be able to cooperatively surround intruders and cut off exits. These advances also bring concomitant protections: robots' internal maps typically permit "forbidden areas" to be defined to prevent robots from autonomously entering certain regions. Outdoor navigation Outdoor autonomy is most easily achieved in the air, since obstacles are rare. Cruise missiles are rather dangerous highly autonomous robots. Pilotless drone aircraft are increasingly used for reconnaissance. Some of these unmanned aerial vehicles (UAVs) are capable of flying their entire mission without any human interaction at all except possibly for the landing where a person intervenes using radio remote control. Some drones are capable of safe, automatic landings, however. SpaceX operates a number of Autonomous spaceport drone ships, used to safely land and recover Falcon 9 rockets at sea. Outdoor autonomy is the most difficult for ground vehicles, due to: Three-dimensional terrain Great disparities in surface density Weather exigencies Instability of the sensed environment Open problems in autonomous robotics There are several open problems in autonomous robotics which are special to the field rather than being a part of the general pursuit of AI. According to George A. Bekey's Autonomous Robots: From Biological Inspiration to Implementation and Control, problems include things such as making sure the robot is able to function correctly and not run into obstacles autonomously. Reinforcement learning has been used to control and plan the navigation of autonomous robots, specifically when a group of them operate in collaboration with each other. Energy autonomy and foraging Researchers concerned with creating true artificial life are concerned not only with intelligent control, but further with the capacity of the robot to find its own resources through foraging (looking for food, which includes both energy and spare parts). This is related to autonomous foraging, a concern within the sciences of behavioral ecology, social anthropology, and human behavioral ecology; as well as robotics, artificial intelligence, and artificial life. Societal impact and issues As autonomous robots have grown in ability and technical levels, there has been increasing societal awareness and news coverage of the latest advances, and also some of the philosophical issues, economic effects, and societal impacts that arise from the roles and activities of autonomous robots. Elon Musk, a prominent business executive and billionaire has warned for years of the possible hazards and pitfalls of autonomous robots; however his own company is one of the most prominent companies that is trying to devise