Large Halloween asteroid at lunar distance

Publication date: 
Main author: 
Müller, T. G.
IAA authors: 
Duffard, R.;Santos-Sanz, P.
Müller, T. G.;Marciniak, A.;Butkiewicz-Bąk, M.;Duffard, R.;Oszkiewicz, D.;Käufl, H. U.;Szakáts, R.;Santana-Ros, T.;Kiss, C.;Santos-Sanz, P.
Astronomy and Astrophysics
Publication type: 
The near-Earth asteroid (NEA) <ASTROBJ>2015 TB145</ASTROBJ> 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<SUB>R</SUB> = 18.6 mag (standard H-G calculations) or H<SUB>R</SUB> = 19.2 mag and H<SUB>V</SUB> = 19.8 mag (via the H-G<SUB>12</SUB> 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<SUP>-2</SUP> s<SUP>-0.5</SUP> K<SUP>-1</SUP> (retrograde rotation) or above 500 J m<SUP>-2</SUP> s<SUP>-0.5</SUP> K<SUP>-1</SUP> (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<SUP>-2</SUP> s<SUP>-0.5</SUP> K<SUP>-1</SUP>; D<SUB>eff</SUB> = 644 m, p<SUB>V</SUB> = 5.5% (prograde rotation with 2.939 h); regolith grain sizes of ≈50-100 mm; (ii) thermal inertia Γ = 400 J m<SUP>-2</SUP> s<SUP>-0.5</SUP> K<SUP>-1</SUP>; D<SUB>eff</SUB> = 667 m, p<SUB>V</SUB> = 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. Analysis is also based on observations collected at the European Southern Observatory, Chile; ESO, DDT proposal 296.C-5007(A).The data of the visual lightcurves are only available at the CDS via anonymous ftp to <A href=''></A> (<A href=''></A>) or via <A href=''></A>
ADS Bibcode: 
minor planets;asteroids: individual: 2015 TB145;radiation mechanisms: thermal;techniques: photometric;infrared: planetary systems;Astrophysics - Earth and Planetary Astrophysics