Operations Challenges for Missions with Significant Round-Trip Light Times

New Horizons (NH), a NASA-managed New Frontiers mission en route to a first flyby of the Pluto system with a potential extension to study one or more Kuiper Belt Objects, launched on January 19, 2006 and will make its closest approach to Pluto on July 14, 2015. New Horizons is currently Earth’s third-farthest operational spacecraft. At Pluto closest approach, the round trip light time (RTLT) for New Horizons will be nearly nine hours. This lengthy RTLT impacts many facets of mission operations (MOps), which is managed by The Johns Hopkins University Applied Physics Laboratory. This paper will discuss the RTLT considerations that are already being addressed in flight and those that still need to be planned for ― especially those that impact the Pluto encounter. Some examples of these considerations include real-time commanding limitations, DSN antenna pointing, ranging strategies, autonomy, and command verification. Still operating at the far reaches of our solar system are the twin Voyager spacecraft, Voyager 1 at approximately 113 AU from Earth and Voyager 2 at approximately 92 AU from Earth. The Voyager operations team has decades of experience flying two spacecraft at distances away from Earth with significant RTLTs. Where applicable, this paper will make comparisons between the NH and Voyager teams’ approaches to RTLT considerations.