Data Set Overview
    =================
 
This data set presents the detailed plate shape model of comet 9P/Tempel 1,
as derived from the images of the comet that were obtained by the Deep Impact
spacecraft and by the Stardust spacecraft around the times of their
respective closest approaches.  Additional information about the model can be
found in Thomas et al., 'The Nucleus of comet 9P/Tempel 1 from two flybys',
Icarus 222, 453-466, 2013.  [THOMASETAL2013B].
 
The Deep Impact (DI) spacecraft consists of two parts, an impactor and a
flyby spacecraft, that encountered comet 9P/Tempel 1 on July 4, 2005.  During
this encounter, the impactor collided with the nucleus to produce a crater
and expose sub-surface material that could be observed by the flyby
spacecraft.
 
During the final approach to the nucleus, three cameras, the High Resolution
Instrument (HRI), the Medium Resolution Instrument (MRI) and the Impactor
Targeting System (ITS) obtained hundreds of images of the nucleus.  After the
impact, in which the ITS was destroyed, the MRI and HRI continued to image
the nucleus for 800 seconds, until the spacecraft entered shield mode and
stopped tracking the nucleus.  While in shield mode, the spacecraft passed
its closest approach to the comet (500 km).  Thirty minutes after closest
approach, the spacecraft again reacquired the nucleus and obtained additional
images as it departed.
 
The Stardust spacecraft visited comet Tempel 1 during its extended mission
(Stardust New Exploration of Tempel 1, or Stardust NEXT).  On February 14,
2011, the spacecraft flew by the comet at a distance of 182 km, obtaining 72
NAVCAM images, centered on the time of closest approach.
 
The data set IDs for the images used to produce the shape model are:
     DIF-C-HRIV-2-9P-ENCOUNTER-V1.0**
     DIF-C-MRI-2-9P-ENCOUNTER-V1.0
     DII-C-ITS-2-9P-ENCOUNTER-V1.0
     SDU-C/CAL-NAVCAM-2-NEXT-TEMPEL1-V1.0
 
     ** HRI data contributed in very early Deep Impact work, including
        determining how the nucleus rotated and analyzing limbs in the
        lookback images.
 
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 2.10 km to a maximum 3.97
km.  The shape is such that it is not well represented by a triaxial
ellipsoid, and the center of figure is not exactly coincident with the origin
of the coordinate system.  (This issue may be corrected in a future version
of the shape model, but the current offset is smaller than the uncertainties
in the vertex measurements.)
 
The plate model was derived from a combination of DI and Stardust images,
using 480 different control points to constrain the shape of the visible
portions of the nucleus (~70%).  Additional constraints were obtained using
the silhouette of the limb, though the uncertainty on these determinations is
larger, due to the ambiguity in where the limb lies along the line of sight.
 
The orientation of the spin axis, derived as part of the solution, is such
that the positive pole points to a right ascension of 255 deg and a
declination of +64.5 deg (J2000), with an uncertainty of approximately 3 deg
(a position about 16 deg different from the orientation presented in version
1 of the shape model).  This result may not represent an instantaneous spin
vector, but instead represents the orientation that best reproduces the views
in both the DI and Stardust images.  The prime meridian is defined by the
center of the 350 meter crater located to the west of the DI impact site (in
geographic coordinate).  Thus, because longitude increases in the direction
defined by the right hand rule around the positive pole, the longitude of the
impact site is at about 16 degrees.  The latitude is -28 degrees.
 
Two versions of the shape model are included.  The first is the original
model (TEMPEL1_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.TXT 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.
Additional information is provided in the document
TEMPEL1_SHAPE_DESCRIPTION.ASC.
 
A second version of the model (TEMPEL1_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 nucleus as it would appear during various key times during the
two encounters, 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:                         108 km^2
  Volume:                       95.2  km^3
  Radius of eq. volume sphere:  2.83 +/- 0.1 km
  Radius range:                 2.10-3.97 km
  Gravity:             0.030-0.038 cm s^2 (for mean density 470 kg m^3)
  Axis Orientation:    RA: 255 deg,  Dec: 64.5 deg (J2000)
  Model moment ratios (A,B,C are the principal moment of inertia axes):
                       A/C: 0.688
                       B/C: 0.930
  Body model moment orientations:
                       A:  -2.7 deg,   16.8 deg E
                       B:  -3.4 deg,  107.0 deg E
                       C: +80.6 deg,  247.5 deg E
 
 
Author
------
The descriptions in this file were written by Tony Farnham, based on
information from [THOMASETAL2013B] and communications with Peter
Thomas, the shape model creator.

Confidence Level Overview
    =========================
 
The uncertainty in the pole orientation is approximately 3 deg.
 
In the regions that are well-defined by control points, the radial
uncertainty of the plate model is approximately 60 m.  (These errors are
dominated by uncertainties in the geometries from the three spacecraft that
introduce inconsistencies in the feature positions in common regions.)  In
regions not defined by control points, silhouettes provide the constraints,
with uncertainties of ~100 m dominated by the ambiguity of where the limb
lies along the line of sight.  Points unconstrained by either of these
methods may have errors 100-300 m.
 
    Review
    ======
The plate model was reviewed March 5, 2013.