Radar Imaging of Near-Earth and Apollo Asteroids
Lance A. M. Benner, PhD
Jet Propulsion Laboratory
California Institute of Technology
Radar is the most powerful astronomical technique for characterizing near-Earth objects and refining their orbits.
Whereas near-Earth asteroids (NEAs) look like unresolved points through ground-based optical telescopes, the Arecibo and Goldstone radars can image NEAs with resolutions as fine as several meters. These images reveal the object's size, shape, spin state, topography, and multiplicity, i.e., whether or not it is a binary or triple system.
Radar can determine the masses of binary NEAs and in some cases solitary NEAs, and is sensitive to surface roughness, porosity, and metal abundance. Radar has produced the best physical characterization yet of a binary small body.
Radar echoes from NEAs have revealed both stony and metallic objects, featureless spheroids and shapes that are elongated and irregular, objects that must be monolithic pieces of rock and objects that must be unconsolidated rubble piles, small-scale morphology ranging from smoother than the lunar surface to rougher than the rockiest terrain on Earth, objects with craters and linear structures, rotation periods ranging from a few minutes to several weeks, objects whose rotation periods are accelerating, non-principal axis spin states, contact binaries, and binary and triple systems.
Radar is invaluable for refining orbits of potentially hazardous NEAs and is responsible for our most accurate orbits for potentially hazards asteroids. Range-Doppler measurements provide line-of-sight positional astrometry with precision as fine as 10 m in range and 1 mm/s in velocity, with a fractional precision typically 100 to 1000 times finer than with optical measurements. Radar reconnaissance adds decades or centuries to the interval over which we can predict close Earth approaches and dramatically refines collision probability estimates based on optical astrometry alone.
Spacecraft operations close to a small asteroid are extremely difficult due to the complexity of the gravitational environment, which depends on the object's size, shape, spin state, and mass distribution. If it turns out to be necessary to have a sequence of missions beginning with physical reconnaissance and ending with a deflection, then a radar-derived physical model would speed up this process, reduce its cost, decrease complexity in the design and construction of the spacecraft, and improve odds of successful mitigation.
Lance Benner is a Research Scientist at the Jet Propulsion Laboratory, California Institute of Technology, in Pasadena. He specializes in radar imaging of near-Earth asteroids using the Arecibo Observatory (Puerto Rico) and NASA's Goldstone Solar System Radar (California). He has authored more than 50 papers on asteroids and comets and has participated in radar observations of more than 200 near-Earth asteroids. He served on the National Research Council panel that authored the the report "Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies" which was released by the National Academy of Sciences in 2010. Lance received an A. B. in Physics in at Cornell 1987 and a Ph.D. in Earth and Planetary Sciences at Washington University in St. Louis in 1994. He has been at the Jet Propulsion Laboratory since 1995.