tions, when feasible to make (see below), can extend reliable orbit prediction by centuries and, for threatening objects, can distinguish between a potential hit and miss much sooner than is possible with optical observations alone. For objects observed only when discovered, radar has added an average of 300 years to the interval over which accurate orbit prediction is possible (Ostro and Giorgini, 2004). Even for objects observed for many years, radar distance and radial velocity measurements can reduce uncertainties significantly and improve NEO orbits (Ostro and Giorgini, 2004).
A radar telescope is not an instrument that can be used to discover NEOs; however, it requires that the orbits be known well enough to “point” the telescope in four dimensions.1 It is a powerful tool for rapidly improving the knowledge of the orbit of a newly found object and thus for characterizing its potential hazard to Earth. In addition to orbit improvement, the interaction of radar signals with the surface of the NEO yields information about its physical characteristics. For example, radar observations can be used to estimate the roughness of the top several tens of centimeters of a NEO’s surface. Radar reflectivity measurements can distinguish between stony and metallic compositions and may be used to estimate the porosity of NEO surfaces.
Understanding asteroid composition is important for developing mitigation techniques. Radar observations have been used not only to estimate asteroid compositions but also to distinguish smoothly rotating from tumbling asteroids, as well as objects that appear to be monolithic fragments broken off from an originally larger parent. Some targets appear to be weakly bound “rubble piles,” while others display either spheroidal, highly elongated, or irregular shapes.
Similarly, radar observations yield direct information as to whether the NEO has a satellite and, if so, provide data about the size, rotation, and surface scattering properties of each member. In many cases where the echo is strong enough, radar may provide detailed images of an asteroid’s shape at both large and small scales (Figures 4.1, 4.2, and 4.3).
When observations of many rotational phases and geometrical aspects can be obtained, radar images can be used to reconstruct an asteroid’s size, shape, and spin state with a level of detail otherwise obtainable only by a spacecraft rendezvous. An asteroid’s shape provides fundamental information on its origin and geologic history and provides clues to its internal structure and bulk porosity. Three-dimensional shapes are available for about 25 NEOs from radar data, while several dozen more are potentially obtainable from data already in hand.
Detailed three-dimensional models open the window to other useful scientific investigations, such as estimated surface slopes and regolith distributions, as well as enabling the advance planning of spacecraft missions in close orbit about an NEO. These investigations may enhance spacecraft navigation and targeting on the NEO and are useful for realistic simulations of impacts and orbit-change scenarios involved in mitigation planning.
The Arecibo Observatory, located near Arecibo, Puerto Rico, is part of the National Astronomy and Ionosphere Center (NAIC) operated by Cornell University under contract with the National Science Foundation (NSF). Its chief feature is a fixed 305-meter-diameter spherical antenna, of which 225 meters are illuminated by radar waves in a way that allows coverage within 20° of directly overhead. Due to its location 18° north of the equator, Arecibo can observe objects between latitudes of −1° and +38°, and about 33 percent of the sy may be observed by allowing Earth’s rotation to move the telescope to point toward the desired celestial target. Arecibo can track an individual object for up to 2.9 hours per day. When combined with its 900 kilowatt (kW) of average transmitting power of waves with a length of 13 centimeters, this system is by far the most sensitive research radar in the world—about 20 times more sensitive than the Goldstone Solar Radar System described below, but at the cost of significantly reduced sky coverage.
Figures 4.1, 4.2, and 4.3 show examples of the quality of imagery that can be obtained with Arecibo’s radar. These images contain thousands of pixels covering the target NEO; their highest resolution greatly exceeds that available from any optical telescope on the ground or in near-Earth space and is matched only by “flyby” and