Hajime Okamoto

A correct understanding of the dynamical effect of solar radiation exerted on fluffy dust particles can be achieved with assistance of a light scattering theory as well as the equation of motion. We refor-mulate the equation of motion so that the radiation pressure and the Poynting–Robertson effect on fluffy grains are given in both radial and nonradial directions from the center of the Sun. This allows nu-merical estimates of these radiation forces on fluffy dust aggregates in the framework of the discrete dipole approximation, in which the first term of the scattering coefficients in Mie theory determines the polarizability of homogeneous spheres forming the aggregates. The nonsphericity in shape turns out to play a key role in the dynamical evolution of dust particles, while its consequence de-pends on the rotation rate and axis of the grains. Unless a fluffy dust particle rapidly revolves on its randomly oriented axis, the nonradial radiation forces may prevent, apart from the orbital ec