Orbital Evolution of Interplanetary Dust

The two most important dynamical features of the zodiacal cloud are: (i) t he dust bands associated with t he major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with th e Eart h. Oth er important dynamical features include the offset of th e center of symmetry of th e cloud from the Sun, the radial gradient of the ecliptic polar brightness at th e Earth, and th e warp of th e cloud. The dust bands provide th e st rongest evidence th at a substantial and possibly dominant fraction of the cloud originate s from aster oids. However, the characteristic diameter of these asteroidal particles is probably several hundred microns and the migration of th ese large particles towards th e inner Solar System due to Poynting Robert son light drag and their slow passage through secular resonances at the inner edge of the asteroid belt result s in large increases in th eir eccent ricities and inclinations. Because of these orbital changes, the dividing line between asteroidal and comet ary type orbits in the inner Solar System is probably not sharp, and it may be difficult to distinguish clearly between ast eroidal and cometary particles on dynamical grounds alone.