  Observations of Pluto's companion planet Charon have so far yielded no
compelling evidence for a persistent atmosphere. However, with the upcoming
encounter of New Horizons at the Pluto-Charon system, sensitivity to detection
of a possible Charon atmosphere will increase by orders of magnitude. In
particular, it has long been planned for New Horizons to use its Alice
ultraviolet spectrograph during solar occultation observations of Charon on
July 14, 2015, for this purpose. But in the days before closest encounter, the
Alice instrument will also acquire spectra of reflected sunlight from Charon's
surface. We examine the effect of absorption by several possible atmospheres
around Charon composed of N$_2$, CH$_4$, and CO, with plausible surface
pressures of 0.1, 1, and 10 nanobar (nbar). We show that this reflectance
technique is sensitive to surface pressures on Charon down to the 0.1 nbar
level, and should provide significant evidence of a possible atmosphere on
Charon before closest encounter. It may also be the only way to detect a local
atmosphere domed over the subsolar region on Charon.
