Large Halloween asteroid at lunar distance

The near-Earth asteroid (NEA) 2015 TB 145 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 - G 12 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.