  With the first direct detection of merging black holes in 2015, the era of
gravitational wave (GW) astrophysics began. A complete picture of compact
object mergers, however, requires the detection of an electromagnetic (EM)
counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the
Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the
binary neutron star merger GW170817. The bright, rapidly fading ultraviolet
emission indicates a high mass ($\approx0.03$ solar masses) wind-driven outflow
with moderate electron fraction ($Y_{e}\approx0.27$). Combined with the X-ray
limits, we favor an observer viewing angle of $\approx 30^{\circ}$ away from
the orbital rotation axis, which avoids both obscuration from the heaviest
elements in the orbital plane and a direct view of any ultra-relativistic,
highly collimated ejecta (a gamma-ray burst afterglow).
