Hayabusa-2 mission target asteroid 162173 Ryugu (1999 JU₃): Searching for the object’s spin-axis orientation^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, Postfach 1312, 85741 Garching, Germany
e-mail: tmueller@mpe.mpg.de
² Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
³ Department of Physics and Astronomy, Seoul National University, Gwanak, 151-742 Seoul, Korea
⁴ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁵ Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, 305-348 Daejeon, Korea
⁶ European Space Astronomy Centre (ESAC), European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
⁷ Center for Planetary Science, Graduate School of Science, Kobe University, 7-1-48, Minatojima-Minamimachi, Chuo-Ku, 650-0047 Kobe, Japan
⁸ Konkoly Observatory, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences ; Konkoly Thege 15-17, 1121 Budapest, Hungary
⁹ IMCCE, Observatoire de Paris, UPMC Paris-06, Université Lille1, UMR 8028 CNRS, 77 Av. Denfert Rochereau, 75014 Paris, France
¹⁰ Laboratoire Lagrange, UNS-CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire-CS 34229, 06304 Nice Cedex 4, France
¹¹ Earth and Planetary Science Department & Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996, USA
¹² Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, 229-8510 Kanagawa, Japan
¹³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 65, Cambridge, MA 02138-1516, USA
¹⁴ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Honjo 3037-5, Kamogata, Asakuchi, 719-0232 Okayama, Japan
¹⁵ Carnegie Observatories, Las Campanas Observatory, Casilla 60, La Serena, Chile
¹⁶ Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Rd., Laurel, MD 20723, USA
¹⁷ Northern Arizona University, Department of Physics and Astronomy, Bldg. 19, Rm. 209, Flagstaff, AZ 86011, USA
¹⁸ Bisei Spaceguard Center, Japan Spaceguard Association, 1716-3 Okura, Bisei-cho, Ibara, 714-1411 Okayama, Japan
¹⁹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²⁰ Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
²¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany

Received: 17 June 2016
Accepted: 25 October 2016

Abstract

The JAXA Hayabusa-2 mission was approved in 2010 and launched on December 3, 2014. The spacecraft will arrive at the near-Earth asteroid 162173 Ryugu (1999 JU₃) in 2018 where it will perform a survey, land and obtainsurface material, then depart in December 2019 and return to Earth in December 2020. We observed Ryugu with the Herschel Space Observatory in April 2012 at far-infrared thermal wavelengths, supported by several ground-based observations to obtain optical lightcurves. We reanalysed previously published Subaru-COMICS and AKARI-IRC observations and merged them with a Spitzer-IRS data set. In addition, we used a large set of Spitzer-IRAC observations obtained in the period January to May, 2013. The data set includes two complete rotational lightcurves and a series of ten “point-and-shoot” observations, all at 3.6 and 4.5 μm. The almost spherical shape of the target together with the insufficient lightcurve quality forced us to combine radiometric and lightcurve inversion techniques in different ways to find the object’s spin-axis orientation, its shape and to improve the quality of the key physical and thermal parameters. Handling thermal data in inversion techniques remains challenging: thermal inertia, roughness or local structures influence the temperature distribution on the surface. The constraints for size, spin or thermal properties therefore heavily depend on the wavelengths of the observations. We find that the solution which best matches our data sets leads to this C class asteroid having a retrograde rotation with a spin-axis orientation of (λ = 310°−340°; β = −40° ± ~ 15°) in ecliptic coordinates, an effective diameter (of an equal-volume sphere) of 850 to 880 m, a geometric albedo of 0.044 to 0.050 and a thermal inertia in the range 150 to 300 J m^-2 s^-0.5 K^-1. Based on estimated thermal conductivities of the top-layer surface in the range 0.1 to 0.6 W K^-1 m^-1, we calculated that the grain sizes are approximately equal to between 1 and 10 mm. The finely constrained values for this asteroid serve as a “design reference model”, which is currently used for various planning, operational and modelling purposes by the Hayabusa-2 team.