Fishing boats kept washing up in Japan with dead North Koreans on board. Dozens were documented every year, but they spiked in 2017, with more than 100 boats found on the northern coasts of Japan. No one could explain the appearance of these ghost ships. Why were there so many?

An answer arrived in 2020. Using a swarm of satellites orbiting Earth, a nonprofit organization called Global Fishing Watch in Washington, DC, found that China was fishing illegally in North Korean waters, “in contravention of Chinese and North Korean laws, as well as UN sanctions on North Korea,” says Paul Woods, the organization’s cofounder and chief innovation officer. As a result, North Korean fishermen were having to travel further afield, as far as Russia, something their small ships weren’t suited for. “They couldn’t get back,” says Woods. China, caught out, promptly halted its activities.

The alarming discovery was made possible by the DC-based firm Spire Global, which operates more than 100 small satellites in Earth orbit. These are designed to pick up the radio pings sent out by boats across the globe, which are primarily used by vessels to avoid each other on the seas. Listening out for them is also a useful way to track illegal maritime activity.

“The way they move when they’re fishing is distinct,” says Woods of the boats. “We can predict what kind of fishing gear they’re using by their speed, direction, and the way they turn.” Of the 60,000 vessels that emit such pings, Woods says 5,000 have been found conducting illegal activities thanks to Spire, including fishing at restricted times or offloading hauls of protected fish to other vessels to avoid checks at ports.

Satellite constellations like Spire’s have seen huge growth in recent years, and novel uses like this are becoming more common. Where once satellites would be large, bulky machines costing tens of millions of dollars, technological advances mean smaller, toaster-sized ones can now be launched at a fraction of the cost. Flying these together in groups, or constellations, to conduct unique assignments has become an affordable prospect. “It’s now economically viable to deploy many, many more satellites,” says Joel Spark, cofounder and a general manager at Spire.

Before 2018, no constellations of more than 100 active satellites had ever been launched into Earth orbit, says Jonathan McDowell, a satellite expert at the Harvard-Smithsonian Center for Astrophysics in the US. Now there are three, with nearly 20 more constellations in the process of being launched and some 200 more in development. It is a “boom in constellations,” says McDowell.

The reasons for flying constellations are numerous. The most notorious is to beam the internet to remote locations, made famous by SpaceX’s Starlink mega-constellation. This vast swarm of 3,000 satellites accounts for nearly half of all those in orbit, and it will swell further to 12,000 or more. Others, like Amazon, have plans for vast space internet constellations of their own. Many are worried about launching so many satellites into orbit, significantly raising the risk of collisions and producing dangerous space junk.