The Wright brothers' first flight at the start of the 20th century marked a new phase in human history. Since then, the number of aircraft in the sky has grown. More than 4,000 airplanes are in the air right now. While the majority of these planes have pilots and carry passengers, some do not. Simply put, drones are unmanned planes and ships, guided either by remote control or onboard computers. The most common type of drone is an unmanned aerial vehicle (UAV). A UAV is a flying robot controlled either by a pilot on the ground or by an onboard computer. Autonomous drones do not require a pilot to actively send directions. Instead, they are programmed to fly a set route. Some autonomous drones can change course along the way if they encounter problems or need to avoid obstacles. Drones are not limited to the sky. Divers and historians use submersible vehicles to explore shipwrecks, plane crashes, coral, and the many yet-to-be-discovered mysteries of the deep sea. Some aquatic drones are also used to fight climate change by cleaning and policing harbors. Aerial drones are also used to police harbors and find oil and chemical spills in the water.
