
We don't know how bright it will get. Astronomers generally just don't know as much about comets as we would like. The comet has undergone outbursts of brightening and dimming, while generally tending to get brighter as it enters in to the inner solar system ISON may become entirely disrupted or get brighter and brighter - the comet of the century. The reason for all this uncertainty is that comets and this object in particular are not well studied. But also it is that comets are not dense rocks, but rather they are loose aggregations of dust, rubble, and ices. Tidal forces and heating of the ices can literally unbind entire comets. I haven't been able to see it for myself yet, it hasn't been quite visible because it rises so late I think. Currently, if you want to see comet ISON from North America look east right before dawn as it passes Mercury. Read the rest of my post on comet ISON over at Common Observer...