  The outer solar system provides a unique, quiet vantage point from which to
observe the universe around us, where measurements could enable several niche
astrophysical science cases that are too difficult to perform near Earth.
NASA's New Horizons mission comprises an instrument package that provides
imaging capability from ultraviolet (UV) to near-infrared (near-IR) wavelengths
with moderate spectral resolution located beyond the orbit of Pluto. A
carefully designed survey with New Horizons can optimize the use of expendable
propellant and the limited data telemetry bandwidth to allow several
measurements, including a detailed understanding of the cosmic extragalactic
background light; studies of the local and extragalactic UV background;
measurements of the properties of dust and ice in the outer solar system;
confirmation and characterization of transiting exoplanets; determinations of
the mass of dark objects using gravitational microlensing; and rapid follow-up
of transient events. New Horizons is currently in an extended mission designed
to focus on Kuiper Belt science that will conclude in 2021. The astrophysics
community has a unique, generational opportunity to use this mission for
astronomical observation at heliocentric distances beyond 50 au in the next
decade. In this paper, we discuss the potential science cases for such an
extended mission, and provide an initial assessment of the most important
operational requirements and observation strategies it would require. We
conclude that New Horizons is capable of transformative science, and that it
would make a valuable and unique asset for astrophysical science that is
unlikely to be replicated in the near future.
