We report on investigation and development of kinetic micro-penetrators onto airless solar system bodies. Our goal is to design a technically feasible penetrator of low mass (~2-5Kg) capable of surviving a high velocity impact and performing useful scientific measurements. We present a rational program of development beginning with generic studies and ground demonstration of impact survivability, through a technical demonstrator mission to high profile scientific missions. We aim to take advantage of existing experience from the UK defense sector, and current advances in high technology (e.g. MEMS, holey optical fibres). Key aspects discussed include the scientific case; a comparison of kinetic penetrators and landers as a platform for exploration; penetrator technology (orbiter support, attitude control, de-orbiting, impact survivability, power, communications, mass); scientific instruments (temperature, heat flow, seismic, chemical); and future mission opportunities.