Spectral and rotational properties of near-Earth asteroid (162173) Ryugu, target of the Hayabusa2 sample return mission

Abstract

Context. The JAXA Hayabusa2 mission will perform the first ever sample return from a primitive asteroid. The target near-Earth asteroid (162173) Ryugu will be reached in mid-2018 and its samples will be returned to the Earth by the end of 2020. Aims. We want to improve the current knowledge of the compositional and rotational properties of Ryugu, which are still presenting some uncertainties that might affect the mission operations and scientific return. Methods. We acquired high-quality photometric time-series data with the FORS2 instrument at the Very Large Telescope of the European Southern Observatory (ESO-VLT, Chile). We also acquired four FORS2 visible spectra and three X-shooter spectra in the 0.35−2.15 µm range, at different rotational phases. Results. We obtained the currently highest-quality visual light-curve of Ryugu. A best solution of ∼7.63 h is found for the rotational period, while a short-period solution (i.e., P ≈ 3.8 h) is ruled out by the clearly non-symmetric light-curve. The obtained spectra are generally similar and featureless, but present a drop-off of the reflectance at <0.45 µm, suggesting the presence of aqueously altered minerals on Ryugu. The best meteorite analogs for Ryugu are represented by thermally altered CM carbonaceous chondrites. Conclusions. Our new photometric data help to refine the target reference model used by the Hayabusa2 team for the mission preparation and implementation, improving our knowledge of Ryugu's spin properties. Our new spectra constrain the compositional and geological context of the Ryugu's surface in order to prepare the planning of mission observations and support the working group for the selection of possible landing and sampling sites.

An evaluation method of reflectance spectra to be obtained by Hayabusa2 Near-Infrared Spectrometer (NIRS3) based on laboratory measurements of carbonaceous chondrites

“…Ground-based observations of asteroid Ryugu indicated that its surface materials would possibly consist of unheated CM chondrite (Vilas 2008) based on an observation of the 0.7-μm band, or heated carbonaceous chondrite because its Vis-NIR spectrum corresponds with the spectrum of Murchison CM2 chondrite heated at 550-900 °C. Their spectral slopes around 0.7 μm are very similar to each other (Sugita et al 2013;Perna et al 2017).…”

“…We can conclude that NIRS3 has sufficient ability to evaluate the surface property of the CM-chondrite-like target asteroid because it has sensitivity for the wide range of wavelength up to 3.2 μm. In the previous observations, spectral heterogeneity of Ryugu was reported; Vilas (2008) suggested that a 0.7-μm absorption band exists; on the other hand, spectra recorded by Bus and Binzel (2002), Lazzaro et al (2013), Sugita et al (2013), and Perna et al (2017) showed no such absorption. The results may indicate that either the surface of Ryugu is extremely heterogeneous or a short-lived resurfacing event excavated hydrous minerals showing the 0.7-μm absorption band.…”

The CM carbonaceous chondrite regolith Diepenveen

“…), near‐IR slope (Perna et al. ; Le Corre et al. ) and the intensity of the absorption at 0.7 μm (Vilas ).…”

“…Studies of suitable meteorite analogs for Ryugu suggest that unusual or heated CM and CI carbonaceous chondrites may be best‐fitting to the asteroid's spectral characteristics (Perna et al. ; Kitazato et al. ).…”

“…; Perna et al. ; Kitazato et al. ), or even the OSIRIS‐REx mission to B‐type asteroid (101955) Bennu (Clark et al.…”