As ISON gets ever closer to the sun, solar energy is heating up its nucleus, causing rapid sublimation of the ices contained within. (Sublimation occurs when an ice turns directly from a solid to gas without passing through the liquid phase - an everyday example of this would be carbon dioxide ice - also known as "dry ice" - venting gas at room temperature.) The comet, which is thought to be making its very first inner solar system appearance after originating from the Oort Cloud, will make closest approach to the sun on Nov. 28 (Thanksgiving), coming within 1.2 million kilometers from its surface.
Currently, the nucleus is retaining its structure, but that could change at any time. Should the nucleus succumb to the searing heat of the sun's corona, it could break apart and fragment.
But will Comet ISON truly become the Comet of the Century? It will have to overcome its trial by fire and swing past the far side of the sun before we find out. In short, all bets are off.
Read more about the ESO's observation of ISON, including high resolution imagery of the comet.