Thermal infrared and optical observations of near-Earth asteroids.

Abstract

Increased physical characterisation of Near-Earth Asteroids (NEAs) is important for understanding their origin and evolution, the links between meteorites and their parent bodies, and for assessing the impact hazard. NEAs are also representative of small main belt asteroids. Optical observations of 13 NEAs taken at the Jacobus Kapteyn Telescope in 2001 and 2002 are presented, from which composite lightcurves, mean magnitudes, absolute visual magnitudes, rotation periods and lightcurve amplitudes are derived. Thermal infrared photometry and spectrophotometry of 10 NEAs taken at the United Kingdom Infrared Telescope (UKIRT) in March and September 2002 are presented. The Standard Thermal Model (STM), Fast Rotating Model (FRM) and Near-Earth Asteroid Thermal Model (NEATM) have been fitted to the measured fluxes to derive geometric albedos (pv), effective diameters (Deff) and beaming parameters (rj). The NEATM assumes zero thermal emission on the night side of an asteroid, affecting the best-fit rj, overestimating Deff and underestimating pv at large phase angles. The Night Emission Simulated Thermal Model (NESTM) is introduced. NESTM models the night side temperature (Tnight) as an iso-latitudinal fraction if) of the maximum day side 1/4temperature (Tmax calculated for NEATM with rj = 1): Tnight = fTmax cos (f), where (j) is the latitude. A range of / is found for different thermal parameters, which depend on the surface thermal inertia (T). NESTM is tested on thermal IR fluxes generated from simulated asteroid surfaces with different T. NESTM, NEATM and radar diameters are compared and it is found that NESTM removes a systematic bias of NEATM that overestimates asteroid diameters. From these tests, it is suggested that a version of the NESTM which assumes T = 200 J m-2 s-1/2 K-1 (f~ 0.6) is adopted as a default model when the solar phase angle is greater than 45°