162173 Ryugu, a mission target of Hayabusa 2, has just been observed during a night between 11 and 12 July using the Very Large Telescope (VLT) of the European Southern Observatory (ESO, Chile). The FORS2 instrument mounted on the 8.2-m Antu telescope has been used by an international team led by Dr. Davide Perna and Antonella Barucci (LESIA - Observatoire de Paris, France), in collaboration with Japanese (JAXA, NAOJ) and Korean (SNU, KASI) members, to acquire a full rotational lightcurve of the asteroid, as well as visible spectra at different rotational phases. Once analyzed, the newly acquired lightcurve will secure the determination of the rotational period and pole orientation of Ryugu: indeed, the current best solution for the rotational period of this asteroid is P=7.625±0.003 hours, assuming a double-peaked lightcurve (Kim, M. J. et al. Astronomy & Astrophysics 550, L11, 2013). However a possibility of single-peaked lightcurve (i.e., P~3.8 h) cannot be ruled out at present because of poor accuracies of the previous measurements. It is expected that the new high-quality VLT data will settle the debate over the rotational period.
Moreover, this and other lightcurves acquired in July/August 2016, under viewing geometries complementary with those of 2007 and 2012 campaigns, will permit to significantly improve the current inversion models to constrain the spin orientation of Ryugu. As for the newly acquired spectral data, they will help to characterize the surface composition and eventual heterogeneity of Ryugu, looking in particular for the presence of hydrated minerals that may provide information on the thermal history of the asteroid. These observations are part of an international campaign during Summer 2016, as this is the last occasion to obtain a detailed characterization of Ryugu prior of the arrival of Hayabusa 2. These new observations are hence very useful to help optimizing the planning of Hayabusa 2 mission operations.
July 16, 2016
Ground-based Observation Team, Hayabsua2 Project