A ”once in a lifetime” comet is expected to light up the night sky as it passes by Earth later this week. Comet Tsuchinshan-ATLAS will be bright enough to view by the naked eye for most of October. Here’s how to make sure you don’t miss the rare sighting. What is Comet Tsuchinshan-ATLAS? [time-brightcove not-tgx=”true”] NASA defines comets as “cosmic snowballs” made of debris from the formation of the solar system.