NASA's Planetary Defense Expert Warns of a Hidden Threat: 15,000 Undetected 'City-Killer' Asteroids
A senior NASA official, Kelly Fast, has issued a stark warning about humanity's vulnerability to a silent danger lurking in space. She revealed that approximately 15,000 near-Earth asteroids, known as 'city killers,' remain undetected, posing a significant threat to our cities and populations. These asteroids, ranging from 140 meters to 1 kilometer in size, have the potential to unleash catastrophic consequences, including massive blast waves, fires, and tsunamis, if they strike populated areas.
Fast's presentation at the American Association for the Advancement of Science (AAAS) annual meeting in 2026 highlighted the stark contrast between the known and unknown threats. While Hollywood-style 'planet killers' larger than 1 kilometer are largely tracked and pose minimal risk, the real danger lies in the mid-sized asteroids that could devastate entire cities. An impact from a 140-meter asteroid releases energy equivalent to tens or hundreds of megatons of TNT, far surpassing the power of the most powerful nuclear weapons ever tested.
Despite progress in planetary defense, such as NASA's Double Asteroid Redirection Test (DART) mission, which successfully demonstrated kinetic impact deflection, the lack of a ready-to-deploy deflection system for imminent threats remains a critical concern. Nancy Chabot, a planetary scientist involved in the DART mission, emphasized the limitations, stating that the technology is not readily available if a threat were to arise.
The challenge extends beyond immediate action. Deflection missions require years, ideally a decade or more, of preparation. This leaves limited options in short-warning scenarios, emphasizing the need for civil defense measures like evacuation. To address this, NASA plans to launch the Near-Earth Object Surveyor (NEO Surveyor) in 2027, aiming to discover at least 90% of near-Earth asteroids 140 meters and larger within a decade, significantly improving detection and early warning capabilities.
International collaboration, such as ESA's Hera mission, further strengthens global efforts to study the long-term effects of deflection missions. NASA's Planetary Defense Coordination Office continues to refine response protocols through hypothetical impact exercises with federal, state, and international partners, acknowledging that mid-sized undetected asteroids represent the highest uncertainty in near-term hazard assessments.
Fast's remarks have sparked discussions on funding priorities for planetary defense. While the overall risk remains low, with most large objects tracked and no imminent threats identified, the potential impact of undetected mid-sized asteroids demands sustained support for detection and deflection technologies to ensure preparedness.
