NSSDCA ID: PSSB-00545
Availability: Archived at NSSDC, accessible from elsewhere
This description was generated automatically using input from the Planetary Data System.
This data set presents the detailed plate shape model of comet 103P/Hartley 2, as derived from the images of the comet that were obtained by the Deep Impact spacecraft around the time of closest approach. Additional information about the model can be found in Thomas et al., 'Shape, density, and geology of the nucleus of Comet 103P/Hartley 2', Icarus 222, 550-558, 2013 [THOMASETAL2013]. The Deep Impact Flyby (DIF) spacecraft encountered comet 103P/Hartley 2 on November 4, 2010. It approached to within 694 km of the nucleus, imaging throughout the approach, encounter and departure time frame. Two cameras, the High Resolution Instrument (HRIV) and the Medium Resolution Instrument (MRI) obtained hundreds of images of the nucleus, providing stereo information that was used to derive the shape of the nucleus. The images used for producing the shape model are contained in the PDS archive of EPOXI Data for Hartley 2. The data set IDs are: DIF-C-HRIV-5-EPOXI-HARTLEY2-DECONV-V1.0 DIF-C-MRI-2-EPOXI-HARTLEY2-V1.0 PLATE MODEL The shape model was derived in planetocentic coordinates with vertices at intervals of 2 degrees in both latitude and longitude. The radial distance is given in kilometers and ranges from a minimum of 0.31 km to a maximum 1.26 km. The center of the model is about 5 m from the coordinate origin in the direction of 19.6N, 240.4E. (This may be corrected in a future version of the model, but it is currently smaller than the uncertainty in the vertex positions). The nearly cylindrical shape is such that it is not well represented by a triaxial ellipsoid. The nucleus is in a state of non-principal axis rotation (Belton et al. 'The complex spin state of 103P/Hartley 2. Kinematics and orientation in space', Icarus 222, 595-609, 2013 [BELTONETAL2013]) that limits any mapping convention that would be tied to rotation. Thus, the coordinate system has been defined such that the 'poles' extend along the long axis of the body (with positive latitudes toward the smaller lobe). The prime meridian was defined by a dark feature, part of which is a block shadow, near the center of the illuminated face of the nucleus. The orientation of the nucleus at the time of closest approach is defined in the table below. Most of the useful MRI data were obtained at <1600 km range (16 m/pixel), and a few deconvolved HRI images were useful out to ranges of ~2500 km. These data were obtained during an interval when a simplified model of rotation predicts that the object orientation changed by ~0.6 deg. The routine mapping was done with a simple rotation model that introduces relative position errors of ~0.3 deg between ends of the object. Determination of the shape and accurate relative positioning of the images relies upon a set of 244 stereo control points distributed around the 50% of the nucleus that was illuminated and observed during the encounter. Limb outlines provide additional constraints on the shape, but have an ambiguity on where the surface is intersected by the line of sight. Two versions of the shape model are included. The first is the original model (HARTLEY2_2012_PLAN.TAB) derived as noted above, given in planetocentric coordinates. The table includes a list of 16022 vertices, with connectivity for forming 32040 plates. The format follows the standard PDS shape model definition (see PLATE_SHAPE_DEFINITION.ASC in the documents directory). In addition, each vertex includes a code that denotes the primary constraint on its derivation (control points, limb silhouette, or not well constrained) providing a measure of the accuracy of the derived radius at that point. A second version of the model (HARTLEY2_2012_CART.WRL), was derived from the planetocentric version and presents the vertices in cartesian coordinates. In this coordinate system, the X axis is defined in the direction of the intersection of the equator and the prime meridian, the Z axis lies along the positive pole and the Y axis completes the right-hand coordinate system. The cartesian coordinate version of the model is presented with a VRML wrapper that allows it to be displayed with existing VRML viewers that are freely available (e.g., INSTANT PLAYER, OCTAGA, CORTONA, etc.) Different camera angles and illumination conditions have been embedded in the WRL file to display the nuclues as it would appear during the approach, closest approach point, and departure, with the associated solar illumination (though the twist angle around the line of sight may not match what is recorded in the corresponding images). There are also embedded viewpoints from all of the principal axis directions, with the 'headlight' illumination. Note that the capabilities of different viewers may limit the ability to display some or all of these viewpoints. Look for 'cameras' or 'navigation' items in the menus. TABLE: Shape Model Characteristics Area: 5.24 km^2 Volume: 0.809 +/- 0.077 km^3 Radius of eq. volume sphere: 0.58 +/- 0.018 km Radius Range: 0.31-1.26 km Gravity: 0.0019-0.0044 cm s^2 (for mean density 300 kg m^3) Model moments: 5.302E4, 31.26E4, 31.92E4 m^2 Model moment ratios (A,B,C are the principal moment of inertia axes): A/C: 0.166 +/- 0.004 B/C: 0.979 +/- 0.002 Body model moment orientations: A: 89.73 deg, 207.56 deg E B: 0.05 deg, 106.14 deg E C: 0.25 deg, 16.14 deg E Time of Closest Approach: 2010-11-04T13:59:47.7 Directions of Axes at Closest Approach: RA (deg) Dec (deg) (J2000) +Z (Long) axis: 226.12 39.37 +X axis: 237.10 -50.10 +Y Axis: 320.59 5.42 Author -----The descriptions in this file were written by Tony Farnham, based on information from [THOMASETAL2013] and communications with Peter Thomas, the shape model creator.
These data are available on-line from the Planetary Data System (PDS) at:
http://pdssbn.astro.umd.edu/holdings/dif-c-hriv_mri-5-hartley2-shape-v1.0/
Questions and comments about this data collection can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Michael F. A'Hearn | Data Provider | University of Maryland | |
Dr. Peter C. Thomas | General Contact | Cornell University |