At any given time, around 7,000 aircraft are flying over the United States. For the past 40 years, the same computer system has controlled all that high-altitude traffic—a relic of the 1970s known as Host. The core system predates the advent of the Global Positioning System, so Host uses point-to-point, ground-based radar. Every day, thousands of travelers switch their GPS-enabled smartphones to airplane mode while their flights are guided by technology that predates the Speak & Spell. If you’re reading this at 30,000 feet, relax—Host is still safe, in terms of getting planes from point A to point B. But it’s unbelievably inefficient. It can handle a limited amount of traffic, and controllers can’t see anything outside of their own airspace—when they hand off a plane to a contiguous airspace, it vanishes from their radar.