TO THE LONG list of things that can cause airport chaos, add another. On December 19th Gatwick airport, Britain’s second-biggest, was forced to close due to several sightings of drones flying near its runways. Airlines delayed, diverted or cancelled over a thousand flights, disrupting the travel plans of 110,000 people on December 20th alone. The airport only reopened on the morning of December 21st, after what is by far the biggest disruption to commercial aviation in history by a drone.

The potential for an incident of this scale has been recognised for some years now. The falling price of small drones in recent years has resulted in the number flying dangerously near aircraft to rise sharply. There were no incidents involving drones and civil airliners in Britain in 2013, according to the UK Airprox Board, a regulator; last year there were 93 and this year at least 117 even before the latest events at Gatwick.

Such trends are replicated elsewhere. According to America’s Federal Aviation Administration (FAA), there are around 100 cases each month of drones potentially endangering an aircraft in the United States. Two collisions have already happened in North America. In September a drone collided with a helicopter near Staten Island in New York, and the following month an aircraft was struck by a drone in Québec City (both aircraft managed to land safely).

This is a threat that the authorities have to take seriously. Recent research, published by the Alliance for System Safety of UAS through Research Excellence, a think-tank, suggests that small unmanned aerial vehicles (UAVs) can be much more damaging than birds—a surprisingly common cause of aircraft crashes—at the same impact speed, even if they are a similar weight. The researchers found that the drones’ rigid and dense materials—such as metal, plastic and lithium batteries—can put aeroplanes at much greater risk than a bird carcass. If a drone were to hit an aircraft’s fan blades when it is operating at its highest speed, the blades could shatter and power to the engine could be lost.

The extended closure of Gatwick was a justified response to this threat. But it also underlined the potential for disruption that drones afford. The British police do not think that terrorism was the motivation behind the latest incident at Gatwick. According to the Daily Telegraph, a British newspaper, environmental activists are suspected of being behind the attack. That is not entirely without precedent. Protestors previously closed down London City Airport in September 2016 by storming the runway.

As attention turns to what can be done to prevent a repeat, two solutions stand out. The first is regulation. Regulators in America and Britain already ban drones from flying too close to airports. America has introduced a compulsory registration scheme for drones and Britain plans to follow suit. BALPA, a trade union for pilots in Britain, suggests that more high-profile prosecutions and longer jail sentences could force more compliance with the existing regulations.

But as the chaos at Gatwick shows, even harsh punishments will not stop those intent on deliberately causing harm. Technology is the second, and more important, answer to the threat that drones pose to airports and other sensitive locations. In the United States the FAA has experimented with a system that cuts off communication between drones that fly too close to an airport and their controllers. Drones can have pre-programmed software that keeps them away from prohibited areas, an approach known as “geofencing”. A set of British engineers have come up with a system that catches drones with a net and then softly lands them with a parachute. (Dutch police have even attempted to train eagles to catch drones in the sky and return them to their trainers as if they were song birds.) The closure at Gatwick will give ideas such as these a mighty shove forward.