FEW things exercise green sensibilities more these days than marine plastic litter. The detritus looks unsightly when it washes up on beaches, and cruel when it chokes photogenic sea creatures. Scientists estimate that perhaps 8m tonnes of plastic waste enters the ocean each year, discharged by rivers or shed from ships. Plenty stays close to shore. Some, though, is carried by currents to mid-ocean gyres.

The biggest of these is located halfway between California and Hawaii—and so littered with flotsam that it has been nicknamed the Great Pacific Garbage Patch. A study published last March in Scientific Reports by Laurent LeBreton of the Ocean Cleanup, a Dutch charity, and colleagues, found that it contains between 45,000 and 129,000 tonnes of plastic debris spread over an area roughly the size of Alaska.

The idea of sweeping it all up might sound fanciful. To Boyan Slat it seemed merely ambitious. What if, he wondered in 2012 (then aged 18), you could build a massive bow-shaped floating barrier, anchor it to the seabed and let currents shuffle the litter into the scoop? Despite his youthful age and madcap scheme, Mr Slat set up the Ocean Cleanup to put it into practice. Six years, €20m ($23m) and several prototypes later, the device set sail from San Francisco on September 8th, escorted by a Coast Guard vessel, a shipload of camera crews and a flotilla of curious boaters.

System 001, as the contraption has been christened, is a hollow cylinder 600 metres long and 1.2 metres in diameter, itself made of plastic (polyethylene). It was moulded together into a seamless whole from 12-metre segments at a shipyard across the San Francisco Bay in Oakland. A three-metre-deep skirt (made of sturdy polyester) dangles beneath the boom to prevent litter from escaping under it; buoyant plastic tends to float within a metre of the water’s surface. The device is even simpler than Mr Slat’s original idea, having dispensed with the anchor. Instead, it relies on the observation that the boom, which is driven by the current as well as by waves and wind, always moves faster relative to the plastic, which is propelled by the current alone. It therefore scoops the litter up as it drifts.

A straight boom will first be towed 250 nautical miles off the coast of California for a fortnight of tests, before embarking on a three-week voyage to its final destination. There it will be turned into a U-shape, with its ends fastened in place using metal lines, and set adrift. Satellite tracking and other electronics will allow its progress to be monitored remotely. Light beacons will alert the two dozen ships which cross the gyre each week to its presence. Some time next year another vessel will be dispatched to fish out the collected rubbish, which the charity hopes to sell to recyclers.

If System 001 succeeds, Mr Slat wants to deploy another 60 booms, measuring 1km or more. Corporate sponsors would foot the bill of €5m apiece for construction and three years’ operation, Mr Slat hopes. He already enjoys the backing of deep-pocketed endowments and of tycoons like Marc Benioff, founder of Salesforce, and Peter Thiel, a noted investor.

The system can do little about plastic that has fragmented into microscopic particles, but these make up just 8% of plastic in the gyre. Mr LeBreton reckons that a fleet of booms could, by 2040, sweep up virtually all the non-tiny detritus, but only if plastic leakage into the sea is stanched. If it continues unabated, the incoming debris would outweigh the fleet’s capacity to skim it within a few years. The ocean’s plastic problem cannot really be solved without better waste management on land.