The optical brightness of an NEO also provides a very rough estimate of its equivalent diameter, as noted in Chapter 3. For example, the albedo (reflectivity) of the NEO must be known or assumed in order for its size to be estimated. The variations in albedo from one NEO to another (Binzel et al., 2002) are such that the average assumed value leads, in “extreme” cases, to an uncertainty in diameter of about a factor of two. Furthermore, because small asteroids can be irregular in shape, it is possible to get a biased idea about the size of a small asteroid if it is observed on only one or two occasions from atypical vantage points as the asteroid rotates.
Radar investigations are exploring the physical properties of individual NEOs, including their sizes, shapes, surface roughness, rotation periods, and rotation pole orientations, as well as whether they have satellites. In addition, time variations of brightness as NEOs spin (“light curves”) are being used to identify body shapes, rotation periods, pole orientation, and the presence of satellites.
The change in the amount of light reflected by an NEO as a function of wavelength (color) of the light provides information on the composition of the NEO. Such “spectra” range in precision from the measurement of the brightness in a few broad wavelength bands, a technique permitting a classification of NEOs into a small number of groups of similar composition, to studies that acquire brightness information over a large number of narrow wavelength intervals. Such spectra can be compared to suites of laboratory spectra of meteorites and minerals to accurately determine the composition of the surface of an NEO. Which technique can be used is determined by the brightness of the object, the size of the telescope used for observation, and the time devoted to such observations. Classification and detailed spectral studies have begun to yield information on the types of minerals present in these objects, which thus lends qualitative insights into their physical strengths, internal structures, and bulk densities.
NEOs are more challenging to observe than are planets and their moons. NEOs tend to come into telescopic range for only short times (approximately a few days to weeks) and they often appear either low in the sky, along the star-crowded Milky Way, or during times when the Moon creates background light; conditions at discovery are thus not always optimal for detailed characterization efforts. Nevertheless, the best opportunity to characterize a given NEO occurs when it is optically bright during close Earth approaches, often when the NEO is discovered. Because the telescopes used to discover NEOs spend their time searching for them, follow-up observations for characterization must be done by other telescopes that can afford to devote the necessary time to this effort. However, few optical telescope facilities routinely provide observing time for the physical characterization of NEOs. (Radar characterizations of NEOs are discussed below.) Even these few efforts are not well coordinated. Therefore, many observable NEOs are not characterized in the detail necessary to allow the development of a better understanding of these objects as a population or the study of the individual objects that present the greatest threats to Earth.
Finding: The best opportunities for the physical characterization of most NEOs occur during close Earth approaches when these objects are optically bright. Existing programs of ground-based optical observations for the characterization of NEOs are few in number and are not coordinated among different observing teams. Many observable NEOs are not characterized.
Radar observations are complementary to optical measurements. The power of radar derives principally from the precision of its measurements: In optimum conditions, radar can determine the distance (“range”) to a target many millions of kilometers away with approximately 10-meter accuracy, and simultaneously measure speed in the direction toward Earth (“radial velocity”) to within 1 millimeter per second, while optical techniques locate the object’s angular position in the sky to about a few tenths of a second of arc (the angle formed by a penny viewed face on from about 15 kilometers away) under the best conditions. Both radar-derived range and velocity data and optically derived angular positions are used to estimate the orbit, which enables the computation of past and future trajectories.
Optical data alone, taken over a span of a few days after an asteroid is discovered, typically yield orbital predictions whose accuracy in distance and radial velocity can be improved by factors of up to several thousand when combined with radar data from the same interval. This rapid improvement provides an early and accurate assessment of future threat and is one of the most important roles for radar observation of NEOs. Radar observa-