As an update to last week's blog post (Martian Encounter), the first images of Comet ISON taken from Mars have been released. Click on the image accompanying this blog post to see them in high resolution. But, even then, these are the kind of images that only an astronomer can love. Perhaps some explanation will help others to appreciate them as well.
As expected, these images came from the High Resolution Imaging Science Experiment (HiRISE) camera of the Mars Reconnaissance Orbiter (MRO). The rovers Curiosity and Opportunity also took images, but no detections have been announced. The rover images are routinely posted in raw form, and a discussion thread among astronomy image enthusiasts has been examining those in detail to try to spot Comet ISON.
The MRO images show not much more than a dot, but dots in astronomy can be really important. Remember that images of planets around other stars and of distant galaxies in the farthest reaches of the universe are similarly also just dots. Not to imply that Comet ISON rises to that level of importance, but rather to exemplify that the cutting edge of science is filled with such dot-like observations.
Here, the dot of Comet ISON shows the image did not capture much of the coma, the gaseous cloud surrounding the icy and rocky nucleus. Part of the reason is the exposure time of the image, and part is that Comet ISON is not as bright as expected. When you recognize that Comet ISON, at this time, is almost 200 million miles from Earth and only 7 million miles from Mars, these close up observations provide a great constraint on the overall brightness.
The size of the dot also helps constrain the size of the comet's nucleus. The resolution of the MRO images is approximately 8 miles per pixel. The nucleus has previously been estimated at sizes smaller than that, so the true size is hidden in the pixelation of the image. However, better estimates of the size can be obtained through deeper analysis of these shots.
More observations from Mars are ongoing, though the view from Mars will see less and less reflected light as Comet ISON passes farther inside Mars' orbit. These observations confirm, augment, and refine the size and brightness estimates. Generally, there is a somewhat pessimistic feeling that Comet ISON is not going to develop into a spectacular visual comet. But unpredictability is a hallmark of comets, and there is the important unanswered question of whether it will break up or survive its passage by the Sun. We do not yet know whether ISON will end in a bang or a whimper.