1. OSIRIS Mars flyby : 3D reconstruction of Phobos and Deimos
O. Groussin (UMD)
P. Lamy (LAM)
L. Jorda (LAM)
M. A’Hearn (UMD)
G. Bonello (LAM)
OSIRIS flyby working group meeting
January 30-31, 2006
Padova, Italy

2. Introduction
We intend to perform a 3D reconstruction of Phobos and Deimos during the Mars flyby.
This is our only opportunity to develop and test 3D reconstruction algorithms before the asteroids flyby and more important, the 67P approach phase.
 We must not miss that opportunity!

3. Geometry of the flyby (1/4)
(from SPICE)
From Agay documents (Hviid, Koschny and Thomas)

4. Geometry of the flyby (2/4)
Distance (from SPICE)
Phobos
Deimos
Phobos hidden by Mars
Deimos hidden by Mars
Phobos in Mars’s shadow
Rosetta in Mars’s shadow

5. Geometry of the flyby (3/4)
Phase (from SPICE)
Phobos
Deimos
Phobos hidden by Mars
Deimos hidden by Mars
Phobos in Mars’s shadow
Rosetta in Mars’s shadow

6. Geometry of the flyby (4/4) Elongation (from SPICE)
Phobos hidden by Mars
Deimos hidden by Mars
Phobos in Mars’s shadow
Rosetta in Mars’s shadow
60 degrees constraint
Phobos
Deimos

7. Windows (W) for Phobos and Deimos (1/2)
Deimos W1
Deimos W2
Phobos W1
Phobos W2
Phobos hidden by Mars
Deimos hidden by Mars
Phobos in Mars’s shadow
Rosetta in Mars’s shadow

8. Windows for Phobos and Deimos (2/2)
Pro and Cons
Deimos W1 : best at the end of the window
Phase angle, Distance, Scattered light from Mars, no look back
Deimos W2 : only the first 20 min of the window (elongation)
Phobos W1 : best at the end of the window
Phobos W2 : not possible because of elongation < 60 deg

9. Some numbers
Closest approach from Mars surface : 257 km on 25 Feb 2005, 01:55 UT
Closest approach from Phobos CM : 4346 km on 25 Feb 2005, 02:06 UT
Closest approach from Deimos CM : km on 25 Feb 2005, 02:20 UT
Window 1 for Deimos (15x12x11km) :
Before 25 Feb 2005, 01:49 UT
Best distance = km (~29 NAC pixels for 15km)
Window 2 for Deimos :
After 25 Feb 2005, 01:58 UT & Before 25 Feb 2005, 02:20 UT
Best distance = km (~34 NAC pixels for 15km)
Window 1 for Phobos (27x21x19km) :
Before 25 Feb 2005, 00:42 UT
Best distance = km (~29 NAC pixels for 27km)
1 NAC pixel = arcsec = microrad

10. Conclusions on a possible flyby strategy
Images during approach for Phobos and Deimos, as long as we can, i.e., until they are hidden by Mars (~29 NAC pixels). Ignore the scattered light, since we do not want to do photometry but just get the limb.
Images after approach for Deimos, as soon as Deimos becomes visible again (~34 NAC pixels), and until elongation becomes a problem (~20min window). Can we observe while Rosetta is in the Mars’s shadow?
Image of Phobos after approach is not possible, because of the constraint on the elongation.

11. How to perform the 3D reconstruction ?
Step #1 is to get images with different phase angles.
Step #2 is to use the limb and the geometry of the flyby. We will start with a spherical shape and modify it as long as necessary to match all the images in all the different geometry. The solution may not be unique in that case. We are developing IDL routines to do this.
Step #3 is to compare the results with the real shape of Phobos and Deimos and see how we can improve our algorithms.
Comments : with our resolution of only NAC pixels, we cannot expect to use an accurate method with control points like P. Thomas (Cornell) did for example, on 19P/Borrelly (DS1) and 9P/Tempel 1 (DI). However, we will learn how to proceed that way for future asteroids and 67P observations.

12. Real shape of Phobos and Deimos
We have obtained the shapes of Phobos and Deimos on the PDS. We have IDL routines to read them and play with them.
Phobos : 27x21x19km
Deimos : 15x12x11km

13. Conclusions
Observations of Phobos and Deimos offer a unique opportunity to prepare the 67P approach phase and the 3D reconstruction of the nucleus shape.
Those observations are possible and must be performed, in good coordination with Mars observations.
We already have a few tools to perform 3D reconstruction. We will learn and develop new tools in the next months, in order to be ready for the Mars flyby.