Messier 102 is one of the "missing" Messier objects that have not been identified unambiguously. Its original discoverer, Pierre Mechain, claimed that M 102 was a duplicate observation of M 101, in a 1783 letter to Bernoulli in Berlin. But there are historical and observational reasons to believe otherwise.
Owen Gingerich has suggested NGC 5866 as a worthy object with which to fill this blank in Messier's catalogue. This galaxy appears to closely match both Mechain's object description in the 1781 printed version of the Messier Catalog of 1781, and the object position listed by Charles Messier in his hand-written notes.
NGC 5866 is a fine, bright galaxy with an apparent magnitude of 9.9 and an angular size of 5.2' x 2.3', elongated northwest to southeast, and containing a broad bright 3.25' x 1.25' core. The galaxy is distinctively spindle-shaped. An inconspicuous stellar nucleus is just discernible at the center of the grainy envelope. A narrow dust lane cuts across the core.
Due to its appearance, NGC 5866 is often called the "Spindle Galaxy". In fact, two different galaxies are known by this moniker; the other Spindle Galaxy is NGC 3115 in Sextans.
Like NGC 3115, NGC 5866 is a lenticular galaxy (type S0) seen almost edge-on. It lies about 40 million light-years away. NGC 5866 is part of a group of galaxies (the NGC 5866 group), which also contains NGC 5907 and 5879, as well as many fainter galaxies. From the movements of the galaxies in the group, NGC 5866's mass has been estimated at 1 trillion solar masses.