The 3d Figure and Surface of Pallas from Hst

Introduction: Spatially resolved images provide the ability to examine both the surface and the shape of planetary objects. Observing from Earth provides limited spatial or shape information for most asteroids due to their small extent. However, remote sensing has advanced by leaps and bounds our understanding of the largest asteroids [e.g. 1,2]. We observed 2 Pallas with Hubble Space Telescope’s (HST) WFPC2 camera in September of 2007, obtaining 45 spatially resolved photometric images in 5 filters from the NUV through the Visible. Observing from space offers the advantage of a stable thermal environment, so that our images achieve resolution near WFPC2’s 0.046” per pixel, resulting in about 73 km/pixel scale in the final images. These images were then deconvolved using the Maximum Entropy Method (MEM) [3]. 3D Modeling: Initially, we derive a triaxial mean shape model of 291 x 278 x 250 +/9 km using only the raw data [3]. This method precludes any artifacts that may be introduced by deconvolution. Using this model as a basis, we then took two approaches to constructing and validating a three dimensional model. Comparing the deconvolved to the unprocessed images, Pallas’ shape and the details of its terminator are more clear after the image processing. From the images we observe Pallas’ terminator to be irregular, and its shape changes significantly through the course of its rotation. In order to visualize how the changing terminator and rotation affect the appearance of Pallas at this epoch and to interpolate between frames, we utilize the Maya digital modeling software. We began with a digital triaxial ellipsoid of the dimensions given above and our pole solution, RA 42 DEC -12 [4]. We then overlay the processed image with the digital figure and fit the edge of the model to Pallas’ limb by hand. We then rotate the model to the next sub-earth longitude and fit the model to the next image, repeating for each of the images and creating a full 3D asteroid from our data. In Figure 1 we show sample views of the processed images along with the digital model. We also extracted contours from both the raw and processed images to compare and refine the model. A similar technique was used to build a digital topography model for Vesta [1]. We utilize the mean shape and contours as a comparison for the digital model. However, our image resolution is less than that of [1] since Pallas is further from the Earth than Vesta. But these two techniques are complementary and yield a model that fits our data well.