  Goldstone Radar Research of Near-Earth AsteroidsSteven J. Ostro
Our planet orbits the Sun amidst a swarm of small asteroids whose existence and importance have been realized only during the past decade. Whereas fewer than 300 of these Earth-orbit-crossing asteroids (ECAs) have been found, so far, the population statistics of objects waiting to be discovered have been established, reliably, from the success rate of optical searches and from analysis of the crater-size distribution of craters in the lunar maria. We are convinced that there are more than 1500 ECAs larger than 1 km, more than 100,000 larger than 100 m, and more than 100,000,000 larger than 10 m. These objects come primarily from the main asteroid belt between Mars and Jupiter, but about 10 percent may be inactive comet nuclei and a few may be pieces of the Moon or Mars. Many ECAs might share (or be) the sources of some of our several thousand meteorites, all of which obviously were ECAs before they fell to Earth. The relationships between ECAs, meteorites, mainbelt asteroids, and comets are central to our understanding of the chemical and dynamical history of the solar system. The proximity of ECAs presents a double-edged sword of danger and opportunity. Collisions with ECAs constitute a low probability but ultra-high-consequence hazard: there is a 1/1000 chance that human civilization will be threatened by a catastrophic collision during the next century. On the other hand, ECAs also include the cheapest destinations of robotic or piloted space missions. Potential resources such as industrial metals, precious metals, complex organics, chemically bound water and perhaps subsurface ice motivate the exploration of these objects. Radar is the most powerful ground-based technique for post-discovery investigation of ECAs, for reasons summarized by Table 1. The table illustrates state-of-the-art capability for radar reconnaissance of a fairly large asteroid during a close Earth approach. Radar synthesizes spatial resolution from the distribution of echo power in time delay (range) and Doppler frequency shift (line-of-sight velocity). In principle, it is not hard to resolve a strong target into 10,000 imaging pixels. Moreover, precision of delay-Doppler positional measurements can improve by several orders of magnitude the accuracy of trajectory predictions (e.g., for close Earth approaches or to assist spacecraft navigation).
Considerable echo strength is needed to be able to take advantage of high-resolution waveforms. Nonetheless, radar reconnaissance of even very weak targets has provided unique information about physical properties. At this writing, radar echoes have been detected from 40 near-earth asteroids. Of those, the Goldstone Solar System Radar (GSSR) has detected 25, and has made the initial detection of 16. Whereas sensitivity is the primary factor that limits a radar system's capabilities in astrometry or imaging, it is not the only one. The orientational and directional coverage possible with the fully steerable 70-m antenna at Goldstone greatly enhances shape reconstruction and orbit refinement. Similarly, bistatic systems using the 70-m and 34-m antennas have enormous advantages for ECAs that are close, small, or slowly rotating. For all these reasons, the GSSR is, and will remain, a unique instrument for ECA reconnaissance. The upgraded Arecibo telescope will have twice the range and will see three times the volume of Goldstone, but Goldstone, which is fully steerable, will see twice the plane-of-sky solid angle, and will cover three times more hour angle than Arecibo, which cannot point more than ~20 degrees from the zenith.
In 1995, observations of Golevka (asteroid 6489; provisionally designated 1991 JX) during its closest approach for at least two centuries included delay-Doppler imaging, two-station (Goldstone-VLA) astrometry, and the first intercontinental planetary radar experiments, from Goldstone to Russia and Japan. Less than 0.6 km across, Golevka is the smallest solar system object imaged so far, and its three-dimensional shape is faceted and angular. Most recently, GSSR images of asteroid 1982 TA showed a ~4-km object with a quasi-triangular equatorial cross section. The diversity of ECAs is not surprising, given the likelihood that each has suffered a unique, complex history of collisions. Now that realistic physical models of ECAs are being provided by radar, computer simulations can begin to explore the effects of impacts into these bodies as a function of projectile energy, impact location, and target cohesiveness and tensile strength. At present, the results suggest that ECAs include unconsolidated rubble piles, single cohesive fragments, and a few contact binaries. However, high-resolution radar reconnaissance of a much larger sample of objects is needed. The frequency of ECA radar opportunities is expected to increase dramatically during the next few years, as new optical search programs become operational and more objects are discovered. Bibliography
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