Jet packs for satellites. According to Daniel Campbell of Effective Space, the British and Israeli firm which is building them, that is the way to think of the robotic spacecraft his company plans to start launching in 2020. The purpose of Effective Space’s devices, which it calls space drones, is to prolong the lives of communications satellites (com-sats) that would otherwise be decommissioned for lack of fuel for station-keeping—in other words, for maintaining their proper orbits.

At the moment, about two dozen big geosynchronous com-sats (those with orbits exactly 24 hours long, which thus hover continuously over the same spot on Earth) are retired each year, most commonly because of fuel exhaustion. Mr Campbell, the head of Effective Space’s British operation, proposes to do something about that...