Abstract First in NASA's New Frontiers series of missions, the New Horizons spacecraft was successfully launched on January 19, 2006. New Horizons is scheduled to perform a gravity assist at Jupiter on February 28, 2007, arrive at Pluto on July 14, 2015; completing an extended mission to at least one Kuiper Belt Object in subsequent years. The concept of operations requires a star tracker that operates autonomously both in a standard three-axis “staring” mode during operations involving instrument observations and in a spin stabilized mode during cruise operations with nominal rotational rates up to 5 rpm. With the support of the Johns Hopkins University Applied Physics Laboratory, Galileo Avionica redesigned their Autonomous Star Trackers (ASTR) to use time-delayed integration techniques to provide autonomous spacecraft attitude estimates at 10 Hz at spin rates up to 10 rpm. This paper will present the performance of the upgraded ASTR in both inertial stare and spin stabilized modes for the first six months of the mission. In addition, effects of the vehicle motion on performance, effects of stray light and direct Sun blinding on tracking and performance, and unanticipated “features” or characteristics of the ASTRs will be discussed.