A senior NASA planetary defense official has issued a stark warning that humanity remains vulnerable to thousands of undetected near-Earth asteroids capable of devastating entire cities. Kelly Fast, acting Planetary Defense Officer at NASA’s Planetary Defense Coordination Office, delivered this assessment during a presentation at the American Association for the Advancement of Science (AAAS) annual meeting in Phoenix on February 16, 2026.
Fast’s primary concern centers on approximately 15,000 unaccounted-for objects that measure at least 140 meters (460 feet) in diameter—roughly the size of a football stadium. These “city killers” represent a massive blind spot in our current planetary defense network.
Why “City Killers” Keep Scientists Up at Night
While massive “planet killers” larger than 1 kilometer are largely tracked and pose minimal near-term risk, mid-sized asteroids are much harder to identify.
The Gap in Our Surveillance
- Detection Shortfall: Currently, NASA and international partners have cataloged only about 40% of potentially hazardous objects in the 140-meter size range.
- Invisible Threats: These asteroids are often dark, reflecting little sunlight, or they approach from the sunward direction, making them virtually invisible to traditional optical telescopes.
- Regional Devastation: An impact from a 140-meter asteroid would release energy equivalent to tens or hundreds of megatons of TNT—far exceeding the most powerful nuclear weapons ever tested.
Defenseless Against a Surprise Strike
Experts at the AAAS session highlighted a sobering reality: if a city-killer were detected on a collision course today, humanity would have no way to actively deflect it. While NASA’s DART mission in 2022 successfully demonstrated kinetic impactor technology, there is currently no “ready-to-go” spacecraft standing by for an immediate threat.
Latest Updates: The 2026 Planetary Defense Outlook
The Near-Miss of Asteroid 2024 YR4
Recent attention has focused on 2024 YR4, an asteroid roughly 90 meters across that initially showed a small chance of colliding with Earth in 2032. While further analysis has downgraded the Earth impact risk to zero, it currently has a 4% chance of striking the Moon. If this occurs, the flash would be visible from Earth with the naked eye.
Upcoming Solutions: NEO Surveyor
NASA is pinning its hopes on the Near-Earth Object Surveyor (NEO Surveyor) infrared space telescope, which is on track for a launch as soon as September 2027.
- Thermal Imaging: Unlike ground telescopes, NEO Surveyor will use heat signatures to find asteroids regardless of how much sunlight they reflect.
- Efficiency: The mission aims to discover at least 90% of near-Earth asteroids 140 meters and larger within a decade of operation.
Frequently Asked Questions (FAQs)
What is a “city-killer” asteroid?
A “city-killer” is an asteroid measuring at least 140 meters (460 feet) in diameter. While not large enough to cause global extinction, its impact could level an entire urban center and cause widespread regional damage.
How many of these asteroids are still missing?
NASA estimates there are roughly 25,000 such objects in our cosmic neighborhood, but about 15,000 remain undetected and unaccounted for.
Can we stop an asteroid today?
No. While we have proven we can “nudge” an asteroid via the DART mission, we do not have a standby defense system. Deflection requires at least a decade of lead time to plan and launch a mission.
What happens if we find one on short notice?
In short-warning scenarios, the only current options are civil defense measures, such as mass evacuations of the predicted impact zone.
When will our defense improve?
The launch of the NEO Surveyor in late 2027 will be a “game-changer,” significantly closing the detection gap and providing the early warning needed to use deflection technologies.