Evidently, M102 is the last possibly "missing", or doubty, Messier object. This means that, based on the observation record of his friend, Pierre Méchain, Charles Messier included this object in his catalog without verifying before publication, and it happens that Méchain's published position is erroneous. Two opinions on the identity of this object are common:
1. M102 might be a duplication of the Spiral Galaxy M101 (NGC 5457) in Ursa Major, due to a possible error of either the catalog author Messier, or its `discoverer' Méchain, a view brought up by Méchain in a letter of May 1783.
2. Perhaps more probably (due to historical evidence), M102 may be the Lenticular Galaxy NGC 5866 in Draco, also sometimes called the Spindle Galaxy, as Messier's catalog description indicates, together with the position he added later by hand in his personal copy.
The authors of this page think that there is sufficient evidence that both Méchain and Messier have probably observed NGC 5866 in context of this catalog entry. Therefore, it should be considered to take this last missing object back into the catalog.
Please note our discussion of more details on this topic.