Hale-Bopp has provided a wealth of new data for determining the physical properties of cometary dust. As with all remote observations, these have to be interpreted with the aid of models based on known scattering and emitting properties of small grains. We find a clear correlation of higher polarization, higher albedo, stronger silicate feature, higher 3-5 jim flux, and higher infrared color temperature. Much work remains to be done to synthesize all of the observed correlations into a dust model consistent also with what we know about interstellar dust and IDPs of likely cometary origin.
The origin of the crystalline cometary silicates remains puzzling. If they condensed in the inner solar nebula, then their presence in comets requires extensive mixing in the solar nebula. If they are circumstellar in origin, then one has to understand why their spectral features are not seen in interstellar dust.
Future progress will come from several directions. Continued work on the scattering by irregular and aggregate particles will allow better interpretation of the correlations outlined here. NASA's space infrared telescope, SIRTF, will extend long-wavelength spectroscopy and photometry to a number of comets. Although some very interesting results have come from analysis of a small number of likely cometary IDPs, the IDPs have not been exploited as fully as they could be to aid the interpretation of remote sensing data. For example, a survey of many chondritic porous IDPs should be conducted to quantify the relative abundance of crystalline olivine, crystalline pyroxene, and glassy silicate grains and their typical dimensions. Ultimately, we can look forward to ESA's Rosetta mission and to the return of cometary samples, beginning with NASA's STARDUST mission, now en route to encounter with comet PAVild 2 in January 2004 .
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