Visible and Near-Infrared Spectral Observations of 4179 Toutatis

Asteroid 4179 Toutatis had a particularly favorable opposition in the winter of 1992/1993, offering a rare opportunity to study a near-Earth asteroid in detail. On January 4, 1993, we measured the 0.5- to 3.5-μm spectrum of near-Earth asteroid 4179 Toutatis almost simultaneously from three separate telescopes to investigate its surface composition. We combined nearly simultaneous visible photometry, CCD spectra from 0.5 to 0.97 μm, CVF photometry from 1.3 to 2.4 μm, J, H, and K photometry, and photometry of the 3-μm region into a composite visible and near-infrared reflectance spectrum of 4179 Toutatis, in an effort to constrain its surface mineralogy and compositional type. A pyroxene absorption band is seen centered at 1.966 ± 0.012 μm, which implies an average pyroxene composition of Wo_5-10En_50-35Fs_45-55. The absorption band near 1 μm is centered at a wavelength longer than 0.96 μm, which implies some olivine must also be present. At a heliocentric distance of 1.158 AU, the thermal flux component of 4179 Toutatis at wavelengths longer than 3 μm is significant. Modeling the thermal contribution to the photometry in the 3- to 3.5-μm region indicates that the surface of 4179 Toutatis has a high thermal inertia compared to the moon. Although the uncertainties in removing the thermal flux are large, no evidence for surface hydration is seen.