The dust velocity is often simply scaled with heliocentric distance as l/v^rh)- However, more realistic results are obtained when computing the dust terminal velocity from a model of the dust coma dynamics [212]. Another important parameter is the upper limit of the integration over the dust size distribution. As most of the mass is usually contained in the large particles, it is important to derive the upper size limit, amax as accurately as possible. How to derive the upper limit of particles that can just be lifted from the nucleus is explained in the chapter by Dave Jewitt [125]. But the uncertainty remains whether such large dust particles really exist, leading to a large error in dust production rates.

Another often unknown parameter is the scattering phase function, D(0), of the cometary grains. The geometry is illustrated in Fig. 54. The scattered light intensity is not isotropic. Instead, a strong forward and backward intensity peak is usually observed. The exact scattering properties depend on the particle size, shape, and composition. Often, the particles are approximated by a sphere because then Mie scattering theory [63,158] can be applied. To describe the scattering properties and related theory of dust grains in detail is beyond the very limited space of this chapter. We refer to [115] for scattering theory.

Here, we also provide a note on the meaning of albedo. In general, the single scattering albedo, A, is defined as the ratio of the energy scattered into all directions to the energy removed from the incident beam by extinction. Extinction includes reflection, absorption, diffraction, and refraction. Therefore, the albedo of a particle is, of course, a function of its material properties.

The Bond albedo, AB, provides the incident light scattered in all directions. It does not take into account diffraction. Thus, it is defined as the ratio of [100]:

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