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Upon approach to asteroid 4 Vesta, the Dawn mission included a dedicated satellite search observation of the operational sphere of the spacecraft around Vesta. Discovery of moons of Vesta would constrain theories of satellite f()rmation. The sequence using the framing camera and clear filter includes three mosaics of six stations acquired on July 9-10. 2011. Each station consists of four sets with three different exposures, 1.5,20 and 270 s. We also processed and scanned the optical navigation sequences until Vesta filled the field of view. Analysis of images involves looking for moving objects in the mosaics and identifying catalogued stars, subtracting them from the image and examining residual objects for evidence of bodies in orbit around Vesta. Celestial coordinates were determined using Astrometry.net, an astrometry calibration service (http://astrometry.net/use.html). We processed the images by subtracting dark and bias fields and dividing by a Hatfield. Images were further filtered subtracting a box car filter (9x9 average) to remove effects of scattered light from Vesta itself. Images were scanned by eye for evidence of motion in directions different from the background stars. All objects were compared with Hubble Space Telescope's Guide Star Catalogue and US Naval Observatory's UCAC3 catalog. We report findings from these observations and analysis, including limits of magnitude, size and motion of objects in orbit around Vesta. We gratefully acknowledge modifications made to Astrometrica http://www.astrometrica.at/ for purposes of this effort.

During the period spanning the second Enceladus flyby in July 2005 through the eleventh Titan encounter in January 2006, the Cassini spacecraft was successfully navigated through eight close-targeted satellite encounters. Three of these encounters included the 500 km flybys of the icy satellites Hyperion, Dione and Rhea and five targeted flybys of Saturn's largest moon, Titan. This paper will show how our refinements to Saturn's satellite ephemerides have improved orbit determination predictions. These refinements include the mass estimates of Saturn and its satellites by better than 0.5%. Also, it will be shown how this better orbit determination performance has helped to eliminate several statistical maneuvers that were scheduled to clean-up orbit determination and/or maneuver-execution errors.

This article describes a fit of numerically integrated orbits for the eight outer Jovian satellites to Earth-based astrometric observations.

The first 2 irregular satellites of Uranus, Caliban and Sycorax, were discovered in late 1997. Subsequently, pre-discovery observations of both satellites were found on plates taken by D. Cruikshank in June of 1984. Recently, P. Nicholson, D. Tholen, and W. Offutt provided observations which they made in late 1998 at Palomar Mountain, Mauna Kea, and Cloudcroft, respectively. I fit a numerical integration perturbed by the Sun, Jupiter, Saturn, and Neptune to the set of available observations. For the 47 observations of Caliban the respective rms values of the Delta alpha (cos delta) and Delta delta residuals axe 0".60 and 0".32, and for the 103 observations of Sycorax the analogous values are 0".57 and 0".59. I extended the integration to span a 6000 year period and computed osculating orbital elements at yearly intervals. An included table contains the mean values of the elements over the 6000 years, the sidereal period, and the precession periods of the argument of periapsis and longitude of the ascending node. The osculating elements (except for a) exhibit a significant long period oscillation with a period roughly half that of the argument of periapsis.

This article provides the reduced astrographic observations of the major Uranian satellites derived from star-satellite imaging data acquired by the Voyager 2 spacecraft. The data set contains 445 sets of spacecraft-centered right ascension and declination observations and includes all of the observations used in Voyager encounter operations. The conversion process from imaging to astrographic observations was identical to that used for the Neptunian satellites (Jacobson 1991). The effect of using the astrographic rather than imaging form in ephemeris improvement is evaluated.

This article describes the reduced astrographic observations of Triton and Nereid derived from Voyager 2 imaging data. The data set contains 496 sets of spacecraft-centered fight ascension and declination observations and includes all of the observations used in Voyager encounter operations. The details of the conversion process from imaging to astrographic observations are given. The effect of using the astrographic rather than imaging form in ephemeris improvement is evaluated.

The determination of improved orbits for the Neptunian satellites Triton and Nereid is discussed. The primary results are the final set of model parameters which generate orbits that best fit both the earth-based satellite observations and data acquired by the Voyager spacecraft during the Neptune encounter. The accuracy of the improved orbits is assessed and they are compared with the orbits generated prior to the encounter. Mean elements are also provided as a geometrical representation for the orbits.

This article presents the results of a fit of numerically integrated Neptunian satellite orbits to earth-based astrometric observations and early Voyager spacecraft observations. Ephemerides based on these orbits were used by the Voyager project as the final pre-encounter ephemerides. As a by-product of the orbit fits, estimates of the Neptune mass, the second zonal harmonic of Neptune, and the pole orientation of Neptune were also obtained.

The Voyager project used Neptunian satellite ephemerides to support both navigation and acquisition of scientific data. The development of postencounter ephemerides for the satellites Triton, Nereid, and 1989N1 is discussed. Primary results are the final set of model parameters which generate orbits that best fit both the earth-based satellite observations and data acquired by Voyager. The ephemerides are compared with those generated preencounter, and the accuracy of the final ephemerides is assessed. Mean orbital elements are also provided as a geometrical representation for the satellite orbits.

The General Uranus Satellite Theory GUST (Laskar, 1986) is used for the construction of an analytical ephemeris for the Uranian satellites. The theory is fitted against earth-based observations from 1911 to 1986, and all radio and optical data obtained during Voyager encounter with Uranus. Earth-based observations alone allow the determination of masses which are within 15 percent of the values determined by the Uranus flyby. The analysis of all the observations confirm the values of the masses obtained during the encounter (Stone and Miner, 1986) and give a complete set of dynamical parameters for the analytical theory. An analytical ephemeris, GUST86, with an estimated precision of about 100 km with respect to Uranus is obtained.

Uranian satellite ephemerides were needed by the Voyager project to support both navigation and acquisition of scientific data. This paper outlines the mathematical modeling approach taken to generate those ephemerides and discusses their pre-encounter development, real time updating, and post-encounter refinement. The results presented include the final set of model parameters which generate the ephemerides that best fit the Voyager optical data as well as the earth based observations of the satellites.

For the planned flyby missions to Comet Halley in March 1986, the comet's ephemeris uncertainties completely dominate the spacecraft-comet miss distance. In an effort to determine realistic Comet Halley ephemeris uncertainties, a statistical covariance analysis was conducted using the actual data in 1909-1910-1911 and simulated data in 1984-1985-1986. In 1985-1986, Comet Halley's ephemeris uncertainties are very sensitive to the comet's orbital position, the optical data noise, data schedule, and whether or not the old data is included in the orbital solutions. The comet's ephemeris uncertainties in March 1986 are relatively insensitive to reasonable center of light/center of mass offsets and also to possible radar data taken in late November 1985. Accurate Space Telescope observations made in early March 1986 might significantly improve upon the comet's position uncertainties for the various intercepting spacecraft.