4 - Approach safety and collision avoidance

Summary

The objective of this chapter is to explain the requirements for trajectory safety, to discuss the causes for trajectory deviations due to the orbital environment and to imperfections and errors of the onboard system, and to investigate the possibilities of employing protection against trajectory deviations. The discussions concerning trajectory deviations and trajectory safety concentrate on the rendezvous phases, since the mission phases of launch and phasing are generally controlled by operators or computer functions on ground. In the rendezvous phases the two spacecraft are relatively close together, their orbital planes are well aligned and the trajectory of the chaser, by definition, leads toward the target, so that any deviation from the planned trajectory can potentially lead to a collision, directly or after one or more orbital revolutions.

Trajectory safety – trajectory deviations

Rendezvous and docking is in fact a ‘planned collision’ of two spacecraft, which is controlled by considering the geometric location of the contact points on the two vehicles and the linear velocities and angular rates at contact. To achieve the contact conditions within the allowed margins, the trajectories have to be maintained within close tolerances prior to contact. Any deviation from such tolerances may lead either to a loss of the rendezvous and mating opportunity or even to the danger of collision of the two spacecraft at unsuitable points and dynamic conditions, with the risk of serious damage. For this reason, rendezvous operations, and all functions and systems involved in them, have to comply with failure tolerance and safety requirements.