It's common for a comet's coma to be almost as large as the Sun -- or even larger than the Sun.
However, a comet's coma is extremely thin and tenuous, and most of that coma is a virtual vacuum. There is no way a comet's coma would block the Sun -- not even a coma larger than the Sun.
edit on 9/26/2011 by Soylent Green Is People because: grammar
