where the subscripts G and °° refer to the values within the gravitational field and at infinity (respectively), F is the flux of meteoroids, and v is their velocity. This equation is only valid for an isotropic flux traveling at a single velocity. Using this equation in conjunction with an isotropic flux distribution such as that of Grün et al. , it is straightforward to compute and average flux on a spacecraft or instrument. For many purposes this simple equation is adequate, but for many applications directional and velocity distribution information may be preferred.
Divine  introduced a method of defining the interplanetary meteoroid environment in terms of orbit families. As described in , the distributions that Divine introduced were not the "classical" distributions. For values of perihelion (n), inclination (i), and eccentricity (e), the classical distribution D(ri,e,i) drj de di (note the variables need not be separable) is related to Divine's separable distributions in perihelion (NO, inclination (p;), and eccentricity (pe) by
D(rpe,i) dTj de di = 2n2 r2 sini (l Nt pe p; dr, de di. (2)
The spatial density in interplanetary space from a distribution of meteoroids can be represented as
71 r -y/r —r, ^/(l + e)rt -(l-e)r Vsin2 i - sin2 X
where r and X are the range and latitude (respectively) relative to the Sun and ecliptic plane (cf. Figure 1). These formulae can be used (in conjunction with the velocity of each orbit distribution) to compute the directional flux in interplanetary space under solar gravity alone for a defined orbital distribution. Such information is useful if a dust-measuring instrument or a susceptible spacecraft part maintains a particular orientation in space and one wishes to compute the expected flux of meteoroids on it.
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