Design and Implementation of the Juno Eclipse Avoidance Maneuver

The Juno spacecraft is the first solar-powered spacecraft to travel to the outer planets and is powered by three, nearly-9-meter solar arrays with an average output of over 500 W. As such, it is critically important to maintain sunlight on the solar arrays and the Juno reference trajectory design included only a single, 19-minute eclipse during a 550-km Earth gravity-assist in October 2013. Previous iterations of Juno’s polar, 32-orbit science campaign leveraged orbital periods of 11 and 14 days with mission end times in October 2017 and February 2018, respectively, and included no science phase solar eclipses. However, the decision in October 2016 to cancel Juno’s Period Reduction Maneuver (PRM) and remain in the 53-day capture orbit indefinitely necessitated Juno traversing a Jovian “eclipse season” to complete its required 32 science orbits.1 Without intervention, Juno would have encountered a nearly-12-hour eclipse in early November 2019 that would have almost certainly been mission-ending. Fortunately, the Juno Navigation Team was able to identify a deterministic maneuvering strategy to avoid solar eclipse entirely and add years to Juno’s operational lifetime. While Juno’s notional eclipse avoidance maneuver strategy was previously summarized by Pavlak et al.1 primarily in the context of selecting Juno’s science orbit period following PRM cancellation, this paper details the operational considerations and complexities associated with designing and implementing Juno’s comprehensive eclipse avoidance campaign. This discussion includes topics such as single-burn vs. multi-burn approaches, backup maneuver opportunities and associated V costs, burn timer selection methodology, and eclipse duration as a function of burn completion percentage. Lastly, this paper analyzes Juno’s eclipse avoidance maneuver as-executed and assesses the performance and delivery accuracy against pre-maneuver predictions.