Radio astrometry of the Cassini spacecraft with the very long baseline array

The planetary ephemeris is a fundamental tool of astronomy that is essential for dynamical studies of the solar system, pulsar timing, tests of general relativity, occultation and eclipse predictions, and interplanetary spacecraft navigation. Since Jupiter and Saturn dominate the dynamics of our solar system, improved knowledge of their orbits will result in a global improvement in the accuracy of the ephemeris. The Cassini spacecraft has been orbiting Saturn for over a decade, a third of Saturn's orbital period. This has provided an unprecedented opportunity to improve all components of Saturn's orbit by combining periodic very long baseline inferterometry (VLBI) measurements of Cassini's sky position with respect to background radio sources, which in turn can be tied to the inertial International Celestial Reference Frame (ICRF). The orbit of Cassini about the center of mass of Saturn is determined from Doppler tracking by the Deep Space Network. Combining these observations, we obtain the barycenter position of the Saturn system in an inertial frame at multiple epochs, with typical uncertainties of 0.3 milli-arcseconds in right ascension and 0.4 milli-arcseconds in declination. These results are then provided to JPL's ephemeris group for inclusion in future ephemeris solutions. At most epochs the largest component of the error budget is uncertainty in the ICRF position of the phase reference radio source used. These source positions are being continuously improved through additional VLBI observations. These VLBA observations have improved our knowledge of Saturn's orbit by nearly an order of magnitude. This technique will be expanded to include astrometric observations of the Juno spacecraft as soon as it enters Jupiter orbit in mid-2016. Although the orbital phase of the Juno mission is expected to last only a bit over one year, it will still allow a significant improvement in Jupiter's orbit. Previous missions to Jupiter have been single-epoch flybys with the exception of Galileo, for which the accuracy of VLBI position measurements was severely limited by failure of the high gain antenna. The Juno mission is scheduled to end in February 2018, five months after the scheduled end of the Cassini mission. At the ends of their missions the Juno and Cassini spacecraft will be destroyed in the atmospheres of Jupiter and Saturn to eliminate the possibility of a future crash onto any of the liquid-containing moons of these planets that may be habitats of life.