Research Note

Thermo-physical properties of 162173 (1999 JU3), a potential flyby and rendezvous target for interplanetary missions

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: tmueller@mpe.mpg.de
² Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, V Holešovičkách 2, 180 00, Praha 8, Czech Republic
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
⁴ Department of Physics and Astronomy, The University of Western Ontario, 1151 Richimond St. London, ON, N6A 3K7, Canada
⁵ Institute of Astronomy, National Central University, 300 Jhongda RD., Jhongli, Taoyuan 32001, Taiwan
⁶ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, 3037-5 Honjo, Kamogata, Asaguchi, Okayama 719-0232, Japan
⁷ Bisei Spaceguard Center, Japan Spaceguard Association, 1716-3 Okura, Bisei, Ibara, Okayama 714-1411, Japan
⁸ Kiso Observatory, Institute of Astronomy, The University of Tokyo, 10762-30 Mitake, Kiso, Nagano 397-0101, Japan
⁹ Tokyo Metropolitan Government, 2-8-1 Nishishinjyuku, Shinjyuku, Tokyo 163-8001, Japan
¹⁰ Aichi Toho University, 3-11 Heiwagaoka, Meito, Nagoya, Aichi 468-8515, Japan
¹¹ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 183-301, Pasadena, CA 91109, USA
¹² Korea Astronomy and Space Science Institute, 61-1 Hwaam-dong, Yueing-gu, Daejeon, Taejon 305-348, Korea
¹³ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85721-0092, USA
¹⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 17 August 2010
Accepted: 5 November 2010

Abstract

Context. Near-Earth asteroid 162173 (1999 JU3) is a potential flyby and rendezvous target for interplanetary missions because of its easy-to-reach orbit. The physical and thermal properties of the asteroid are relevant for establishing the scientific mission goals and also important in the context of near-Earth object studies in general.

Aims. Our goal was to derive key physical parameters such as shape, spin-vector, size, geometric albedo, and surface properties of 162173 (1999 JU3).

Methods. With three sets of published thermal observations (ground-based N-band, Akari IRC, Spitzer IRS), we applied a thermophysical model to derive the radiometric properties of the asteroid. The calculations were performed for the full range of possible shape and spin-vector solutions derived from the available sample of visual lightcurve observations.

Results. The near-Earth asteroid 162173 (1999 JU3) has an effective diameter of 0.87  ±  0.03 km and a geometric albedo of 0.070  ±  0.006. The χ²-test reveals a strong preference for a retrograde sense of rotation with a spin-axis orientation of λ_ecl =  73°, β_ecl = −62° and P_sid = 7.63 ± 0.01 h. The most likely thermal inertia ranges between 200 and 600 J m^-2 s^-0.5 K^-1, about a factor of 2 lower than the value for 25143 Itokawa. This indicates that the surface lies somewhere between a thick-dust regolith and a rock/boulder/cm-sized, gravel-dominated surface like that of 25143 Itokawa. Our analysis represents the first time that shape and spin-vector information has been derived from a combined data set of visual lightcurves (reflected light) and mid-infrared photometry and spectroscopy (thermal emission).