1. Determination of photometric properties of Steins
Sofie Spjuth, MPS, Lindau
Laurent Jorda, LAM, Marseille
OSIRIS Team Meeting , September 18-20, 2007

2. Motivation Understanding the regolith Asteroids:
- Constrain the physical properties of the regolith
(composition, SS albedo, particle size, roughness)
- Identify areas with different photometric properties
- Compare the derived properties with that of other asteroids
Comet nucleus:
- Constrain the physical properties of different terrains identified
at the surface of the comet nuclei (smooth terrains, etc.)

3. Method BDR - Distance → Solar irradiance (F)
Camera characteristics (filter, exposure time)
→ reflected radiance (I)
BDR = I/F
SIMULATOR
I = reflected radiance exiting from a surface in a particular direction
F = irradiance incident on the surface
Fitted against models
HAPKE
Least square fit → Hapke‘s parameters

4. SCHEDULE SIMULATOR SETUP Time (ET) Exposure time Filter(s) Shape
Positions
...
Observed,
calibrated
Images (n)
‚schedule‘
# i
‚devel‘
Image matching
‚shapecomp‘
s-file
g-file
Synthetic images
‚sim2map‘
Rotated image, # i
imap_
emap_
...
‚sim2flx‘
fmap_
Distances
Filter(s)
Solar irradiance
...
Fit Hapke
Parameters
(n images)
‚hapke‘ no
BDR
i < n
yes

5. Test case: simulator Steins BDR
~ 3000 measurements
Measurement distance [km]
Mean error [%]
Min error [%]
Max error [%]
Mean error
RMS error
0.23
4e-005
0.73
0.0082
0.0108
0.14
0.41
0.0038
0.0045

6. Test case: simulator Steins
Distance range [km] w h g B0 θ
“TRUE”
PARAMS
0.40
0.02
-0.35
0.97
0.00
750 -
0.32
-0.53
1.38 23
3000
0.44
0.95
(0.20)
(0.10)
(-0.11)
(0.40)
(20)
5000 -
10000
0.47
fixed
-0.29
4x4°

7. Test case: simulator Steins
measurements (750 – 3000 km)
Fitted BDR
Measured BDR

8. Test case: simulator Steins
measurements (750 – 3000 km)
Fitted/Measured BDR
Incidence Angle [°]
Emission Angle [°]

9. Test case: simulator Steins
Map of SSA
10x10° partitions
25 measurements in each image (2x2° shape model)
Latitude
Longitude

10. Test case: simulator Steins
Map of error in SSA
Latitude
Longitude

11. Test case: simulator Steins
# of measurements
Latitude
Longitude

12. Test case: simulator Steins
# of measurements with resolution > 200 m/px
Latitude
Longitude

13. Test case: simulator Steins
Phase angle range of measurements
Latitude
Longitude

14. Ongoing/Future
Perform the photometry on earlier targets (Tempel1,Mathilde,Ida etc.)
Simultaneous determination of shape and photometric properties such that the shape and its reflectance parameters best agree with the input images
Comparison of the extracted BDR with laboratory measurements

15. THE END