Review of guidance techniques for landing on small bodies

A renewed scientific interest has been growing in the exploration of small asteroids in addition to larger planetary bodies such as Mars, since their weaker gravitational field makes them more easily accessible. However, such exploratory missions are very challenging from an engineering perspective, particularly when striving for optimal propellant consumption. This is mostly due to the perturbed and poorly known characteristics of small planetary bodies but also, as shown by the European Rosetta mission, to the long-time degradation of spacecraft subsystems. In order to address this challenge, it has been long recognised the need for robust descent algorithms.

However, Space guidance and control communities have different understandings, restricting the integration of scientific advances and even constraining their capabilities. To incite such an integration and guide engineers in the development of planetary descent algorithms, this survey gathers state-of-practice guidance and control techniques and presents them in an instructive fashion. In addition, it clarifies and reconciles different concepts from both guidance and control perspectives. The survey and reconciliation of concepts then lead to the identification of an underlying parametric generalisation of guidance techniques, suitable for the application of systematic optimisation tools. Albeit simple, this structural identification is very important as the latter tools have shown great promise and have already been employed, for example, for Rosetta's critical control update. Finally, special emphasis is placed on the robustness of those techniques against uncertainties.