The filmmakers do not reveal how the comet was filmed other than to say it was a practical effect. See more

Goofs

(at around 7 mins) Right after Hugh asks everyone at the table about their phone reception but before he starts talking about the comet from the 1930s there is a brief shot of Lee saying something. A cameraman can be seen crouched behind the sofa in the living room to the left of Lee's head, and he stays there for quite awhile. See more