It’s only a matter of time before a drone takes down a passenger plane

Gatwick Airport, the second-busiest in the UK, has come to a standstill after one or more drones were spotted flying over airport grounds, forcing authorities to stop all flights in and out. As of Thursday afternoon, tens of thousands of travelers have been affected. (Update: on Friday the airport briefly had to suspend flights again.)

It is not yet known who is operating the drones, or why. The Gatwick episode has renewed calls for “anti-drone” technology. But drones present a threat to commercial air traffic that cannot be waved away with a technological fix. They’ve become too cheap, and too capable. Passenger jets are vulnerable targets and will remain so. Neither technologies like geofencing that seek to hobble drones nor things like guns, nets, radio-jamming systems, or even eagles can reliably protect airline traffic.

This might sound alarmist. But drone countermeasures can only go so far.

The cheapest drones are mass-produced consumer models, which can be bought for as little as $25. These are relatively easy to guard against—for instance, by jamming the radio signals that allow operators to control them. Slightly more expensive drones have GPS chips. Such drones can come programmed with geofencing, a fancy-sounding word for instructing the software that controls the drone to prevent it from flying within specified geographical coordinates.

This can effectively constrain drones that the average consumer might buy from Walmart or Amazon. It can prevent, say, drunks from accidentally flying their drones onto White House grounds.

But it’s easy for someone with moderate technical sophistication to build a drone, and homemade drones need not have GPS chips on board. They also don’t necessarily need to communicate with their operator in order to stay aloft, which would render radio jamming systems ineffective. Stronger jammers could simply fry on-board electronics, but such strong signals risk causing problematic interference with systems on the aircraft they are meant to protect.

To be sure, it’s not all that simple to take out an aircraft with a single drone. That’s because even on takeoff and landing, airplanes are moving pretty fast, usually between 150 and 200 miles per hour. Very few drones can fly that fast—widely available consumer drones top out at 50 to 70 mph. But even faster drones can be evaded by commercial aircraft. Furthermore, airplanes are designed to withstand bird strikes and are more durable than is widely appreciated. If a single drone hits an airplane accidentally, the plane will probably be only slightly damaged and is likely to land safely.

Things can change, though, if drones fly in swarms. While it is quite difficult for a single drone to target an aircraft, put 30 drones in an aircraft’s flight path and the odds shift. (Given the relative prices of drones and countermeasures, it’s going to be easier to add more drones to an attacking swarm than it is to bolster defense.) If you program the swarm to seek out a plane’s engines (either through infrared sensing or through image recognition), the risk of catastrophic damage from an impact grows further still. Add a small amount of explosives, and collisions are likely to prove deadly.

Anti-drone systems like nets might protect high-value targets like the White House, 10 Downing Street, or the immediate vicinity of major airports. But there is too much airspace around to protect it all—and from a technical perspective a drone can fly 2,000 feet up just as easily as it can fly near the ground. There is simply no good technical countermeasure to a swarm of semi-autonomous drones attacking an airliner.

That’s the bad news. The good news is that the vast majority of people with the necessary technical skills are not willing to commit mass murder. As a result, there’s little reason to be afraid that drones will decimate air travel any time soon. An attack will come, eventually, just as bombings sometimes take place in cities—as Londoners know better than most—because the technologies behind making both bombs and drones are too widely available to be effectively embargoed. There is no technological shield to be had—only a moral one. Fortunately, that’s good enough—most of the time.