The "Sky Spirits", while apparently capable of warp travel, had to make use of a generational ship system in order to visit Earth from their home world in the Delta Quadrant, due to the sheer distances involved. It took more than two of their generations just to travel one way. The species would do this several times over thousands of years. (VOY: "Tattoo") A generational starship also appears in the Pocket DS9 novel Objective: Bajor.