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By Associated Press

A collision between a British Airways passenger jet and a drone over London has left the plane undamaged but the aviation industry deeply shaken.

British police and air accident authorities were investigating Sunday's incident, in which an Airbus A320 carrying 137 people struck an object believed to be a drone at a height of about 1,700 feet (518 meters) while it was approaching Heathrow Airport.

Read More: British Airways Pilot Reports Drone Strike Near London Airport, Police Say

The plane landed safely and was cleared to fly again after an inspection by engineers. But the incident has focused attention on the growing number of unregulated drones in the sky and the potential for disaster if they hit a plane — either accidentally or on purpose.

How many drones are out there?

The authorities don't know exactly, since small drones bought for private use often don't have to be registered, but the market is growing fast as drones become cheaper and easier to operate.

In Britain alone, electronics stores sold thousands during the 2015 Christmas season. Phil Finnegan, director of corporate analysis at aerospace research company Teal Group, estimates there are "several million" drones in the United States.

Tony Tyler, director-general of the International Air Transport Association, told an aviation conference in Denmark on Monday that drones "are here to stay."

What are the rules?

Many countries distinguish between commercial drone operators, which must be licensed, and those used recreationally.

In Britain, operators don't need a license to use a small drone weighing less than 44 pounds for recreational purposes.

Read More: Drone Strikes British Airways Jet. How Bad Could It Have Been?

Drones must not be flown above 400 feet, must remain within sight of the operator and must and kept away from planes, helicopters, airports and airfields. Violators can receive six months in prison and a fine — though prosecutions have been rare — but endangering the safety of an aircraft carries a maximum life sentence.

What could a drone do to a plane?

The biggest worries are a drone damaging the windshield or fuselage of a plane — with the additional danger that lithium batteries on the drone could ignite — or getting sucked into an engine.

Aviation systems expert Philip Butterworth-Hayes says there has been relatively little research on the impact of such a collision.

"We don't know what happens when a drone gets inside an engine," he said. "The only thing you can do [to find out] is fly it into an engine, which is horrendously expensive. It's not a test you want to do all the time."

What can be done to cut the risks?

The British government is considering introducing a drone-registration scheme so the aerial vehicles can be traced back to their owners.

In the U.S, the FAA introduced a registration plan in December, with owners facing a fine if they don't comply. More than 400,000 people had registered by the end of March.

Read More: FAA to Require Recreational Drone Operators to Register

While new rules and better education are part of the solution, technology will also be a key factor. Drones can be fitted with "geo-fencing" software that prevents them from entering restricted areas. But geo-fencing technology is not a legal requirement for many drones, and criminals or those bent on terror could potentially disable the software.