The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
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(a) Phobos’ leading side
(b) Phobos’ trailing side
Figure 5: Shape model from triangulation of the control
point network. The x-axis (dot-dash) points
towards Mars, the y-axis (long-dashed) points in
the direction of motion and the z-axis marks the
rotation axis. Note that the y-axis in Figure 5(a)
points in the direction of motion and is in the
negative y direction of the IAU frame. In Figure
5(b) the positiv y-axis is displayed
Object Pom! Accuraoes over varying Ubrabon Amplitudes
— X-Coordmate
YCoordinate
¿Coordinale
1.0 15 2,0
Ubrawn Amplitude *n Deg
Figure 6: Observed residuals of object points for varying forced
libration amplitudes. The red vertical line indicates
the observation by (Duxbury, 1991) , the green line
indicates the minimum of SRC observations.
points, indicate an amplitude of 1.2° for the force libration (see
Figure 6). The observed amplitude differs slightly from the
amplitude of 0.80 ±0.3° observed and computed by (Duxbury
and Callahan, 1988, Duxbury, 1991), but is in well agreement
with the amplitude of 1.2° derived from a topographic model
(Borderies and Yoder, 1990). The topographic model of the
later reference was approximated through a Kriging
interpolation of the 98 control points from the 1989 control
point network (Duxbury, 1989). A correction for the Stickney
crater was made in the topography model. A deviation of the
forced libration amplitude from previous observed amplitudes
could have its cause in the mass distribution model. Current
studies assume a homogeneous mass distribution for Phobos.
Mass concentrations could significantly change inertia figures
which can be related to the forced libration amplitude in a first
approximation. Equation (1) shows this relation, where 0 A is the
forced libration amplitude, e equals the excentricity of Phobos,
and A < B < C are the moments of inertia.
r=
B-A
C
0 A =
2e
1 — y
3
(i)
5. CONCLUSIONS AND FUTURE WORK
We observed Phobos’ positions over a three year time span
through astrometric measurements in SRC image data. A new
reduction method was established which can be applied to any
planetary body and may prove to be useful for analysis of future
planetary data sets. The ongoing MEX mission with further
close flyby maneuvers will make more observations available,
adding constraints to the Phobos orbit models.
The control point network was primarily used to observe the
forced libration amplitude. Estimates of the volume and
moment of inertia factors are yet to come, and may further
constrain the bulk density and mass distribution of Phobos.
Currently, studies of HRSC stereo data are under way to derive
a global digital terrain model (DTM) in high resolution. A DTM
derived from Viking orbiter images will be needed to fill the
unobserved area on the anti-Mars side of Phobos.