In a stark and widely reported warning, NASAPlanetary defense chiefs have revealed that Earth currently has no reliable way to prevent thousands of “city-destroying” asteroids that could one day strike our planet, sparking urgent concerns in the scientific community and beyond. These insights come from remarks by Dr. Kelly Fast, NASA’s acting planetary defense officer, at the American Association for the Advancement of Science (AAAS) meeting in Phoenix, Arizonashe highlighted the huge gaps in the global response to near-Earth objects (NEOs).Fast explained that scientists have discovered about 25,000 asteroids that are large enough (about 140 meters or more) to cause regional damage if they hit the Earth, but shockingly, current detection systems have only classified about 40% of them. This leaves approximately 15,000 urban killers untracked, orbiting silently near Earth, potentially making an impact without warning. “What keeps me up at night is the asteroids we don’t know about,” Fast said. He emphasized that while tiny meteoroids are harmless and the largest “planet killers” are already being monitored, these medium-sized rocks are a blind spot for planetary defenses.
Why medium-sized asteroids are the focus of attention
NASA defines “city-killer asteroids” as objects about 140 m (460 ft) or larger that are too small to be easily seen with conventional visible-light telescopes but large enough to flatten entire metropolitan areas and cause fires, infrastructure collapse and widespread casualties on impact. They are different from:
- Tiny meteoroids burn up in the atmosphere every day,
- Planet killers, massive celestial bodies, have been cataloged and monitored extensively.
The challenges are physical and technical: Many asteroids are dark and slowly reflective, blending into the background sky, and some follow Earth-like orbits, making them especially difficult to detect until they get very close. Even with the best ground-based observatories, detection is still incomplete.
NASA admits we have no defense against 15,000 “city-killing” asteroids
The danger is not just theoretical. History shows that relatively small asteroids can have a huge impact. The 1908 Tunguska event, in which an object approximately 100 meters below the surface exploded above the Earth, leveled approximately 2,000 square kilometers of forest in Siberia. Similar effects on modern cities would result in catastrophic regional impacts.
Defenses are not yet ready to stop these asteroids from hitting Earth
NASA and international partners have made progress in asteroid defense technology, but current systems are not yet up to the level of protecting Earth from a sudden onslaught of city-killer rocks. The most famous test to date is NASA’s 2022 DART mission, which successfully nudged an asteroid into its orbit by crashing a spacecraft into its small moon. While this shows that deflection is in principle possible, experts warn that this approach requires years of notice and the right targeting conditions, which we may not have for many untracked asteroids.NASA officials acknowledge that no dedicated deflection spacecraft are currently on standby, and that global planetary defense systems lack the funding and infrastructure to detect hazardous asteroids on short notice to perform immediate response missions. This flaw creates a fragile gap between detection and response capabilities.
NASA’s plan to better detect asteroids
Positive developments are on the horizon. NASA is preparing to launch the NEO Surveyor, an infrared space telescope specifically designed to detect dark asteroids by spotting their previously unseen thermal signatures. If successful, this mission will greatly improve the identification of city-killer asteroids and bring the detection rate of such objects closer to the goal of cataloging 90% of such objects within a decade.Expanded ground-based surveys, such as those conducted by international collaborative networks such as the Vera Rubin Observatory and the International Asteroid Warning Network (IAWN), are also expected to enhance early warning capabilities, which will be critical for any future deflection programmes.
what does this mean for humans
NASA’s warning reveals a straightforward fact: Earth remains vulnerable to medium-sized asteroids that we have yet to detect. While the likelihood of any asteroid hitting Earth in the near future remains low compared to common natural disasters, the cumulative risk from unseen objects increases, highlighting the clear need for continued investment in planetary defense, international cooperation and early detection technology.
Are we ready? NASA reveals Earth is no match for thousands of ‘city-killer’ asteroids
Astronomers emphasize that increasing detection and cataloging efforts is the first and most effective step. With more comprehensive tracking, scientists can measure trajectories more quickly and, with enough preparation time, potentially perform deflection missions to guide hazardous objects away from Earth.NASA scientists have warned that Earth is vulnerable to thousands of “city-killer” asteroids that could devastate metropolitan areas but have yet to be tracked. There are an estimated 25,000 asteroids larger than 140 m, of which approximately 15,000 have yet to be discovered, making early detection difficult. Current technology is not equipped to protect against threats that arrive suddenly, especially without years of notice. Upcoming missions like NEO Surveyor and global collaborations promise to provide better detection and preparedness over the next decade.This latest warning demonstrates that while we have made important progress in planetary defense, Earth’s safety from celestial hazards still depends on scientific, technological and international efforts, and in many ways depends on asteroids that we are not even aware of yet.
