Lunar and planetary missions launched from a geosynchronous transfer orbit

Considerable cost and mass savings are possible by launching small spacecraft into lunar and planetary space as secondary payloads. The Ariane, for example, provides a platform for several such payloads on each of its monthly launches of placing communication satellites into geosynchronous orbits. Specifically, the second stage injects into a geosynchronous transfer orbit (GTO), and can accommodate eight auxiliary spacecraft, each weighing about 80 kg. This paper addresses the question of how and for what propellant cost can a spacecraft be injected from a GTO orbit to the Moon or to other deep space bodies. The analysis begins with a discussion on the restrictions due to the highly elliptic and specifically oriented GTO. The importance of target choice and wait time in GTO, perhaps for months, are noted. Only these are suitable for single burn using a solid propulsion stage. Specific application to Venus, Mars and near-Earth bodies are presented. More versatility is gained using a liquid stage where multiple burns are possible, allowing plane changes, and swingbys of the Earth and Moon. This greatly expands the mission possibilities to all targets, as well as to Mars and small bodies. Scenarios can include direct and indirect lunar flybys, Apollo type circumlunar trajectories, the lunar flip trajectory, and Belbruno's use of the weak stability boundary beyond the Moon to aid or control the direction of escape. It is concluded that GTO secondary launches can play a significant role in solar system exploration, but that it will depend on having a small restartable propulsion system, and a small spacecraft capable of flying intricate trajectories in the Earth-Moon system.