Messier 102

Messier 102 (M102) is an entry in the Messier Catalogue whose exact identity remains uncertain. The object was originally observed by Pierre Méchain, but two years later he withdrew the discovery, stating it was simply a mistaken repeat observation of Messier 101. More recent historical analysis suggests that M102 is most likely the galaxy NGC 5866, though several other galaxies have also been proposed as candidates. NASA officially treats M102 as identical to NGC 5866.

The Messier Catalogue was created by Charles Messier between 1771 and 1781 as a list of objects that were not comets, since he was primarily searching for comets and found these other objects a nuisance. Méchain assisted Messier in compiling the catalogue. M102 was observed by Méchain in late March or early April 1781 and was added to the final published version of the catalogue that same year. However, Messier did not include coordinates for M102, which led to confusion about which object was meant. His description read: "Nebula between the stars Omicron Boötis and Iota Draconis: it is very faint; near it is a star of 6th magnitude."

In 1783, Méchain wrote to J. Bernoulli retracting the discovery, explaining that M101 and M102 were actually the same nebula, mistakenly recorded as two separate objects due to an error in the sky charts. This letter was later published twice: first in French in the Memoirs of the Berlin Academy for 1782, and then in German translation by Johann Elert Bode in the Berliner Astronomisches Jahrbuch for 1786.

Several galaxies have been proposed as the true M102. Méchain himself claimed it was a duplicate of M101 (the Pinwheel Galaxy, NGC 5457), a face-on spiral in Ursa Major. However, NGC 5866—a lenticular galaxy in Draco, also known as the Spindle Galaxy—closely matches both Méchain’s description and the position Messier noted in his personal handwritten list. Other nearby galaxies, such as NGC 5879, NGC 5907, NGC 5905, and NGC 5908, have also been suggested, but none are as bright or have as high a surface brightness as NGC 5866, making them less likely candidates. NGC 5928, a 14th-magnitude galaxy, was proposed by J. L. E. Dreyer on the possibility that ι Serpentis was mistaken for ι Draconis, but it is too faint to have been observed by Messier or Méchain, whose faintest catalogue objects are around 10th magnitude.