Astronomy and Astrophysics, volume 598A, 63-63 (2017/2-1)
Large Halloween asteroid at lunar distance.
MULLER T.G., MARCINIAK A., BUTKIEWICZ-BAK M., DUFFARD R., OSZKIEWICZ D., KAUFL H.U., SZAKATS R., SANTANA-ROS T., KISS C. and SANTOS-SANZ P.
Abstract (from CDS):
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±0.02mag. A second rotation period of 4.779±0.012h, with an amplitude of the Fourier fit of 0.10±0.02mag, also seems compatible with the available lightcurve measurements. We estimate a V-R colour of 0.56±0.05mag 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 HR=18.6mag (standard H-G calculations) or HR=19.2mag and HV=19.8mag (via the H-G12 procedure for sparse and low-quality data), with large uncertainties of approximately 1mag. 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 250-700J/m2/s0.5/K (retrograde rotation) or above 500J/m2/s0.5/K (prograde rotation). We find that the NEA has a minimum size of approximately 625m, a maximum size of just below 700m, and a slightly elongated shape with a/b~=1.1. The best match to all thermal measurements is found for: (i) thermal inertia Γ=900J/m2/s0.5/K; Deff=644m, pV=5.5% (prograde rotation with 2.939 h); regolith grain sizes of ~=50-100mm; (ii) thermal inertia Γ=400J/m2/s–0.5/K; Deff=667m, pV=5.1% (retrograde rotation with 2.939h); regolith grain sizes of ~=10-20mm. A near-Earth asteroid model (NEATM) confirms an object size well above 600m (best NEATM solution at 690m, 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.