The existence of the nongyrotropic velocity distribution of newborn ions is a very important feature for comet Grigg-Skjellerup (P/G-S); this kind of distribution may be a reservoir of free energy and play a rather important role in the generation of plasma waves observed at P/G-S, especially close to the comet. In this paper we predict the distribution function of the newborn ion population, taking into account the source term for this population (implantation from neutrals) and the loss term (pickup, i.e., velocity diffusion leading to assimilation in the solar wind). We integrate the equation backward along the solar wind plasma flow line modified by the ion gyration motion using the observations of the Giotto spacecraft at P/G-S on July 10, 1992. We obtain the nongyrating zero-order nongyrotropic velocity distribution of the newborn ions which depends on the average pickup time of implanted ions and on the distance from the comet. The stability of the above equilibrium is then studied by adding linear perturbations and solving numerically the inferred dispersion equation. We show that this kind of distribution function triggers a left-handed instability which can be called ''nongyrotropic ring instability'' and which arises from the coupling between the two transverse electromagnetic modes and the longitudinal electrostatic mode. ¿ American Geophysical Union 1995