  NASA's New Horizons spacecraft will conduct a close flyby of the cold
classical Kuiper Belt Object (KBO) designated (486958) 2014 MU69 on January 1,
2019. At a heliocentric distance of 44 AU, "MU69" will be the most distant
object ever visited by a spacecraft. To enable this flyby, we have developed an
extremely high precision orbit fitting and uncertainty processing pipeline,
making maximal use of the Hubble Space Telescope's Wide Field Camera 3 (WFC3)
and pre-release versions of the ESA Gaia Data Release 2 (DR2) catalog. This
pipeline also enabled successful predictions of a stellar occultation by MU69
in July 2017. We describe how we process the WFC3 images to match the Gaia DR2
catalog, extract positional uncertainties for this extremely faint target
(typically 140 photons per WFC3 exposure), and translate those uncertainties
into probability distribution functions for MU69 at any given time. We also
describe how we use these uncertainties to guide New Horizons, plan stellar
occultions of MU69, and derive MU69's orbital evolution and long-term
stability.
