The near earth asteroid rendezvous mission to asteroid 433 Eros: A milestone in the study of asteroids and their relationship to meteorites

A milestone in the study of asteroids occurred on 14 Feb 2000, when the NEAR spacecraft entered orbit around the asteroid 433 Eros for a year of detailed observation of the geology, mineralogy and chemistry of the surface, before landing on the surface on 12 Feb 2001 and conducting the first science on the surface of a small Solar System body. This paper reviews the scientific results of THE NEAR MISSION. Eros is irregularly shaped, can be described by a best fit ellipse measuring 35 by 10.2 by 10.2 km, with a complex surface that includes a global regolith, asymmetrically distributed population of boulders, heavily cratered surface at diameters above 200 m, and grooves and ridges thought to record broad scale tectonic deformation. Significant albedo differences are associated with steeply sloping surfaces, indicating downslope movement of "relatively dark" regolith, leaving behind "fresher," "less-altered" material. The spectrally-derived mineral compositions and olivine: pyroxene ratio, absence of a global magnetic field, and the Mg/Si, Al/Si and Ca/Si ratios and K concentrations point to a primitive, chondritic body (most likely an ordinary chondrite), while a strong apparently-global depletion in sulfur and lower Fe/Si and Fe/O ratios at the landing site relative to ordinary chondrites are likely due to metal and sulfide migration due to partial melting throughout the entire asteroid or, more likely, through processing of the regolith layer by micrometeorite bombardment, sputtering and/or size/density segregations. Synthesis of the spectral and chemical data suggest the most likely meteoritic analog is either an ordinary chondritic composition that experienced alteration at the surface or less likely a primitive achondrite formed from an ordinary chondritic precursor. The markedly different compositions from the X-ray and gamma-ray experiments can be reconciled with a common silicate mineralogy through variable abundances of metal and/or sulfide. As we begin our detailed exploration of the diverse group of small bodies that make up the asteroid belt, NEAR has provided both a technical and scientific blueprint for the future.