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Although the entire nucleus was not imaged - the spacecraft flew past and not completely around the comet - enough was seen to give a good idea of the shape. Using data collected from the instruments, co-Investigator Peter Thomas of Cornell University has worked with his team to derive the shape. The MRI imaged about 25% of the nucleus. Discernible features, usually circular ones, that can be recognized in multiple images (red + signs, left frame) are marked as 'control points.' Input to the grid-making program typically uses 189 control points from 70 images. The control points are then pieced together into a shape model consisting of plates that define the three dimensional figure of the nucleus. There are gaps of course, because we didn't see the entire nucleus. The right frame shows the grid representing the shape of the nucleus. From this model we then have some dimensions. The mean radius is 3.0 ± 0.1 km, the longest dimension is 7.5 km and the shortest is 5.0 km. In addition to the shape, Dr. Thomas also derived the orientation of the comet's spin pole, which points to a place in the sky designated at Right Ascension 293.8° and declination 772.6° with an uncertainty of ±5° (these are equivalent to latitude and longitudes on the celestial sphere).
Knowing the shape and the orientation of the pole about which the comet rotates, are critical pieces of information upon which the rest of our analysis is built.
Photo Credit: NASA/UM/Cornell/Peter Thomas
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