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Observability of asteroidal or cometary near-Earth object (4015) Wilson-Harrington. Maxima in brightness correspond to closest Earth approach. As indicated by current limits for visible and near-infrared spectroscopy, NEOs such as Wilson-Harrington make infrequent, close approaches that allow measurements of their physical properties with 2-m-class telescopes occasionally available for NEO studies. Previously, the best opportunity for physical measurements often coincided with the discovery apparition, where discovery was enabled by an extremely close approach to Earth. Most modern NEO discoveries by CCD surveys are at magnitudes well below the physical measurement capabilities of telescopes generally available to NEO observers.
Apparition circumstances for known near-Earth objects over the years from 1996 to 2001. Lines on top indicate current limits for physical measurements. Arrows indicate potential new limits for NEO physical measurements arising from NASA's participation in operation of the Keck telescopes.
infrared wavelengths, recognizable spectral absorptions arise from the presence of the silicate minerals pyroxene, olivine, and sometimes feldspar, as well as nickel-iron metal, spinel, primitive carbonaceous assemblages, and organic tholins. Water-bearing minerals such as phyllosilicates also exhibit distinct absorption features at near-infrared wavelengths. Seemingly dormant comets can have their activity revealed through the detection of fluorescence emission bands. Complementing these optical techniques are radar albedo measurements, which give primarily diagnostic information on the presence and abundance of metal phases.
At visible wavelengths, multiple-filter photometry measurements in the 1980s have given way to CCD spectrographs in the 1990s. The transition to CCD spectrographs is occurring more slowly at near-infrared wavelengths, where new instruments with capabilities for mineral spectroscopy are only now becoming available. Currently, visible-spectrum observations are the most common physical measurements being made of near-Earth objects. Access to larger telescopes, which improve the observational limits for visible-spectrum physical measurements, will correspondingly provide the opportunity for compiling a substantial sample of measurements at wavelengths other than visible.
Although the most reliable mineralogical interpretations require measurements extending into the near infrared, measurements in the visible wavelengths allow preliminary characterization according to the taxonomic groups established for main-belt asteroids. Many near-Earth asteroids fall into taxonomic categories over the same range as asteroids in the inner main belt. Most common in the inner asteroid belt and among NEOs are objects