Large Halloween asteroid at lunar distance^⋆,⋆⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstraße, 85741 Garching, Germany
e-mail: tmueller@mpe.mpg.de
² Astronomical Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
³ Instituto de Astrofísica de Andalucía (CSIC) C/ Camino Bajo de Huétor, 50, 18008 Granada, Spain
⁴ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ Konkoly Observatory, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary

Received: 25 August 2016
Accepted: 20 October 2016

Abstract

The near-Earth asteroid (NEA) 2015 TB₁₄₅ had a very close encounter with Earth at 1.3 lunar distances on October 31, 2015. We obtained 3-band mid-infrared observations of this asteroid with the ESO VLT-VISIR instrument covering approximately four hours in total. We also monitored the visual lightcurve during the close-encounter phase. The NEA has a (most likely) rotation period of 2.939 ± 0.005 h and the visual lightcurve shows a peak-to-peak amplitude of approximately 0.12 ± 0.02 mag. A second rotation period of 4.779 ± 0.012 h, with an amplitude of the Fourier fit of 0.10 ± 0.02 mag, also seems compatible with the available lightcurve measurements. We estimate a V−R colour of 0.56 ± 0.05 mag from different entries in the MPC database. A reliable determination of the object’s absolute magnitude was not possible. Applying different phase relations to the available R-/V-band observations produced H_R = 18.6 mag (standard H-G calculations) or H_R = 19.2 mag and H_V = 19.8 mag (via the H-G₁₂ procedure for sparse and low-quality data), with large uncertainties of approximately 1 mag. We performed a detailed thermophysical model analysis by using spherical and partially also ellipsoidal shape models. The thermal properties are best explained by an equator-on (±≈30°) viewing geometry during our measurements with a thermal inertia in the range 250–700 J m^-2 s^-0.5 K^-1 (retrograde rotation) or above 500 J m^-2 s^-0.5 K^-1 (prograde rotation). We find that the NEA has a minimum size of approximately 625 m, a maximum size of just below 700 m, and a slightly elongated shape with a/b ≈ 1.1. The best match to all thermal measurements is found for: (i) thermal inertia Γ = 900 J m^-2 s^-0.5 K^-1; D_eff = 644 m, p_V = 5.5% (prograde rotation with 2.939 h); regolith grain sizes of ≈50–100 mm; (ii) thermal inertia Γ = 400 J m^-2 s^-0.5 K^-1; D_eff = 667 m, p_V = 5.1% (retrograde rotation with 2.939 h); regolith grain sizes of ≈10–20 mm. A near-Earth asteroid model (NEATM) confirms an object size well above 600 m (best NEATM solution at 690 m, beaming parameter η = 1.95), significantly larger than early estimates based on radar measurements. In general, a high-quality physical and thermal characterisation of a close-encounter object from two-week apparition data is not easily possible. We give recommendations for improved observing strategies for similar events in the future.