The asteroids Hygiea, Europa and Bamberga were selected for the basis of the 100 µm calibration because the analysis of ground based observations (Lebofsky, 1984) indicated that the standard asteroid model was a good fit out to 25 µm and could thus reasonably be expected to fit the longer IRAS wavelengths as well. This expectation was in fact borne out by the IRAS observations.
Several other asteroids were observed with IRAS. These are included in Table VI.C.4 in order to show the dispersion in the method. It should be emphasized that those selected as the basis of the 100 µm calibration were those which a priori fit the "standard asteroid model" described below between 12 and 60 µm.
As a check on the above procedure and the simple assumptions concerning asteroid colors, the observed flux densities for all the asteroids measured already, including the three used in the 60 to 100 µm extrapolation, were compared to calculations based on the "standard asteroid model" of Morrison (1973) and Jones and Morrison (1974). The infrared emissivity of the surface was taken to be 0.9 independent of wavelength and the thermal modeling constant was 0.9; the albedo was taken from the TRIAD file (Zellner, 1979). The temperature distribution on the surface was assumed to follow:
|Europa||228 ± 15||0.97 ± 0.12|
|Bamberga||243 ± 10||1.01 ± 0.15|
|Hygeia||232 ± 16||1.01 ± 0.13|
|Eukrate||292 ± 29||1.19 ± 0.12|
|Egeria||259 ± 19||0.95 ± 0.13|
|Ceres||234 ± 12||1.13 ± 0.12|
|Flora||297 ± 40||1.43 ± 0.25|
|Berberic||301 ± 32||1.52 ± 0.26|
|Pallas||232 ± 7||1.05 ± 0.08|
where Tss is the subsolar point temperature and is the zenith angle of the Sun. The temperature of the dark side was taken to be 0 K; this assumption does not lead to a significant error since mainly the sunlit sides of the asteroids were observed by IRAS.
The asteroid diameters were adjusted for each observation to match the 60 µm stellar calibration exactly, i.e. all ratios of observed/model fluxes at 60 µm are identical to unity. The 100 µm calibration was adjusted such that for the asteroids the mean of the ratio of the observed flux to the model flux was equal to unity. The resultant ratios of observed fluxes to those derived from the model are given in Table VI.C.5.
|25 µm||60 µm(2)||100 µm|