Radar and infrared observations of binary near-Earth Asteroid 2002 CE26

We observed near-Earth Asteroid (NEA) 2002 CE26 in August and September 2004 using the Arecibo S-band (2380-MHz, 12.6-cm) radar and NASAʹs Infrared Telescope Facility (IRTF). Shape models obtained based on inversion of our delay-Doppler images show the asteroid to be 3.5 ± 0.4   km in diameter and spheroidal; our corresponding nominal estimates of its visual and radar albedos are 0.07 and 0.24, respectively. Our IRTF spectrum shows the asteroid to be C-class with no evidence of hydration. Thermal models from the IRTF data provide a size and visual albedo consistent with the radar-derived estimate. We estimate the spin-pole to be within a few tens of degrees of λ = 317 ° , β = − 20 ° . Our radar observations reveal a secondary approximately 0.3 km in diameter, giving this binary one of the largest size differentials of any known NEA. The secondary is in a near-circular orbit with period 15.6 ± 0.1   h and a semi-major axis of 4.7 ± 0.2   km . Estimates of the binary orbital pole and secondary rotation rate are consistent with the secondary being in a spin-locked equatorial orbit. The orbit corresponds to a primary mass of M = 1.95 ± 0.25 × 10 13   kg , leading to a primary bulk density of ρ = 0.9 + 0.5 / − 0.4   g cm −3 , one of the lowest values yet measured for a main-belt or near-Earth asteroid.