Complex sentence: Having completed its flyby of Pluto, the "New Horizons" spacecraft has been maneuvered for a flyby of, which will occur on 31 December 2018 or 1 January 2019, at which point it will be 43.4 AU from the Sun in the constellation Sagittarius.
Split into simpler sentences: Having completed its flyby of Pluto, the "New Horizons" spacecraft has been maneuvered for a flyby of. Closest approach will occur at, at which point it will be from the Sun in the constellation Sagittarius.