identification is not possible, these data at 24 wavelengths permit the separation of asteroids with far greater discrimination than is possible in the three-color UBV work. The full value of spectral reflectivity studies will only be achieved, however, once spectral reflectivities of many dozens or several hundred asteroids have been studied and once a comprehensive catalog of meteorite and rock spectral reflectivities has been assembled. Both of these goals can be achieved within a couple years, and we hope to make progress in these directions.

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DISCUSSION

BRATENAHL: What is the limit in apparent magnitude your technique can be pushed to?

CHAPMAN: We have had no difficulty measuring several asteroids per night brighter than 12 mag. The problem is with the interesting wavelength interval beyond the response limit of S-20 photomultipliers (~750 nm). With sufficient time on a large telescope it should be possible to measure 15 and 16 mag objects to some precision out to 1050 nm, using an S-1 tube. Still fainter asteroids could be measured shortward of 750 nm provided they could be accurately located with respect to a guide star.

GEHRELS: Lightcurves have been obtained by direct visual setting on a moving asteroid—with the B or V filter, 1 P21 tube, and an integration time of a minute-down to V - 16.5 with a 154 cm telescope; the precision is about +0.004 mag.

JOHNSON: In comparing spectral reflectivity measurements with standard UBV photometry, several things should be kept in mind: (1) our filters have considerably narrower bandpasses; (2) we must observe sequentially in 24 of them instead of one, two, or three; (3) the S-1 surface has a low quantum efficiency (about 0.1 percent compared to ~10 percent for an S-20); and (4) our program requires frequent measurements of standard stars at all 24 wavelengths.

INFERENCES FROM OPTICAL PROPERTIES CONCERNING
THE SURFACE TEXTURE AND COMPOSITION
OF ASTEROIDS

BRUCE HAPKE
University of Pittsburgh

The optical properties of the asteroids are compared with those of the Moon and various terrestrial, lunar, and meteoritic materials. It is concluded that the surfaces of most of the asteroids are covered with at least a thin layer of unconsolidated, fine-grained powder similar to lunar soil. None of the asteroids appear to have compositions corresponding to pure nickel/iron meteorites.

The picture that most of us have in our minds of a typical asteroid is probably of a large, irregularly shaped chunk of iron, unrusted by exposure to oxygen or water, and with a surface kept clean and dust free by the sandblasting effect of repeated micrometeorite impacts. However, consideration of the known optical properties of asteroids suggests a rather different model. It is the purpose of this paper to review briefly the optical characteristics of asteroids and to compare them with other extraterrestrial and terrestrial materials to obtain information concerning the nature of the outer surfaces of the minor planets.

OPTICAL PROPERTIES OF ASTEROIDS

The optical characteristics of the asteroids that this paper will be concerned with include the visual albedo, UBV color indexes, brightness-phase curve, and polarization-phase curve. These properties are summarized in table I and figure 1. In the tables and figures, the following were used: the visual geometric albedo; U – B and B - V, differences between UBV color indexes relative to the Sun; omy/00, the slope of the apparent visual magnitude my versus phase angle a for 5° 3 o' < 25°; the opposition effect defined as the ratio of brightness at a = 1° to that at a = 5°; the value of polarization at the minimum of the polarization-phase curve; or, phase angle at which the minimum occurs; oo, phase angle (other than 0° and 180°) at which the polarization is zero. The data for table I and figure l are taken primarily from the review paper by Gehrels (1970) and from these additional sources: Dollfus (1961), Harris (1961), Miner and Young (1969), Gehrels (1956), Allen (1963),