Variability of Thermal Infrared Emission from Near-Earth Asteroids

We have measured thermal emission between 2 and 4 microns for several near-Earth asteroids (NEAs) of different taxonomic types with SpeX on the NASA IRTF. Initial results for individual P-, V-, and E-type NEAs were presented at last year's meeting (Howell et al., 2008). Here we present results for three S-type NEAs -- 2005 GN59, 1996 HW1, and 2008 SV11 -- that were both observed with radar and show the effect of irregular shapes at different viewing geometries on thermal emission. Although the heliocentric distance and phase angle of 2005 GN59 changed considerably between observations in September and October 2008, the thermal emission remained nearly constant, meaning the set of thermal model parameters determined from the first epoch are unable to accurately predict the thermal emission at a second epoch. Radar observations show 2005 GN59 has a two-lobed, 0.35- by 1.1-km shape and non-convex surface features that clearly affect the thermal emission. For 1996 HW1, itself a two-lobed object about 1.1 by 2.7 km in size and reminiscent of Itokawa (but larger), the thermal emission from September to October 2008 is reduced by more than is expected from either the standard thermal model or a thermophysical model (Lebofsky and Spencer, 1989). We will also discuss 2008 SV11, another two-lobed object roughly 0.3 by 0.6 km in size that was observed in April 2009. These results demonstrate the need for several observations of the same object under different viewing conditions to determine whether rotation phase, illumination effects (solar phase and shadowing), or regolith differences across the irregularly-shaped surface dominate the thermal behavior and whether the dominant property depends on composition, size, and/or shape. Support for this work comes from the National Science Foundation