For instance , comet Hale - Bopp was last seen in 2013 at magnitude 24 and continues to fade , making it invisible to all but the most powerful telescopes .
Most of these objects are comets on a calculated path and may not be directly observable .
Comet Hale - Bopp was last seen in 2013 at magnitude 24 and continues to fade , making it invisible to all but the most powerful telescopes .
Most of these objects are comets on a calculated path and may not be directly observable .