The Aquarius Formation was a name given by Federation scientists to a large collection of astronomical objects which were located about a 1,000 light years from Federation space.
In the year 2273, Admiral Heihachiro Nogura assigned the USS Enterprise to voyage to the Formation, and chart the unexplored territories leading up to the formation. When the Enterprise returned to Federation space in 2280, they had charted hundreds of planets and dozens of stellar phenomena. (TOS - Crucible novels: Provenance of Shadows, The Fire and the Rose)