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

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 0.1546 s) and 22 images with the WAC red F12 filter (UT 06:07—16:39, exposure 1.0078 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= 2.3243 AU Rosetta-Vesta distance= 0.28591- 0.28368AU Phase: 51.11—51.84 deg. Sub solar latitude= 30.4 deg. From JPL horizon; updated to 22.8 (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.

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= 9.91055e-09, nac_sr= 3.54744e-10) Absolute flux computed as: Rosetta-Vesta distance, and r is Vesta equivalent radius (265 km)

IUE

IUE

From Burbine et al. 2001

From Burbine et al. 2001

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

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)

Vesta lightcurve Data corrected for lightime, mag. Reduced to 1 AU solar and observer distances, Phase corrected to 52.261 using G=0.15 Psyn=5.352±0.025 Ampl.= 0.19± 0.01 From Literature P= 5.342±0.120

Phase function

Phase function

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)