It is an entirely hypothetical creation, courtesy of amateur astronomer and science writer Stuart Atkinson. While the image is based on real data and events it’s less of a prediction and “more like, ‘Wouldn’t it be cool if it looked like this!’” Atkinson wrote in an e-mail.
Officially named C/2012 S1, Comet ISON was discovered in September and is currently visible only with a powerful telescope. Right now it’s passing near Jupiter but, given its brightness at that distance, it has astronomers salivating at the prospect that it might grow into one of the brightest comets ever seen. Like all comets, ISON will sprout a massive tail as it nears the sun, potentially making it bright enough to see during the day.