Radar is a uniquely powerful source of information about asteroid physical properties and orbits. Measurements of the distribution of echo power in time delay (range) and Doppler frequency (radial velocity) constitute two-dimensional images that can provide spatial resolution finer than 10 meters if the echoes are strong enough. With adequate orientational coverage, such images can be used to construct geologically detailed three-dimensional models, to define the rotation state precisely, and to constrain the object's internal density distribution. Moreover, radar wavelengths are sensitive to near-surface bulk density and structural scales larger than a few centimeters.
Radar-derived shape models of asteroids open the door to a wide variety of theoretical investigations that previously have been impossible or have used simplistic models (spheres or ellipsoids). For example, with detailed models of real objects, it is possible to explore the evolution and stability of close orbits, with direct application to the design of spacecraft rendezvous and landing missions, to studies of retention and redistribution of impact ejecta, and to questions about the origin and lifetimes of asteroidal satellites. Such models also allow realistic investigations of the effects of collisions in various energy regimes on the object's rotation state, surface topography, regolith, and internal structure.
Delay-Doppler measurements are orthogonal to optical angle measurements, typically have a fractional precision between 10-5 and 10-9, and consequently are invaluable for refining orbits and prediction ephemerides. A single radar detection secures the orbit well enough to prevent "loss" of newly discovered asteroids, shrinking the instantaneous positional uncertainty by orders of magnitude with respect to an optical-only orbit. In the future, radar could make the difference between knowing that an object will "pass within several Earth-Moon distances of Earth" and knowing whether or not it will hit the Earth. During the past decade, observations of newly discovered asteroids have revealed errors from ~100 km to ~100,000 km in pre-radar range predictions.
A defining feature of radar astronomy is human control of the transmitted signal used to illuminate the target. While virtually every other astronomical technique relies on passive measurement of reflected sunlight or naturally emitted radiation, radar uses coherent illumination whose time/frequency structure and polarization state are designed by the scientist. The general stratagem of a radar observation is to transmit a signal with very well-known characteristics and then, by comparing the echo to the transmission, deduce the target's properties. Hence, the observer is intimately involved in an active observation and, in a very real sense, performs a controlled experiment on the target.
The world's two primary facilities used for planetary radar astronomy are the National Astronomy and Ionosphere Center's Arecibo Observatory in Puerto Rico and NASA's Goldstone Solar System Radar (part of the Deep Space Network) in California. Arecibo has twice the range and can see three times the volume of Goldstone, while Goldstone, whose greater steerability provides twice the sky coverage and much longer tracking times, serves a complementary role. The impact of the Arecibo upgrade on studies of small bodies is likely to be far-reaching. During its first decade of operation, the instrument should provide several-hundred-pixel images of ~100 mainbelt asteroids and several-thousand-pixel images of ~50 near-Earth asteroids. Currently, Arecibo can barely skim the main belt's inner edge, but the upgraded telescope will have access to asteroids throughout the belt.
Proposed programs to discover Earth-orbit-crossing asteroids (ECAs) probably could find ~100,000 objects at least as large as several tens of meters. The rationale for such a search is compelling on many grounds; for example, ECAs include the cheapest destinations of robotic or piloted space missions beyond the Earth-Moon system. Most of the optically discoverable Earth-crossing asteroids will traverse the Arecibo/Goldstone detectability window at least once every few decades. Eventually, the initial radar reconnaissance of a new Earth-approaching world may become an almost daily event.
Click here for a reprint of:
Ostro, S. J. Radar observations of Earth-approaching asteroids. Engineering and Science 60, No. 2, pp. 24-31 (1997).
Dr. Steven J. Ostro
Jet Propulsion Laboratory
Pasadena, CA 91109-8099