S-TYPE ASTEROIDS, ORDINARY CHONDRITES, AND SPACE WEATHERING - THE EVIDENCE FROM GALILEOS FLY-BYS OF GASPRA AND IDA

New observations of the S-type asteroids Gaspra and Ida, especially, b y the Galileo spacecraft demonstrate that a ''space weathering'' proce ss operates, which modifies the reflectance spectra of fresh material to be redder, straighter, and have shallower absorption bands. The wea thering process appears similar to but less potent than that observed on the Moon. It operates in the sense that it would tend to convert sp ectra of ordinary chondrites (OC) to having the spectral traits of S-t ype asteroids. These results appear to resolve the major obstacles of the long standing ''S-type conundrum'' about the provenance of ordinar y chondrite meteorites. A wide body of recent research, which is revie wed here, builds on previous meteoritical evidence to support a new, d eveloping consensus that the larger S-type asteroids are a diverse ass ortment of silicaceous assemblages, which includes the ordinary chrond ite parent bodies. Recent fairly realistic laboratory simulations of s pace weathering processes have changed OC spectra to resemble S-type s pectra. J. F. Bell's highly regarded earlier paradigm that OC parent b odies would be found only among sub-10 km main-belt asteroids has been tested by an extensive observational survey by R. Binzel and his prot eges; the result is that no small, main-belt analog for the OCs has be en found, not even the somewhat CC-like object Boznemcova. This articl e reviews the history of the S-type conundrum, which set the stage for Galileo's historic fly-bys. I review the findings about Gaspra and Id a, including results about their geology, their geophysical structure and probable origins, and about Ida's small moonlet, Dactyl. Density c onstraints on Ida set by Dactyl's orbit rule out (for Ida, at least) t he classic view of S-types as metallic, stripped cores of differentiat ed precursor asteroids. New analysis of the Galileo spectral images of Ida is presented that provides strong evidence that space weathering occurs on Ida, and that Ida, and the Koronis family asteroids in gener al, are plausibly CC-like in composition. After reviewing 1990s develo pments on the S-type conundrum, I advocate a new perspective that the ordinary chondrite parent bodies are among the S-type asteroids, a div erse grouping that also contains other silicaceous and silicate/metal assemblages, presumably including various stony-irons and primitive ac hondrites represented in meteorite collections; Gaspra may be such a m etal-rich assemblage.