The Asteroid-Meteorite Connection: The Discovery of a Main Belt Ordinary Chondrite Asteroid

Details The Asteroid-Meteorite Connection: The Discovery of a Main Belt Ordinary Chondrite Asteroid The Asteroid-Meteorite Connection: The Discovery of a Main Belt Ordinary Chondrite Asteroid

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.324b&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 324

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Asteroid Belt, Kirkwood Gaps, Ll Chondrites, Ordinary Chondrites, Parent Bodies

Scientific paper

Although ordinary chondrites dominate the observed flux of meteorites, a main-belt asteroid source has remained elusive. Two hypotheses have been proposed to account for the lack of observed ordinary chondrite asteroids: space weathering alters the surface of ordinary chondrite bodies and disguises their spectral characteristics; ordinary chondrite bodies exist only among small and previously unsampled asteroids. To test the latter hypothesis, the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) was initiated using a CCD spectrograph attached to the 2.4-m Hiltner telescope of the Michigan-Dartmouth-MIT (MDM) Observatory, located at Kitt Peak, AZ. With this instrumentation, routine spectral measurements are possible down to V magnitude 18, corresponding to main-belt asteroid diameters <10 km. To date, SMASS measurements have been obtained for 50 small asteroids (diameters 5-20 km) located within +- 0.05 AU of the 3:1 resonance Kirkwood gap, believed to be a dynamically viable source region for meteoritic material. We report the discovery that one of these asteroids, (3628) 1979 WD, has a reflectance spectrum nearly identical to that of an LL6 chondrite. This ordinary chondrite reflection spectrum has been confirmed over optical wavelengths (0.5 to 1.0 microns) through multiple independent measurements during the asteroid's 1991 and 1993 apparitions. In addition, we have recently obtained infrared flux measurements of the asteroid using the new University of Massachusetts NICMOS3 camera (NICMASS) at the MDM Observatory. By utilizing simultaneous optical measurements made at McDonald Observatory, we have achieved a flux calibration between the optical and infrared wavelengths. Results from these measurements further confirm the ordinary chondrite spectrum of asteroid (3628) 1979 WD at wavelengths of 1.26- and 1.64-microns. Although the discovery of this ordinary chondrite asteroid finally establishes the existence of ordinary chondrite material within the main belt, the alternative between space weathering and predominance among small asteroids remains unresolved. Because asteroid 3628 is small (estimated diameter 7 km), it could be a relatively "fresh" fragment that has not been long exposed to the space environment. On the other hand, while this discovery is clearly a success for the small source body hypothesis, the SMASS survey statistics (1 out of 50) apparently indicate that ordinary chondrite material is still relatively rare among 5-20 km objects. A goal for future SMASS work is to push the limits of observation to still smaller sizes.

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