NASA has cataloged only a fraction of the near-Earth asteroids large enough to destroy a major city, and the agency’s own data confirms that thousands of these objects remain invisible to current tracking systems. The gap between what Congress has asked NASA to find and what the agency has actually located raises a pointed question: if a 140-meter rock were on a collision course with Earth, would anyone see it in time to act?
Congress Set the Goal, but the Sky Is Still Dark
More than two decades ago, lawmakers directed NASA to find and catalog 90 percent of all near-Earth objects measuring 140 meters or larger. That mandate, codified in the National Aeronautics and Space Administration Act of 2005, gave the agency a 15-year window to complete the survey, a deadline established in House hearing records that spelled out the detect, track, catalog, and characterize requirements. The rationale was straightforward: objects in this size class can cause massive regional devastation on impact, even if they fall short of triggering a global extinction event. A single impact near a dense population center could flatten infrastructure, ignite widespread fires, and disrupt global supply chains in a matter of minutes.
The deadline has long since passed, and the results are sobering. According to NASA’s planetary science division, the agency had found only about 40 percent of near-Earth objects in the 140-meter-and-above category. That leaves the majority unaccounted for. With an estimated population of roughly 15,000 near-Earth asteroids larger than 140 meters, as summarized by the Center for Near-Earth Object Studies at the Jet Propulsion Laboratory, simple arithmetic puts the number of undiscovered objects in the thousands. The shortfall is not hidden in fine print; it is a direct consequence of trying to survey a dynamic, three-dimensional volume of space with instruments limited by weather, daylight, and the intrinsic faintness of small, dark rocks.
What a “City-Killer” Actually Means
The phrase “city-killer” sounds like science fiction shorthand, but NASA applies it to a real and specific threat category. Near-Earth objects around 140 meters in diameter carry enough kinetic energy to level an urban area or generate destructive airbursts over wide regions. A retrospective from JPL on two decades of tracking explicitly notes that objects of this size can cause massive regional devastation, a finding that informed the original Congressional mandate. These are not the planet-ending monsters of Hollywood disaster films, but they do not need to be. A rock the length of a football field, arriving at tens of kilometers per second, converts its velocity into an explosion measured in megatons, more than enough to erase a city center and overwhelm local emergency services.
The recent case of asteroid 2024 YR4 illustrates how the system works when it works. Classified as a city-killer-class object, 2024 YR4 briefly attracted public attention after early orbit calculations could not rule out an Earth encounter in 2032. Further observations resolved the uncertainty: NASA’s fact sheet on the object confirms it has no significant chance of Earth impact in 2032 or in the foreseeable future. The agency’s damage modeling for the hypothetical collision suggested a likely airburst, with limited tsunami potential because of the object’s size and expected entry behavior. That outcome was reassuring, but it also exposed a tension. The system caught 2024 YR4 because it was bright enough and favorably placed in the sky to spot. The thousands of similar-sized objects that remain undiscovered have simply not yet wandered into that observational sweet spot.
Deflection Works, but Only After Detection
NASA proved in 2022 that humanity can alter an asteroid’s trajectory on purpose. The Double Asteroid Redirection Test, known as DART, slammed a spacecraft into the small moon Dimorphos and shortened its orbital period around the larger asteroid Didymos by approximately 32 minutes, according to NASA’s confirmation of the mission results. The agency’s pre-set success criterion was a change of at least 73 seconds, meaning DART exceeded expectations by a factor of more than 25. Follow-up analysis, including a peer-reviewed study in Nature, refined the period change to roughly 33 minutes with tight error bars, providing the strongest technical evidence yet that kinetic impact deflection is a viable tool for planetary defense.
Yet deflection without detection is meaningless. DART targeted a known object with a well-characterized orbit, and mission planners had years of lead time to design, launch, and guide the spacecraft. A real planetary defense scenario against one of the thousands of undiscovered 140-meter asteroids would demand something fundamentally different: early warning measured in years or decades, not months. Current ground-based surveys cannot reliably deliver that kind of advance notice for the majority of unseen objects, particularly those approaching from the direction of the sun or spending much of their time inside Earth’s orbit where they are lost in glare. The DART result is a proof of concept, not a shield. It demonstrated that the physics of deflection are sound, but the operational chain from discovery to response remains incomplete for most of the threat population.
The Detection Gap and What Fills It
NASA’s Near-Earth Object Observations Program coordinates the survey projects, orbit calculations, radar campaigns, and mitigation research that form the backbone of the agency’s planetary defense architecture. The program’s stated objective is still to find, track, and characterize at least 90 percent of predicted near-Earth objects 140 meters and larger, echoing the benchmark Congress set in 2005. The fact that the timeline has stretched well beyond the original 15-year window reflects not indifference but the physical difficulty of the task. Many hazardous objects are dark, irregularly shaped, and spend much of their orbits far from Earth, where even large telescopes see them only as faint points against a crowded star field.
Much of the current discussion around closing the detection gap centers on moving the search above Earth’s atmosphere. Ground-based telescopes must contend with cloud cover, moonlight, and the simple fact that they can only observe at night. A dedicated infrared telescope in space, by contrast, can survey the sky almost continuously and is better suited to spotting the heat signatures of dark, carbon-rich asteroids that reflect little visible light. NASA’s decision to advance a new asteroid-hunting observatory through its planetary science portfolio reflects a recognition that incremental improvements to existing surveys will not be enough to meet the 90 percent goal. Without a step change in capability, the catalog of city-killer-class objects will continue to grow slowly, and the margin for surprise will remain uncomfortably large.
Living With the Remaining Risk
The uncomfortable truth is that for the foreseeable future, humanity will live with a nonzero risk from undiscovered city-killer asteroids. The probability that any particular year will bring a devastating impact is low, but it is not zero, and the consequences of a direct hit near a metropolitan area would be severe. The policy question is not whether to eliminate the risk entirely, that is not possible, but how far to drive it down and how quickly. Investments in better surveys, faster orbit determination, and more realistic impact modeling all serve the same end: turning an unanticipated catastrophe into a manageable engineering and emergency-planning problem.
In that sense, the current state of planetary defense is paradoxical. On one hand, the DART mission has shown that deflection can work, and ongoing tracking efforts have already ruled out impacts from the largest, truly civilization-ending asteroids for the near future. On the other hand, the very class of objects that Congress highlighted (those around 140 meters across) remains only partially mapped, with thousands of potential city-killers still unseen. Until the detection gap is closed by new space-based surveys and sustained ground observations, the planet’s best defense against such an object remains a combination of statistical luck and the hope that, if a threatening rock is finally spotted, it will be early enough for DART’s proof of concept to become an operational reality.
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*This article was researched with the help of AI, with human editors creating the final content.
