1. Asteroid 4 Vesta observed from OSIRIS-ROSETTA
S. Fornasier, S. Mottola, M.A. Barucci And the Osiris team

2. Osiris observations of 4 Vesta
Vesta observed on 1 may 2010 for calibration purposes
22 images with the NAC orange F22 filter (UT 05:52—16:32, exposure of s) and 22 images with the WAC red F12 filter (UT 06:07—16:39, exposure s) covering more than 2 rotational periods
1 spectrophotometric sequence including 12 NAC filters (11:00—11:09) and 9 WAC filters (10:56—10:59).
Observing conditions:
Vesta sun distance= AU
Rosetta-Vesta distance= AU
Phase: 51.11—51.84 deg.
Sub solar latitude= 30.4 deg. From JPL horizon; updated to (Li et al., 2011)
Sub obser. Latitude= 4.8 deg. (horizon); 2.8 updated.
Sub obs longitude (UT 11)= 145( W ) from JPL; 218 (E) IAU def.

3. Data analysis Level 2 images taken from the Osiris archive
Data converted to DU/s using the pipeline abs. calib. factors.
Total flux in DU/s obtained integrating over a radius of 6 pixel. Sky background evaluated in few regions around the target.
Flux converted in W/m2/nm/sr using new abs. Calib. Factors, derived from 3 Vega observations in 2010 (HST solar irradiance used; wac_sr= e-09, nac_sr= e-10)
Absolute flux computed as:
Rosetta-Vesta distance, and r is Vesta equivalent radius (265 km)

4. IUE

5. IUE

6. From Burbine et al

7. From Burbine et al

8. From Hendrix et al. , 2003, Icarus 162, 1

9. Vesta lightcurve Red: WAC F12 (red); 22 images (1 may, UT 06:07—16:39)
Black: NAC F22 (orange); 22 images (1 may, UT 05:52—16:32)

10. Vesta lightcurve
Data corrected for lightime, mag. Reduced to 1 AU solar and observer distances, Phase corrected to using G=0.15
Psyn=5.352±0.025
Ampl.= 0.19± 0.01
From Literature
P= 5.342±0.120

11. Phase function

12. Phase function

13. Conclusion Rosetta data are taken at ‘high’ phase angle (52)
Phase function, H and G parameters can be better constraint
Absolute photometry obtained over the UV-VIS-NIR range
The observed Vesta surface looks similar to howardites
Still some work to do in the building of the phase function (check of literature data);
Need to understand what hemisphere we really observed on Vesta (some mismatches exist between published Earth or Sun subsolar latitude and longitude and the JPL horizon value taken at the same observing conditions
Results will be submitted as soon as possible to A&A (letter)