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HST Mapping of the Shape and Rotation Pole of Ceres

Peter C. Thomas
Cornell University, Ithaca, NY

Of the 259 images taken by the ACS on Hubble, over 200 were useful for determining Ceres’ shape and pole position. The center of the disk and the radius were obtained using limb data. The limb accuracy approached 0.1 pixel accuracy. The shape appears to be rotationally symmetric to the accuracy of the measurement. The standard deviation of these axes is about 1.5 km. Figure 1 shows an example limb profile.

Figure 1. An example limb profile obtained in this study. Click for larger view.

Figure 2 shows images of the one bright spot that was seen. It was observed in 51 images covering 93^o of rotation. The spot lies at 14^oN latitude. This spot enables the spin axis to be determined to be 291^o right ascension and 59^o declination. These values compare favorably with earlier derivations.

Figure 2. Three images of the bright spot used as a control point to obtain the spin axis. Click for larger view.

In short, the HST Ceres observing campaign was extremely successful. The shape and pole position obtained helps planning of the Dawn mission and enables the next stage of processing of the images to be undertaken. Ceres appears to be rotationally symmetric with a pole in approximate agreement with earlier estimates. These new observations are proving to be quite valuable in planning mission operations in addition to their intrinsic scientific value.