Hovering Control of a Spacecraft Over a Binary Asteroid

摘要

In the last two decades studies related to missions to asteroids has gained much attention in the space sciences community. This is due to many reasons: to learn more about the origin of life, they could have played a substantial role in the transport of water to the Earth, asteroids give us clues about the formation of our solar system and they are one of the main interplanetary threat to life on Earth. Among all known asteroids, it is estimated that binaries make up 15% of the total, although they never have been explored before. In the next decade, NASA should launch the DART spacecraft to the Dydimos binary asteroid in order to test a kinect impactor to estimate effects for asteroid deflections. This mission should be followed by the ESA's Hera mission, with the same target, to characterize the pair and the impact site left by DART. Such asteroid missions present challenges to the guidance, navigation and control, due to the low-gravity and strongly perturbed environment. Binaries could imply in additional challenges depending on the mission requirements. For this reason, we plan to study the hovering control of a spacecraft operating in a binary system of asteroids. We choose to apply a sliding mode control strategy in order to deal with the highly perturbed environment. The chattering effects inherently associated to this control approach is eliminated by allowing a convergence boundary nearby the sliding surface, which has the drawback of allowing errors around the desired states. We therefore propose a disturbance-based sliding mode observer to estimate these disturbances, which are fed into an adaptive disturbance-based sliding mode control law to reducc the steady state errors. The last shows a much higher robustness. In addition, the disturbance estimative may be a valuable source for online estimation of the asteroid environment's parameters. This could be highly relevant for an autonomous mission architecture.