** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.

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    Progress in Earth and Planetary Science

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    Space and planetary sciences

    Asteroid Ryugu before the Hayabusa2 encounter

    Wada K et al.Wada K, Grott M, Michel P, Walsh K, Barucci A, Biele J, Blum J, Ernst C, Grundmann J, Gundlach B, Hagermann A, Hamm M, Jutzi M, Kim M, Kührt E, Le Corre L, Libourel G, Lichtenheldt R, Maturilli A, Messenger S, Michikami T, Miyamoto H, Mottola S, Müller T, Nakamura A, Nittler L, Ogawa K, Okada T, Palomba E, Sakatani N, Schröder S, Senshu H, Takir D, Zolensky M and International Regolith Science Group (IRSG) in Hayabusa2 project

    Hayabusa2, Ryugu, Asteroids, Regolith, Physical properties

    Contour plot of thermal inertia (in units of J m−2 s−1/2 K−1) as a function of typical particle diameter and porosity (see text for the regolith parameters assumed in the calculation). The gray-filled region indicates the thermal inertia range estimated for Ryugu by ground-based observations (Müller et al. 2017). From this figure, we can estimate a typical particle size for the Ryugu regolith.

    Asteroid (162173) Ryugu is the target object of Hayabusa2, an asteroid exploration and sample return mission led by Japan Aerospace Exploration Agency (JAXA). Ground-based observations indicate that Ryugu is a C-type near-Earth asteroid with a diameter of less than 1 km, but the knowledge of its detailed properties is very limited prior to Hayabusa2 observation. This paper summarizes our best understanding of the physical and dynamical properties of Ryugu based on ground-based remote sensing and theoretical modeling before the Hayabusa2’s arrival at the asteroid. This information is used to construct a design reference model of the asteroid that is used for the formulation of mission operation plans in advance of asteroid arrival. Particular attention is given to the surface properties of Ryugu that are relevant to sample acquisition. This reference model helps readers to appropriately interpret the data that will be directly obtained by Hayabusa2 and promotes scientific studies not only for Ryugu itself and other small bodies but also for the solar system evolution that small bodies shed light on.