Initial Astrometry of the Juno Spacecraft Orbiting Jupiter

We have been using the Very Long Baseline Array (VLBA) to measure accurate sky positions of the Juno spacecraft since its insertion into orbit around Jupiter in July 2016. These positions are referenced to the inertial International Celestial Reference Frame. During Juno's perijove passes the relative positions of the spacecraft and the Jupiter system barycenter are extremely well determined from Doppler tracking by the Deep Space Network (DSN). Combining our VLBA measurements with DSN tracking gives us accurate Jupiter barycenter positions, which in turn can be used to improve our knowledge of Jupiter's orbit (particularly its orientation) as part of the JPL planetary ephemeris. A problem with the main engine has kept Juno in its current 53.5-day orbit instead of the originally planned 14-day orbit. This will extend the mission duration in Jupiter orbit to nearly 5 years, well over 1/4 of Jupiter's orbital period, which in turn will allow us to obtain stronger constraints on orbital inclination and ascending node longitude. DSN range measurements complement the astrometric measurements by providing the best constraints on orbit semi-major axis and eccentricity. We expect to reduce the uncertainty in the orientation of Jupiter's orbit with respect to the International Celestial Reference Frame from approximately 20 milliarcseconds (mas) to approximately 0.2 mas (1 nrad), based on the improvement in Saturn's orbit obtained from similar VLBA measurements of the Cassini spacecraft. Our initial VLBA observations of Juno, combined with the Ulysses flyby in 1992, have already improved the Jupiter ephemeris accuracy by about a factor of 4.