In aviation and in space, a drone refers to an unpiloted aircraft or spacecraft. Another term for it is an "unmanned aerial vehicle," or UAV. On Earth, drones are often used for military purposes because they don't put a pilot's life at risk in combat zones. In addition, drones don't require rest, enabling them to fly as long as there is fuel in the craft and there are no mechanical difficulties.

Technically speaking, spaceborne drones could include cargo spacecraft, satellites and machines that leave Earth, although they aren't usually referred to as such. Perhaps the best example of a drone in space is the U.S. military's mysterious X-37B spacecraft, which has made multiple flights into orbit for hundreds of days at a time. Its mission is highly classified, leading to speculation about what it is doing.

Drones have been around for almost as long as airplanes have been used in warfare (1911), and that's not even including bomb-filled balloons that were first used by Austria in the mid-1800s. [Rise of the Drones: Photos of Unmanned Aircraft]

The U.S. Navy developed limited "air torpedoes" during World War I, but set aside the concept until World War II. At that time the Navy began a program called Operation Anvil, according to The Nation. Remote-controlled B-24 bombers were used to deliver explosives to German bunkers, but the program was a "disaster," according to the article. Many planes crashed or exploded prematurely.

For decades afterward, the United States focused on using rockets while also working on drone development. The first big demonstration of drones came during the 1991 Gulf War, when the United States deployed UAVs.

The drone market was worth about $20 billion in 2018, according to a global forecast published in researchandmarkets.com. While military applications remain possible, the consumer market has exploded with drone options in recent years, allowing ordinary people to fly these small planes for all sorts of purposes – photography, recreation and in some cases, surveillance. Drones can also be used for applications such as distributing fertilizers to farmers' fields or keeping an eye on remote pipelines.

In the U.S., the Federal Aviation Administration has been cautious about allowing unpiloted vehicles to zip around the skies. There are numerous safety concerns – they could crash into buildings, interfere with airspace or cause other problems. 

In February 2015, however, the FAA moved to allow limited use of drones. Commercial use of drones in the United States was permitted as of 2016.  As of 2018, the drone regulations advise that operators can fly only during daylight or twilight, with altitude and speed restrictions. Also, drones must be kept in the line of sight of an operator.

Certain companies are considering sending drones to deliver goods, which could reduce the cost of using drivers for door-to-door service. Amazon is advertising a future service called "Prime Air," which is intended to send deliveries to customers in 30 minutes or less. The company has written a letter of application to the FAA to begin this service. Amazon performed its first drone deliveries in Britain in December 2016.

Government agencies occasionally use drones for safety reasons, such as monitoring storms and hurricanes without putting pilots at risk. One example was the Hurricane and Severe Storm Sentinel (HS3), which was a five-year test program using unmanned surveillance aircraft called Global Hawks. It was a collaboration among NASA, the National Oceanic and Atmospheric Administration (NOAA) and Northrop Grumman, an aerospace and defense company based in Virginia. Other reported applications of drones have been in search-and-rescue operations and in-air 3D mapping of local terrain, such as forests or roads.

 Artist's illustration of the U.S. Air Force's robotic X-37B space plane orbiting Earth.
Artist's illustration of the U.S. Air Force's robotic X-37B space plane orbiting Earth.
Credit: Boeing

Perhaps the closest thing to a UAV in space is the X-37B, which looks like a miniature version of NASA's now-retired space shuttle. The unpiloted spacecraft completed four clandestine space missions.

Most of X-37B's work is highly classified, making it difficult to speculate about the purpose of its missions. However, the Air Force has released some information about the fourth mission. In an email to Space.com in May 2015, Air Force spokesperson Capt. Chris Hoyler said the X-37B would have an experimental propulsion system and would help engineers investigate how well various materials perform in space.

"We are excited about our fourth X-37B mission," Randy Walden, director of the Air Force Rapid Capabilities Office, said in a statement in May 2015. "With the demonstrated success of the first three missions, we're able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads.”

In 2018, NASA announced it would send a drone (a small helicopter) to Mars along with the Mars 2020 rover mission. The drone will be tested for about 30 days on Mars. The drone will have to fly autonomously because radio signals from Earth take several minutes to arrive on Mars, which precludes live remote piloting from Earth. [Watch: NASA's Tiny Helicopter Drone for Mars]

Although not technically classified as drones (since they operate beyond Earth's atmosphere), it could be argued that several of the spacecraft that visit the International Space Station are unmanned aerial vehicles as they carry only cargo and no pilot. There are several of these vehicles, including Russia's Progress spacecraft, the (now-retired) European Space Agency Automated Transfer Vehicles, SpaceX's Dragon and Northrup Grumman's Cygnus.

From time to time, a space agency will test vehicles in space before putting people on it, which is what happened with the Mercury, Gemini and Apollo programs. (The space shuttle was never tested without pilots on board.) In 2014, NASA sent its Orion spacecraft high into Earth-orbit (3,600 miles or 5,800 km) in an unpiloted test. And in 2019 or 2020, the agency is planning a second test in in which the Orion spacecraft will be sent around the moon.