ZEM/ZEV guidance approach for asteroid touch-and-go sample collection maneuvers

Future asteroid sample return missions (e.g. NASA OSIRIS REx mission to 1999 RQ₃₆) will require the spacecraft to touch down to the asteroid's surface to collect samples for short time periods (∼ 5 sec). Such maneuvers are called Touch-And-Go (TAG) and are critical for effective sampling. Importantly, the final TAG descent to the selected asteroid's surface must be unpowered in order to avoid sample contamination. This paper presents a ZEM/ZEV-based guidance approach with selected waypoints to autonomously reach a point 30m above the desired landing site. The constraints imposed by the navigation system result in a small velocity error at the initiation of the coasting phase, which results in unacceptable landing accuracy. We show that such residual velocity can be reduced to an acceptable level provided the spacecraft is allowed to hover prior to the coasting descent phase. A set of Monte Carlo simulations is executed to validate the guidance approach. These simulations model a novel state estimation algorithm that enables real-time estimation of the spacecraft's state in the asteroid body-centered reference frame, as well as a mapping of the guidance law's commanded thrust to the pulsed thrusters. The simulations demonstrate an unprecedented level of accuracy for the TAG maneuver.