Hamas has used surveillance drones at rallies. Photographer: Said Khatib/AFP/Getty Images
Hamas has used surveillance drones at rallies. Photographer: Said Khatib/AFP/Getty Images

A very small airplane rose over the Gaza Strip last week. It entered Israeli airspace and sped toward the coastal city of Ashdod. Then a Patriot missile blew it up.

The plane, a rudimentary drone launched by the terrorist group Hamas, posed little threat. But Hamas promises more to come -- including some intended for "suicide missions."

That suggests a vexing problem: As drones become more commonplace, what's to stop terrorists from using them?

Drones have an obvious appeal to the extremist mind. They're hard to detect, controlled from afar and capable of flying into crowded or remote places, anywhere from a sports stadium to a power plant. They can be affixed with explosives or chemical agents. And no one has to die to complete the mission.

In short, drones could combine the intimacy and stealth of a suicide bomber with the power and range of an armed aircraft.

Concerns about a terrorist using a drone aren't entirely hypothetical. Hezbollah has been flying them into Israeli airspace for a decade. Hamas claims to have three varieties. Al-Qaeda has planned to use remote-controlled planes for a range of brutal attacks. In 2012, a Massachusetts graduate student was imprisoned for plotting to strap plastic explosives to small drones and fly them into the Pentagon and the U.S. Capitol.

None of these scenarios led to casualties. But other potential uses are unnerving: crop-dusting drones modified to disperse deadly chemicals, unmanned planes used as assassins, drones meant to attack critical infrastructure.

It's somewhat comforting to know that, for now, armed drones -- the kind the U.S. uses in Pakistan and Afghanistan -- probably remain beyond the reach of terrorists. And the payloads of most unarmed varieties on the market, even if modified to do harm, are probably insufficient to cause significant casualties or structural damage. Also: flying a drone isn't a trivial skill and buying one still takes a lot of cash.

But these challenges may not impede committed terrorists forever, as drones get cheaper, better and easier to find.

Is there a way to mitigate this threat?

Simply publicizing the dangers -- without hysterics -- would be a start. Making sure that drone manufacturers and training instructors around the world know their customers and are on the lookout for suspicious behavior should be a priority. And the international arrangements regulating the export of drone technology should be refined and strengthened with terrorism in mind, with special attention on unmanned planes that can evade radar or fly aggressively.

Technology should also be of some help. Many larger drones can be detected by radar and shot down. The U.S. and Israel are improving defenses against smaller ones, too. It's also possible to jam the frequencies drones use for navigation or (as with everything else these days) hack them.

Although there's been some discussion of embedding unmanned aircraft with tracking software or "kill switches," practical problems abound with such an approach. Unfortunately, reliably detecting such small and agile machines will probably be a challenge for years to come.

By 2030, some 30,000 unmanned planes may be hovering overhead in the U.S., most of them devoted to worthy things such as agriculture and emergency response. All the more reason to start thinking now about how best to separate the good ones from the bad.

To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at davidshipley@bloomberg.net.