The first of 2013's two possible Great Comets will reach its peak two months from now.
C/2011 L4 (PANSTARRS) is still far enough away that it could be either a treat or a disappointment. It will make its closest approach to the Earth March 5. Perihelion (its closest approach to the Sun) will be on March 7. Current estimates are that it will be somewhere between magnitude 3 (not even naked eye) and magnitude -0.5- brighter than Comet Hyukatake was in 1996.
Comet ISON, which will be at its best in late in November, may well blow PanSTARRS away, though. A sun-grazer which may boast one of the longest dust tails on record, there is an excellent chance that ISON may be a daylight comet. Yes, that's right: like Comet McNaught in 2006-2007 (which put on its best show in the southern hemisphere), ISON may well be visible in the daytime. Estimates are that ISON may reach somewhere between magnitudes -6 and -13.1. Comet Hale-Bopp in 1997 had an apparent magnitude of -10.5. But there's a catch. Well, two of them, actually. First, ISON will best be viewed after perihelion, and sun-grazers which come as close to our local star as ISON will often break up. And second, its time as a daylight comet may be brief- and happen when it is so close to the sun that it may be hard to look at. ISON reaches perihelion on November 28. at which time we all need to say a prayer and hold our breath that it emerges from its encounter with Old Sol in one piece.
At its closest approach Comet PanSTARRS will be 1.10 AU from Earth (1.1 times Earth's distance from the Sun). ISON will actually be a "near miss" for us earthlings, coming within a mere two million miles of us. . It has been compared to the Great Comet of 1680 (above, right), whose orbit was similar.