  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 with the ESO VLT-VISIR instrument and visual
lightcurves during the close-encounter phase. The NEA has a (most likely)
rotation period of 2.939 +/- 0.005 hours and the visual lightcurve shows a
peak-to-peak amplitude of approximately 0.12+/-0.02 mag. We estimate a V-R
colour of 0.56+/-0.05 mag from MPC database entries. 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 & H_V = 19.8 mag (via the H-G12
procedure), with large uncertainties of approximately 1 mag. We performed a
detailed thermophysical model analysis by using spherical and ellipsoidal shape
models. The thermal properties are best explained by an equator-on (+/- ~30
deg) viewing geometry during our measurements with a thermal inertia in the
range 250-700 Jm-2s-0.5K-1 (retrograde rotation) or above 500 Jm-2s-0.5K-1
(prograde rotation). We find that the NEA has a minimum size of 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 of
900 Jm-2s-0.5K-1; D_eff = 644 m, p_V = 5.5% (prograde rotation); regolith grain
sizes of ~50-100 mm; (ii) thermal inertia of 400 Jm-2s-0.5K-1; D_eff = 667 m,
p_V = 5.1% (retrograde rotation); regolith grain sizes of ~10-20 mm. A
near-Earth asteroid model (NEATM) confirms an object size well above 600 m,
significantly larger than early estimates based on radar measurements. We give
recommendations for improved observing strategies for similar events in the
future.
