| DATA_SET_DESCRIPTION |
Data Set Overview
=================
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.
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