REMOTE SENSING OF ASTEROIDS FROM EARTH ANDSPACE. L. A. McFadden NASA, Goddard Space Flight Center.
E-mail: lucy.mcfadden@nasa.gov.
Introduction:The story of the asteroid belt and its role information of our solar system is very different now than it was
about two decades ago thanks to the complementary information
provided by ground- and space-based studies including recent
sample return. It was 1991 when the Galileo spacecraft made thefirst asteroid flyby of 951 Gaspra (1). The return of samples
directly from 25143 Itokawa (2) in 2010 resulted in 9 orders ofmagnitude increase in the scale at which asteroids are studied!
Orbital Evolution: Asteroid orbital evolution is studied bycomputer simulation (3). When xenolithic material is found in
returned samples we know that dynamical mixing has occurred.
This information constrains models of orbital evolution. Evidence
from comets suggests mixing across the entire solar system (4).
In Situ Studies: Space-based studies numbering 10’s ofasteroids, takes us into the realm of geology. The position of aspacecraft in orbit around an asteroid provides the most precise
mass. Images are the raw data for shape models. With accuratemass and volume, the bulk density is determined contraining bulk
composition and internal structure. Geological context is derived
from surface morphology. Cratering is ubiquitous and crustal
stresses produce ridges or graben placing limits on strength.
Further, surface compositional information is determined fromspectral reflectance measurements spanning ultraviolet to infrared
wavelengths. Some information about mineralogy, surface
texture and scattering properties become known for the top few
microns of regolith (5). Gamma ray and neutron detectors probe alittle deeper and reveal elemental abundances and ratios for O,Mg, Al, Si, Fe, K, Ca, among the rock-forming elements and H,C, N, Cl of the volatile elements (6).
Surveys: Thousands of asteroids are characterized by groundand space-based telescopic surveys. Spatial resolution is limited
compared to in situ studies. Radiometric measurements combinedwith visual magnitude result in derived diameters for >100,000
asteroids (7). Lightcurve measurements (8) and radar imaging(9) yield rotation rate, shape and aspect ratios. The intensity ofthe reflected light or radar signal is proportional to the crosssectional area of the asteroid. In situ exploration refines andextends interpretations from telescopic surveys.
Recent Results and Future Plans: Dawn mission tells us theasteroid belt had multiple episodes of violent collisions (10) andconfirms Vesta’s differentiated state. Dynamical models predictthat asteroids originally condensed in both the inner and outersolar system and now reside between Mars and Jupiter (11).
Direct observational evidence of outer solar system material in
the asteroid belt remains to be seen. Sample return missions areexpected to reveal just how mixed the asteroid belt is.
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C.M. et al. 2006 Science, 313:635-640. [5] Adams, J. B. 1974 J.
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