Shape of Asteroid 433 Eros from Inversion of Goldstone Radar Doppler Spectra

✍ Scribed by David L. Mitchell; R.Scott Hudson; Steven J. Ostro; Keith D. Rosema

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 571 KB
• Volume: 131
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1006/icar.1997.5815

✦ Synopsis

INTRODUCTION We use new analysis techniques to constrain the shape of

Asteroid 433 Eros is the second largest member of the 433 Eros with Goldstone radar data obtained during the asteroid's close approach in 1975. A previous analysis of these data near-Earth population and also one of the most elongated.

(Ostro, Rosema, and Jurgens, 1990, Icarus 84, 334-351) used Only the Earth-crosser 1620 Geographos, roughly oneestimates of the echo's spectral edge frequencies as a function tenth the size of Eros, is known to be more elongated of asteroid rotation phase to constrain the convex envelope of (Ostro et al. 1995). Much of our current knowledge of Eros Eros' pole-on silhouette. Our approach makes use of the echo's is based on extensive observations made when the asteroid full Doppler-frequency distribution (effectively ȁ15 times more passed within 0.15 AU of Earth in 1975(Zellner 1975).

echo data points) and is thus capable of constraining shape

Initial results from those observations were reported in a characteristics, such as concavities, within this convex envelope. special issue of Icarus (Vol. 28, 1976).

The radar echoes are weak and north-south ambiguous, which limits the accuracy of our models. We present two different Eros' visual lightcurves have amplitudes as large as 1.5 approaches, perturbations to an ellipsoid and successive apmagnitudes, with two distinct maxima and two distinct proximations, that help to quantify the model uncertainties and minima, which occur in the order Max 1, Min 1, Max 2, identify features that are likely to be real. Both approaches Min 2, in the convention of Millis et al. (1976). During yield models that are tapered along their lengths, with one or the 1975 apparition, Max 1 was the brighter, or primary, more prominent concavities on one side but not the other. We maximum for solar phase angles less than about 40Њ. Sev-

not have sufficient information to determine the exact nature eral lightcurve analyses have yielded estimates of Eros' of the concavities, and in particular, whether they are craters, pole direction and elongation in the context of axisymmetroughs, or bends in Eros' overall shape. The pole-on silhouette of the successive approximation model is shaped like a kidney tric models (Magnusson 1989 and references therein). bean, which resembles a nearly pole-on optical image derived Eros' elongation is also evident in 3.5-cm Goldstone radar from speckle interferometry (Drummond and Hege, 1989, in echoes (Jurgens and Goldstein 1976); as the asteroid ro-Asteroids II (R. P. Binzell, T. Gehrels, and M. S. Matthews, tates, the echo's Doppler bandwidth varies by about a Eds.), pp. 171-191, Univ. of Arizona Press, Tucson); however, factor of two. A consensus model ellipsoid (Zellner 1976), we cannot exclude shapes, such as the perturbation model, with based primarily on the 1975 lightcurve and radar observamore than one large concavity. Variations in the pyroxene/ olivine ratio over Eros' surface have been inferred from visual tions, has dimensions 36 ϫ 15 ϫ 13 km and a pole direction and infrared observations (Murchie and Pieters, 1996, J. Geowithin a few degrees of ecliptic coordinates ϭ 16Њ and phys. Res. 101, 2201-2214). Correlating these variations with ͱ ϭ ϩ11Њ.

our shape information, we find that the side with concavities The 1976 consensus ellipsoid provides a reasonable first is relatively px-rich compared with the more rounded opposing approximation to Eros' shape, but it is known to be simplisside.