Thermal infrared observations of near-Earth asteroid 2002 NY40

Abstract

We obtained N-band observations of the Apollo asteroid 2002 NY40 during its close Earth fly-by in August 2002 with TIMMI2 at the ESO 3.6 m telescope. The photometric measurement allowed us to derive a radiometric diameter of 0.28+/-0.03 km and an albedo of 0.34+/-0.06 through the near-Earth asteroid thermal model (NEATM) and a thermophysical model (TPM). The values are in agreement with results from radar data, visual and near-IR observations. In this first comparison between these two model approaches we found that the empirical NEATM beaming parameter $η$=1.0 corresponds to a thermal inertia values of about 100 $\mathrm{J m^{-2} s^{-0.5} K^{-1}}$ for a typical range of surface roughness, assuming an equator-on viewing angle. Our TPM analysis indicated that the surface of 2002 NY40 consists of rocky material with a thin or no dust regolith. The asteroid very likely has a prograde sense of rotation with a cold terminator at the time of our observations. Although both model approaches can fit the thermal spectra taken at phase angles of 22$^{\circ}$ and 59$^{\circ}$, we did not find a consistent model solution that describes all pieces of photometric and spectroscopic data. In addition to the 2002 NY40 analysis, we discuss the possibilities to distinguish between different models with only very few photometric and/or spectroscopic measurements spread over a range of phase angles.