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M. Delbo, A. W. Harris (DLR), R. P. Binzel (MIT), J. K. Davies (JAC)
We report on preliminary results from a new program of thermal IR measurements of near-Earth objects (NEOs) and other small bodies carried out with the 10-m Keck 1 telescope on Mauna Kea, Hawaii. Our goals are to physically characterize the NEO and small body populations by determining reliable albedos and sizes for a significant sample. We also aim to improve the thermal models used for interpreting these types of data.
Knowledge of albedos is vital for the interpretation of optical and near-IR reflectance spectra in terms of mineralogy and the correct taxonomic classification of NEOs. Is the mix of taxonomic types consistent with NEOs arising entirely as fragments from collisions between main-belt asteroids, or do other types of objects (e.g. dormant or extinct comets) contribute significantly to the NEO population? As an example of an object with a likely cometary origin, albeit in the outer solar system, is 1999 LD31. One of our important results is the discovery of a very low albedo (0.03) for 1999 LD31 which has a retrograde orbit. Such a low albedo is a strong argument in favor of its cometary origin.
Previous studies of small bodies in the thermal IR have been limited to a relatively small number of IR-bright (large) objects which are not representative of the population as a whole. With the Keck telescope we are able to study much fainter and smaller objects and thus remove some of the observational bias inherent in earlier studies. Results of fitting thermal models to spectrophotometry in the 5 - 20 micron range will be presented and discussed, and reflectance spectra of the objects, where available, will be discussed in the light of albedos derived.
We show how a first order approximation thermal model like NEATM (a modified Standard Thermal Model) can provide reliable results in most cases. However, neglecting the thermal inertia and the irregular shape of the object (which for small bodies are normally unknown) can lead to erroneous conclusions. Numerical simulations indicate that significant errors can arise in particular in the case of objects observed at large solar phase angles. The uncertainties associated with our approach are discussed.