While possible impact locations included patches of empty ocean, the space rock, called 2024 YR4, also had several densely populated cities in its possible crosshairs, including Mumbai, Lagos, and Bogotá. If the asteroid did in fact hit such a metropolis, the best-case scenario was severe damage; the worst case was outright, total ruin. And for the first time, a group of United Nations–backed researchers began to have high-level discussions about the fate of the world: If this asteroid was going to hit the planet, what sort of spaceflight mission might be able to stop it? Would they ram a spacecraft into it to deflect it? Would they use nuclear weapons to try to swat it away or obliterate it completely? At the same time, planetary defenders all over the world crewed their battle stations to see if we could avoid that fate—and despite the sometimes taxing new demands on their psyches and schedules, they remained some of the coolest customers in the galaxy. “I’ve had to cancel an appointment saying, I cannot come—I have to save the planet,” says Olivier Hainaut, an astronomer at the European Southern Observatory and one of those who tracked down 2024 YR4. Then, just as quick as history was made, experts declared that the danger had passed. On February 24, asteroid trackers issued the all-clear: Earth would be spared, just as many planetary defense researchers had felt assured it would. How did they do it? What was it like to track the rising (and rising and rising) danger of this asteroid, and to ultimately determine that it’d miss us? This is the inside story of how, over a span of just two months, a sprawling network of global astronomers found, followed, mapped, planned for, and finally dismissed 2024 YR4, the most dangerous asteroid ever found—all under the tightest of timelines and, for just a moment, with the highest of stakes. “It was not an exercise,” says Hainaut. This was the real thing: “We really [had] to get it right.” IN THE BEGINNING December 27, 2024 THE ASTEROID TERRESTRIAL-IMPACT LAST ALERT SYSTEM, HAWAII Long ago, an asteroid in the space-rock highway between Mars and Jupiter felt a disturbance in the force: the gravitational pull of Jupiter itself, king of the planets. After some wobbling back and forth, this asteroid was thrown out of the belt, skipped around the sun, and found itself on an orbit that overlapped with Earth’s own. “I was the first one to see the detections of it,” Larry Denneau, of the University of Hawai‘i, recalls. “A tiny white pixel on a black background.”