C/2012 S1, Comet ISON, is intriguing because it has never approached the sun before and that is scientifically terrific.
Comet ISON began in the Oort cloud almost a light year away and has traveled for over a million years. Unlike more famous comets, Halley's as an example, it has never come this way before - and that means it is still made of pristine matter from the earliest days of the solar system's formation. Its top layers haven't been lost by a trip near the sun.
To study it as it approached Sol, a vast fleet of solar observation experiments have been watching. Would it break up or slingshot?