A “city killer” asteroid isn’t science fiction, it’s a real risk.
Project Leader at The Aerospace Corporation Nahum Melamed explains that though these events are statistically rare, history shows they can happen. In 1908, a roughly 50-meter asteroid exploded over Siberia in what’s known as the Tunguska event, flattening more than 800 square miles of forest. Had that airburst occurred over a major metropolitan area, the destruction would have been instantaneous. Preventing that kind of devastation requires intercepting an asteroid before it explodes in Earth’s atmosphere. That is the core mission of planetary defense: protecting our planet from hazardous asteroids and comets before they strike.
Planetary defense begins with detection. Powerful telescopes across the United States and around the world continuously scan the skies to discover near-Earth objects as early as possible. Once detected, scientists calculate an object’s orbit to determine whether it poses a collision risk. If the probability crosses a certain threshold, global teams mobilize to pinpoint potential impact zones, estimate the asteroid’s size, composition, and mass, and calculate the energy it would release, since impact energy depends directly on mass and velocity. With enough warning time, missions like NASA’s DART have demonstrated that we can deliberately crash a spacecraft into an asteroid millions of kilometers away to nudge it off course. In more extreme, last-resort scenarios, a nuclear device could be used to push an object off trajectory, though that approach carries risks, including breaking the asteroid into multiple dangerous fragments.