NASA brought the big gun to bear on our lost comet. On December 18, the Hubble Space Telescope slewed to Comet ISON’s expected position and found nothing down to magnitude 25. That’s 100 million times fainter than the faintest star visible with the naked eye.
According to astronomer Hal Weaver, who planned the ISON picture session, that implies that remaining fragments would have to be smaller than about 500 feet (160 meters) in diameter.
Nothing is visible in any of the images in the photo panel above except the trails of stars and galaxies from the time exposures, reflections and the occasional zap of a cosmic ray. After Comet ISON broke apart under the searing heat of the sun and spread into a widening cloud of dust, there was a possibility that its remains would follow a slightly different orbit than the original predicted one. To make sure he was covered, Weaver photographed two possible comet locations, stacking several exposures together to enhance even the faintest objects.
“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”, writes Zolt Lavay, author of the ISONblog at the Hubble site.
Again, nothing shows up in these either. There’s probably something left of the comet, but the pieces are too small for even Hubble to see. Meanwhile, no observations of ISON from the ground have been made for nearly two weeks. Even the few reports from the beginning of the month, when the comet was presumably brighter, were mostly tentative.
What once compelled us to rise before dawn for a glimpse of one of nature’s most fickle yet beautiful creations has dissipated to dust. There’s speculation that a portion of its shards may return as an invisible meteor shower in mid-January, but don’t count on it. Predicting this comet’s near-term future is like guessing what the stock market will do next.