Description of the basic tri-axial ellipsoid shape model of Comet Wild 2 This data set presents the basic shape model of comet 81P/Wild 2, as derived from the Stardust Navigation camera images that were obtained around the time of closest approach to the comet. It gives the dimensions of the three axes, and defines the orientation of the spin axis in space. On 2 January 2004, the STARDUST spacecraft flew past comet 81P/Wild 2 and took 72 images of the nucleus. In these images, the phase angle started at 70 deg during the approach, dropped to 3 deg near closest approach, and then increased to 110 deg during the departure. Most images were obtained within a range of 2,000 km, with the closest at 237 km. The camera took the images using a broadband filter and a CCD that had an angular resolution of 57 microrad/pixel. A tri-axial ellipsoidal model was fit to the limb and terminator in all the Wild 2 nucleus images. The model was constrained by the fact that the viewing geometry changed by 180 degrees during the encounter. Essentially, the entire illuminated hemisphere of Wild 2 was viewed in these images, or 50% of the surface since there was no noticeable rotation during the few minutes that the images were taken. The axial dimensions determined from the visual fit of the model to the limb and terminator were 1.65 x 2.00 x 2.75 km +/-0.05 km (1 sigma). The uncertatinties in these dimensions were estimated by determining how much the value could be changed before overlays of the model were clearly not fitting the observed nucleus in the corresponding images. Changing the radii by more than 0.05 km in any dimension introduces a noticeable mismatch between the model overlays and the position of the limb and terminator. The illuminated limb is reasonably smooth, with the exception of a few large depressions. It is understood that half of the surface was not illuminated and that these unseen surfaces could deviate from the model derived from the illuminated regions. However the model fit to the terminator, even though quite rough due to topographic variations, does give some evidence that the model probably extrapolates to the unseen part of the nucleus to at least a km or better. A body-fixed coordinate system was established with the shortest axis assumed to be the rotation pole, because it is the most dynamically stable rotation axis. The posigrade pole was defined to agree with the corresponding poles from Farnham and Schleicher (2004) [FARNHAM&SCHLEICH2004] and Sekanina (2003) [SEKANINA2003] and is oriented at the position RA=112 deg, dec=-17 deg. The longest axis is used to define the prime meridian. All angles are measured relative to the Earth Mean Equator and Vernal Equinox of J2000 and have an epoch of 2004 Jan 02 19:47 UTC. All planet-fixed coordinates are expressed using IAU right-handed, planetocentric angles. Figure 1 shows the coordinate system used to describe the pole orientation and direction of the prime meridian (angle W is the argument of the prime meridian). The overlays in the examples shown in Figure 2 have latitude grids every 30 deg and longitude grids every 45 deg. In this figure, the red ellipse defines the limb of the triaxial ellipsoid, the green ellipse defines the terminator, label 'A' shows the intersection of the equator and the prime meridian and label 'C' denotes the positive pole. It is noted that the camera used a rotating mirror to track the nucleus during flyby. This produced a unique rotation in each image in Figure 2 as well as flipping them about one axis, making east longitude increase clockwise about the poles in Figure 2. Even though the sizes and orientations of the 3 axes are believed to be well determined, the actual spin axis could vary from the short axis of the model because of non-uniformities in mass distribution within the nucleus and the possibility of wobbles that could not be determined from the STARDUST observations. The images used to derive this shape model are archived at the PDS Small Bodies Node, in the data set SDU-C-NAVCAM-2-EDR-WILD2-V1.0.