Physical characterization of the potentially-hazardous high-albedo Asteroid (33342) 1998 WT₂₄ from thermal-infrared observations

Abstract

The potentially-hazardous asteroid (33342) 1998 WT₂₄ approached the Earth within 0.0125 AU on 2001 Dec. 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT₂₄ stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT₂₄ obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35 ± 0.04 km for the effective diameter, 0.56 ± 0.2 for the geometric albedo, pv, and 100 – 300 J m^-2 s^-0.5 K^-1 for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT₂₄. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β = -52°, λ = 355° (J2000), but we caution that this is uncertain by several tens of degrees.