NASA Planetary Defense Expert Warns of 15,000 Undetected ‘City-Killer’ Asteroids

A senior NASA planetary defense official warned that humanity remains vulnerable to thousands of undetected near-Earth asteroids capable of devastating entire cities, emphasizing that current detection efforts have cataloged only about 40% of potentially hazardous objects in the 140-meter (460-foot) size range and that no ready-to-deploy deflection system exists for an imminent threat.

Kelly Fast, acting Planetary Defense Officer at NASA’s Planetary Defense Coordination Office, delivered the stark assessment during a presentation at the American Association for the Advancement of Science annual meeting in Phoenix on Feb. 16, 2026. She estimated that roughly 25,000 near-Earth objects of at least 140 meters exist, with approximately 15,000 still unaccounted for.

“What keeps me up at night is the asteroids we don’t know about,” Fast said, according to multiple reports from the event. She distinguished the threat from Hollywood-style “planet killers” — massive asteroids larger than 1 kilometer, which are largely tracked and pose minimal near-term risk — and tiny meteoroids that burn up harmlessly in the atmosphere daily. The real concern centers on mid-sized “city killers” that could cause regional devastation, including massive blast waves, fires and potential tsunamis if striking populated areas or oceans.

An impact from a 140-meter asteroid releases energy equivalent to tens or hundreds of megatons of TNT — far exceeding the most powerful nuclear weapons ever tested — capable of leveling urban centers and causing widespread casualties and infrastructure damage without triggering global extinction-level effects.

Fast noted that while NASA’s Center for Near-Earth Object Studies continuously monitors known objects via the Sentry and Scout systems, no significant impact risk exists in the next century for cataloged bodies. However, undetected asteroids remain a blind spot, particularly those approaching from the sunward direction or lingering in observational gaps.
The comments echo ongoing challenges in planetary defense despite progress. NASA’s 2022 Double Asteroid Redirection Test (DART) mission successfully demonstrated kinetic impact deflection by slamming a spacecraft into the moonlet Dimorphos, shortening its orbit around parent asteroid Didymos by about 32 minutes — confirming the technique’s viability in principle.

Nancy Chabot, a Johns Hopkins Applied Physics Laboratory planetary scientist who led DART, addressed the same AAAS session and reinforced the limitations. “Dart was a great demonstration but we don’t have that sitting around ready to go if there was a threat we needed to use it for,” she said. “We would not have any way to go and actively deflect one right now.”

Experts stress that deflection requires years — ideally a decade or more — of lead time to launch a mission, whether kinetic impactor, gravity tractor, nuclear deflection or other methods under study. Short-warning scenarios leave few options beyond civil defense measures like evacuation.

To close detection gaps, NASA plans to launch the Near-Earth Object Surveyor (NEO Surveyor) infrared space telescope in 2027. The mission aims to discover at least 90% of near-Earth asteroids 140 meters and larger within a decade of operations, dramatically improving catalog completeness and early warning capabilities. International collaboration, including ESA’s Hera mission — which will rendezvous with the Didymos system in late 2026 to study DART’s long-term effects — bolsters global efforts.

NASA’s Planetary Defense Coordination Office continues hypothetical impact exercises with federal, state and international partners to refine response protocols. The agency maintains that while the overall risk remains low — with most large objects tracked and no imminent threats identified — mid-sized undetected asteroids represent the highest uncertainty in near-term hazard assessments.

Fast’s remarks have renewed public and scientific discussion on funding priorities for planetary defense amid competing space exploration goals. Proponents argue that incremental investments in telescopes, rapid-response missions and international coordination could substantially mitigate the threat.

No immediate action is required based on current knowledge, but officials urge sustained support for detection and deflection technologies to ensure preparedness if a threatening asteroid emerges.