  We present the discovery of a long-term stable L5 (trailing) Neptune Trojan
in data acquired to search for candidate Trans-Neptunian objects for the New
Horizons spacecraft to fly by during an extended post-Pluto mission. This
Neptune Trojan, 2011 HM102, has the highest inclination (29.4 degrees) of any
known member of this population. It is intrinsically brighter than any single
L5 Jupiter Trojan at H~8.18. We have determined its gri colors (a first for any
L5 Neptune Trojan), which we find to be similar to the moderately red colors of
the L4 Neptune Trojans, suggesting similar surface properties for members of
both Trojan clouds. We also present colors derived from archival data for two
L4 Neptune Trojans (2006 RJ103 and 2007 VL305), better refining the overall
color distribution of the population. In this document we describe the
discovery circumstances, our physical characterization of 2011 HM102, and this
object's implications for the Neptune Trojan population overall. Finally, we
discuss the prospects for detecting 2011 HM102 from the New Horizons spacecraft
during their close approach in mid- to late-2013.
