Improving the Measurement of Pole Orientation of Vesta by Combining Three HST Observations

Details Improving the Measurement of Pole Orientation of Vesta by Combining Three HST Observations Improving the Measurement of Pole Orientation of Vesta by Combining Three HST Observations

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.5304l&link_type=abstract

American Astronomical Society, DPS meeting #41, #53.04

Astronomy and Astrophysics

Astronomy

Scientific paper

The pole orientation of Vesta was previously determined from Hubble Space Telescope (HST) observations obtained in 1994 by tracking five control points and from limb-fitting to be (308°, 48°) by Thomas et al. (1997). The quoted uncertainty is 10° (3-σ) in both RA and Dec, much more uncertain than the accuracy required to precisely deliver the Dawn spacecraft into orbit around Vesta to acquire the planned imagery. We combined the three available HST observations acquired through Wide Field Plantery Camera 2 (WFPC2) with the same filter set obtained in 1994, 1996, and 2007, all covering a complete rotation (5.34 hrs) of Vesta, to improve the determination of Vesta's pole orientation. The RAs and sub-Earth latitudes of the three datasets are 99.4°, 229.6°, and 251.8°, and 25.8°, -9.3°, and -18.8° (from current pole determination), respectively. The separation of RAs and the large span of sub-Earth latitudes are suitable for a better pole orientation determination. We located the surface features and measured their locations on several images as Vesta rotates, fitting the tracks of those features with ellipses. The orientations of the fitted tracks indicate the projected orientations of the spin axis in the plane of the sky at three different RAs, and the axial ratios of the fitted ellipses are associated with the sub-Earth latitudes. With more than five features tracked from three datasets obtained from three significantly different viewing angles, and with the positioning uncertainties of less than 0.5 pixel (19-26 km on Vesta) for the surface features, the uncertainty of the pole position can be determined to better than 1° (1-σ). This will allow us to plan our science observation sequences with greater fidelity as Dawn enters orbit around Vesta in mid 2011.

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