On December 18, 2013, the Hubble Space Telescope tried to observe Comet ISON one last time. As many people expected, there is no evidence of Comet ISON in these images.
The above image shows a composite of Hubble's observations. If there had been a feature that appeared in all of the images at the same place, that would have been very strong evidence of the comet. But all of the features in these images are not repeated at the same place from image to image. Nothing that appears in the images looks like a piece of a comet. Mostly what shows up are stars that are moving across the frame as the telescope tracks the comet's expected position, and a few galaxies, also trailed.
Other features are well-known artifacts produced within the camera, reflected and scattered light from brighter stars, and cosmic rays, which cause bright streaks across the images. Each of the four panels is a combination of two separate exposures. Had Comet ISON actually been present, it would have shown up in the same location in two or more of these frames.
These two images are composites of several exposures each, with Hubble pointing at two different positions. The images have been combined so that features not at the same place in the various images are suppressed. Any comet fragments would show up more clearly in this composite, though stars still show up as faint streaks.
There was some uncertainty in where Hubble should point to recover the comet because no observations showed the comet since soon after perihelion. According to astronomer Hal Weaver, who devised Hubble's strategy, there were two likely locations of the comet, predictions based on previous positions measured when the comet was still visible. Dr. Weaver also estimates that the faintest objects Hubble could see in these images would be about 25th magnitude. This means that Hubble could have seen comet fragments larger than about 500 feet (160 meters) in diameter.
We can't completely rule out the possibility that something is left of the comet. After all, it was seen after its passage close to the Sun, but disappeared not long after. This material would still exist, but is likely very diffuse gas, dust, and very small pieces spread over an extremely large area.