Asteroid Radar Astronomy

Abstract

Ostro et al.: Asteroid Radar Astronomy Asteroid Radar Astronomy Steven J. Ostro Jet Propulsion Laboratory, California Institute of Technology R. Scott Hudson Washington State University Lance A. M. Benner and Jon D. Giorgini Jet Propulsion Laboratory, California Institute of Technology Christopher Magri University of Maine at Farmington Jean-Luc Margot California Institute of Technology Michael C. Nolan Arecibo Observatory Radar is a uniquely powerful source of information about the physical properties and orbits of asteroids. Measurements of the distribution of echo power in time delay (range) and Dop- pler frequency (radial velocity) produce two-dimensional images that can provide spatial reso- lution as fine as a decameter if the echoes are strong enough. With adequate orientational coverage, such images can be used to construct detailed three-dimensional models, define the rotation state precisely, and constrain the object’s internal density distribution. As of May 2002, radar signatures have been measured for 75 main-belt asteroids (MBAs) and 105 near-Earth asteroids (NEAs). We summarize specific results for radar-detected asteroids, which span 4 or- ders of magnitude in diameter and rotation period. Radar has revealed both stony and metallic objects, principal-axis and complex rotators, smooth and extremely rough surfaces, objects that must be monolithic and objects that probably are not, spheroids and highly elongated shapes, contact-binary shapes, and binary systems. Radar also has expanded accurate orbit-prediction intervals for NEAs by as much as several centuries. INTRODUCTION One of the goals of this book is to outline developments since Asteroids II (Binzel et al., 1989), which was completed in 1988. The subsequent 13 years have seen critical devel- opments in technical aspects of asteroid radar astronomy, including increases in sensitivity and versatility of tele- scopes, the evolution and optimization of observational techniques, and the invention of analytical methods to opti- mize extraction of information from radar images. Observa- tions commensurate with those developments have raised the number of radar-detected asteroids from 52 (19 NEAs + 33 MBAs) in mid-1988 (Ostro, 1989) to 180 (105 NEAs + 75 MBAs) in May 2002 and have produced an enormous body of information about the physical properties of aster- oids. An equally significant development since Asteroids II is an increase in the number of persons lead-authoring aster- oid radar papers at an average rate of one person per year. The following sections outline the technical develop- ments and observational highlights of the past 13 years, summarize the major conclusions drawn from the explo- sive increase in data, and discuss current problems and chal- lenges to be faced during the coming decade. See Ostro (2002a) for a list of radar-detected asteroids, Ostro (1993, 2002b) for reviews of planetary radar principles and tech- niques, Ostro (1994) for a discussion of radar’s role in NEO hazard identification and mitigation, and Harmon et al. (1999) for a review of radar observations of comets. 2. STRATEGIES, TELESCOPES, AND TECHNICAL DEVELOPMENTS Telescopes and Observing Strategies The basic strategy of an asteroid radar experiment is to measure the distribution of echo power in time delay and Doppler frequency, usually in the opposite sense (OC) of circular polarization as transmitted as well as in the same sense (SC) as a function of the object’s orientation and plane-of-sky (POS) direction. SC echo would be absent in