Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2003 December 23 Comet Encke Returns Credit & Copyright: Michael Holloway Explanation: It's back. Every 3.3 years, Comet Encke swoops back into our inner Solar System. First officially discovered in 1786, Comet Encke is on its 59 th documented return, making it one of the best-studied comets on the sky. Mysteriously, Comet Encke should have been discovered millennia earlier, since it likely became bright enough to see unaided...