THE NATURE AND ORIGIN OF OUTER SOLAR SYSTEM ASTEROIDS FROM REFLECTANCE SPECTROPHOTOMETRY (CORONAGRAPH).

Abstract

Narrowband spectrophotometric observations of outer solar system (semi-major axis greater than 3.2 AU) asteroids have been used to investigate the surface compositions of the D- and P-class asteroids in an effort to learn about their origins and formation conditions. Spectra of 20 outer solar system asteroids and 2 main-belt D-class asteroids were obtained using two charge-coupled device (CCD) spectrographs. No mineralogical absorption features were evident. The spectra can be divided into four groups based upon four discrete slopes among the spectra. The slope increases (reddens) with increasing heliocentric distance. All of the asteroids observed are locked in orbits driven by Jupiter's gravitational attraction. The distinct slope changes suggest that these asteroids are the remnants of a gradation in composition of planetesimals in the outer solar system, which were selectively retained in location when other material was lost. Three possible compositions based upon organic polymer materials are discussed. A coronograph/spectrograph was designed to be used with a CCD camera for data acquisition. This instrument can be used for both astronomical imaging and spectrophotometry across the 0.46- to 1.0- μm spectral region. Options to reduce the diffracted light from a telescope and block the light from bright astronomical objects permit high resolution imaging of both bright and faint objects. Direct (1:1) or reducing (2.7:1) imaging is available. Two grisms provide ∼40- Å resolution spectral coverage of the full spectral region. The instrument is modular and easily transported to observing sites.