A new comet (magnitude 18.8) has been discovered beyond the orbit of Jupiter.
Comet ISON will get within 0.012 AU of the Sun by the end of November 2013 and ~0.4 AU from of Earth early in January 2014. It may reach very welcome negative magnitudes at the end of November 2013. Forecasts of comet magnitudes have been disappointing in the past. It's clear that comet magnitudes depend on the quantity of dust and ices that are ablated by the Sun's energy, and therefore nucleus size and distance from the Sun and Earth.
My question is, to what extent is a comet's magnitude dependent on the ratio of nucleus dust to ice and on the type of ices?