As delivery drones multiply, they may need to protect themselves

INTERNET SHOPPING makes buying things easier, but has also led to the rise of a new kind of thief: the porch pirate. Porch pirates scour door steps for deliveries that have been made when a householder was out, and nab them. Sometimes, they will stalk delivery vans to do so. Residents of New York City, for example, lose an astonishing 90,000 parcels every day to porch pirates, according to a report in the .Porch piracy is a problem that may be solved by the spread of parcel-delivering drones. Because each drone delivery involves a separate journey, rather than having to be fitted into a round, it will be easier for courier and customer to agree on when a drone should arrive than on the arrival time of a van. However Nirupam Roy and Nakul Garg, a pair of engineers at the University of Maryland, worry that drone deliveries are open to a different sort of piracy—hijacking. A drone in flight is easily upset. A well-aimed stone, baseball or similar missile is enough to bring it down, permitting its payload to be purloined. Nor need such stone-throwers have pecuniary motives. Vandalism, or irritation with the very presence of drones, might also provoke pot shots. High-flying drones, like those employed by the police for surveillance, will normally be out of range of such activity. But parcel drones will have to fly low, at least for part of their journeys. To counter this risk the pair therefore propose to build a lightweight, low-power self-defence mechanism which lets a drone sense a missile fast enough to get out of its way.