Data Set Overview
    =================
 
This data set presents the detailed plate shape model of asteroid 2867
Steins, as derived from the images that were obtained by the Rosetta
spacecraft around the time of its closest approach to the asteroid.
Additional information about the model can be found in Jorda et al., Icarus
221, 1089, 2012 [JORDAETAL2012].
 
The Rosetta spacecraft reached its closest approach to Steins on 5 September
2008 18:38:20 UT at a distance of 803 km. The Optical, Spectroscopic, and
Infrared Remote Imaging System (OSIRIS) instrument (including both the Narrow
Angle Camera (NAC) and the Wide Angle Camera (WAC)) obtained body-resolved
images starting 2 hours before closest approach and continuing until about 2
hours after.  Unfortunately the NAC stopped operating ~10 minutes before
closest approach, when the pixel resolution was ~100 m/pix, but the WAC
coverage continued throughout the encounter.  Images covered about 60% of the
surface, with spatial scales up to 80 m/pix.
 
The shape model presented here was developed from the combination of several
different techniques: Limb profile fitting of 13 images, with stereo control
points, provided the initial constraints on the body's shape.
Stereophotoclinometry, using 55 NAC and WAC images, was used to refine the
topography of the areas illuminated and visible in the Rosetta images.  The
unseen portions of the surface were constrained using the inversion of
photometric lightcurves.
 
The orientation of the spin axis, derived as part of the solution, is such
that the North (positive) pole points to a right ascension of 91 deg and a
declination of -62 deg (J2000) with an uncertainty of 5 deg.  (Note that the
values given in the abstract of [JORDAETAL2012] are incorrect.)
 
The body's coordinate system was defined with the +Z axis in the direction of
the spin axis and the prime meridian (+X axis) is defined to pass through the
center of the crater Spinel, located near the equator (see [JORDAETAL2012]).
The +Y axis completes the right-hand coordinate system.  The body center is
not exactly coincident with its center of gravity, but the offset is within
the uncertainties derived for the surface positions.
 
 
PLATE MODEL
 
The model is given in the standard PDS vertex/triangular plate format (see
plate_shape_definition.txt in the documents directory) with dimensions in
cartesian coordniates.
 
   STEINS_CART.WRL - 10242 vertices forming 20480 triangles
 
The data are presented with a VRML wrapper that allows the file to be
displayed with existing VRML viewers that are freely available (e.g., INSTANT
PLAYER, OCTAGA, CORTONA, etc.)  Different camera angles have been embedded in
the WRL file to display the nucleus from all of the principal axis
directions, with the 'headlight' illumination.  Note that the capabilities of
different viewers and different platforms may limit the ability to utilize
some or all of these options.  Look for 'cameras' or 'navigation' items in
the menus for different viewpoints and search the help information for
turning headlights on/of, etc.
 
SURFACE DISCONTINUITY
 
The shape model contains a flaw in the surface that produces a notable jump
in elevation around the body.  This discontinuity is the seam between the two
portions of the model constrained with the different derivation techniques
(stereophotoclinometry and lightcurve analysis).
 
 
TABLE: Shape Model Characteristics [JORDAETAL2012]
 
  Area:                92 +/- 9 km^2
  Volume:              76 +/- 11 km^3
  Mean radius:         2.70 +/- 0.2 km
  Volume eq. radius:   2.63 +/- 0.2 km
  Radius range:        1.91 to 3.62 km
  Dimensions along the principal axes of inertia:
                       A: (6.83 +/- 0.1) km
                       B: (5.70 +/- 0.1) km
                       C: (4.42 +/- 0.1) km
  Normalized moments of inertia:
                       A: 2.36, B: 3.00, C: 3.68 km^2
  Surface Gravity:     0.08-0.13 cm s^2 (for assumed mean density 1800 kg m^3)
  Axis orientation:    RA:   91 +/- 5 deg
                       Dec: -62 +/- 5 deg
  Rotation Period:     6.04681 +/- 0.00002 hr
 
Note, because the prime meridian is defined with respect to a feature on the
surface, the X, Y and Z axes are not coincident with the axes of inertia A, B
anc C.
 
 
Author
------
The descriptions in this file were written by Tony Farnham, based on
information from [JORDAETAL2012] and discussions with the model creator.

Confidence Level Overview
    =========================
 
Fits of the 45 control points, distributed around the surface, produced an
estimate of 20 m for the mean positional error in the illuminated areas.
Lightcurve analysis, which provided the primary constraints on the portions
of the body unseen by Rosetta, were good to within 20% of the
stereophotoclinometry technique in regions where both techniques could be
applied. This provides confidence that the unseen portions of the model are
valid.
 
    Review
    ======
The plate model was reviewed on 5 March 2013.