Measurements of the thermal emission at far-infrared and submillimeter wavelengths can help us to assess the abundance of large dust particles in the coma, because the emissivity of small particles decreases as X~a 1 < a < 2, while the emissivity of large particles remains essentially constant.
The far-infrared spectral domain of Hale-Bopp was surveyed from the ISO satellite. The on-board photometer (PHOT) measured the thermal flux through filters at 7-160 |im, while the spectrometers (SWS and LWS) recorded the spectrum from 5-160 |im. To fit the slope of the spectral energy distribution with a size distribution of the form n(a) a~a requires a* 3.5, as shown in Figure 3 , For an outflow velocity v(a) a"05, this result implies that the dust production size distribution from the nucleus has a • 4 and that the mass is concentrated in large particles.
Jewitt and Matthews  acquired submillimeter continuum images of Hale-Bopp in 1997. The observed submillimeter spectral index of 0.6 suggests that the emitting particles were millimeter sized. A dust production rate of 1-2 x 10 kg/s near 1 AU was derived, giving a dust/gas mass ratio of at least 5.
Thus, despite the obvious abundance of small grains giving rise to the silicate features and high color temperature at short infrared wavelengths, the mass of the dust emitted from the nucleus of Hale-Bopp appears to have been dominated by large particles. Li and Greenberg  argue that the small grains are incorporated into large, extremely fluffy aggregates with porosity > 97%. At such high porosity, these aggregates would be optically thin in the optical and mid-infrared and would be indistinguishable from a cloud of small grains.
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