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Jupiter.s planet-sized satellite Ganymede is the only known moon with a mini-magnetosphere and two auroral ovals. In its only close flyby of Ganymede comma NASA.s Juno spacecraft is scheduled to pass through the satellite.s magnetotail on June 7 comma 2021. The magnetotail is the region where magnetic reconnection and energization is expected to be strongest. Juno.s flyby provides a unique opportunity to make significant progress about the poorly understood processes which energize Ganymede.s magnetosphere and power its auroral emissions. The flyby occurs over a short time-span of only approximately 30 minutes. We propose 6 HST orbits to observe Ganymede.s aurora which will provide important context and assist with the interpretation of the Juno observations. We request three HST orbits before the Juno flyby to establish the state of the aurora prior to the flyby and three additional orbits after the Juno flyby to monitor the changes in Ganymede.s aurora after the flyby. The moon.s aurora is known to vary on timescales of Juno.s flyby and the proposed HST observations. The collection of HST observations over these timescales will enable an investigation of how conditions and events observed by Juno are related to changes in Ganymede.s aurora observed by HST. The Juno flyby provides a very important and unique opportunity for HST to complement investigations of NASA.s solar system exploration program and improve our understanding of the only magnetosphere in the solar system without a bow shock.