Messier 102 (also known as M102) is a galaxy listed in the Messier Catalogue that cannot be unambiguously identified. Its original discoverer Pierre Méchain retracted his discovery two years after publication and said that it was a duplicate observation of Messier 101. Later historical evidence favors that M102 is actually the galaxy NGC 5866, although other galaxies have been suggested as possible identities. The National Aeronautics and Space Administration (NASA) considers it to be the same as NGC 5866. History The Messier Catalogue was a list of astronomical objects compiled by Charles Messier between 1771 and 1781, in which he briefly described each object and provided their coordinates on the sky. Because Messier was only interested in finding comets, he created this list of non-comet objects that frustrated his hunt for them. Astronomer Pierre Méchain collaborated with Messier on the compilation of his list. M102 was observed by Méchain in late March or early April 1781 and was added by Messier to the final version of his catalogue published in 1781. However, Messier didn't include the coordinates of M102 on his catalogue, leading to confusion about the exact object they observed. His description of the object was the following: In 1783, Méchain retracted his discovery in a letter written to J. Bernoulli and claimed that M102 was actually an accidental duplication of M101 in the catalog. This letter was later published twice: First in original French in the Memoirs of the Berlin Academy for 1782, and second in German translation and somewhat rearranged by Johann Elert Bode in the Berliner Astronomisches Jahrbuch for 1786. Candidate corresponding objects Since the publication of the Messier Catalogue, a number of galaxies have been identified by different historians, professional astronomers, and amateur astronomers as corresponding to M102. Messier 101 Messier 101 (also known as the Pinwheel Galaxy or NGC 5457) is a face-on spiral galaxy in the constellation Ursa Major. In a letter written in 1783 to J. Bernoulli, Pierre Méchain (who had shared information about his discoveries with Messier) claimed that M102 was actually an accidental duplication of M101 in the catalog. This letter was later published twice: First in original French in the Memoirs of the Berlin Academy for 1782, and second in German translation and somewhat rearranged by Johann Elert Bode in the Berliner Astronomisches Jahrbuch for 1786. NGC 5866 NGC 5866 (one of two galaxies commonly called the Spindle Galaxy)