The 2000 Rosetta asteroid targets observational campaign: 140 Siwa and 4979 Otawara

Photometric and spectroscopic observations of 140 Siwa and 4979 Otawara, targets of the Rosetta mission, have been carried out at the European Southern Observatory (ESO, Chile). The complete lightcurves in V and R of both asteroids and the Siwa spectrum in the range 0.95–2.52 μm have been obtained. We determined the precise synodic rotational period of 140 Siwa (18.495 h ± 0.005 h) and we confirmed the synodic rotational period of 4979 Otawara (2.707 h ± 0.005 h). The phase function has been obtained for both asteroids, allowing the determination of the HV and GV parameters: for 4979 Otawara HV = 14.62 mag, GV = 0.34; for 140 Siwa HV = 8.34 mag, GV = 0.17. The near infrared spectrum of Siwa does not show any spectral feature, which is consistent with a C/P type object.