Near Earth Asteroids (NEAs) are fragments of collisions of asteroids from the Main Belt. Characterization of NEAs provides important inputs into models for atmospheric entry, risk assessment and mitigation. The WISE Mission data (cryogenic, 3- or 4-band data) detected a significant number of very low albedo NEAs, which do not show a size-dependence vs. albedo, meaning that their presence in the NEA populations is about 50:50 for diameters ≤1 km and ≥1 km. Often times these dark NEAs are referred to as 'dead comets'. Comet nuclei typically have albedos of 5-6%, higher than the darkest NEAs (albedos as low as 1–3%). So, one wonders, why are these dark NEAs darker than primitive comets? Secondly, cometary nuclei have low thermal inertia, as does the well-studied low albedo NEA Bennu, the target of the OSIRISREx Mission. In suprising contrast, there are some very dark NEAs in the WISE data that also are much cooler than expected for NEAs like Bennu at similar distances and viewing geometries. Cooler NEAs have higher thermal inertias and are thought to be rock-like. Mostly we think of rock-like materials being silicate-bearing. Silicate-bearing asteroids (S-types), however, have much higher albedos (>~10%). We discuss our analyses of NEA 2010 GY6, which is of these relatively dark (albedo 2.8%) and cool (eta=2.3) NEAs from the WISE sample (Mainzer et al. 2011). We address our question: What properties of an NEA allow it to be both dark and cool?
The presence of very low albedo NEAs, in a statistical sense, significantly increases the perceived risk from NEAs. Specifically, an albedo distribution is used to convert the absolute magnitudes (Hmag) distribution into diameters. Effective Diameter is derived from the absolute magnitude using the D(Hmag,pv) relation, D=1329 km * pv^-0.5 * 10^(-Hmag/5) with pv=albedo. Having an NEA population with a greater fraction of low albedo NEAs means a greater relative number of larger bodies for the same observed Hmag distribution. Larger diameters mean larger energies upon entry, and the assessment of risk increases. This highlights the importance of improving our understanding of the members of the NEA population that have very low albedos.