlit crescent of the comet is hard to differentiate from the brightest inner part of the coma.