NASA estimates that roughly 25,000 near-Earth asteroids measuring at least 140 meters across orbit in our planetary neighborhood, but surveys have so far cataloged only about 40% of them. That gap means approximately 15,000 objects large enough to cause severe regional damage remain uncharted, with orbits that have not yet been measured well enough to fully assess their long-term Earth-impact risk. With a congressional detection deadline already missed and the agency’s next-generation space telescope still years from launch, the race to find these so-called city-killers is far from over.
How Many City-Killers Are Still Missing
The scale of the problem comes into focus through a single pair of numbers. An assessment by NASA’s NEO science team puts the total population of near-Earth asteroids 140 meters or larger at about 25,000. Congress directed the agency to detect 90% of those objects by the end of 2020, a benchmark that was never met. NASA’s report estimates that only about 40% of that population has been found so far; the running discovery statistics maintained by the Center for Near Earth Object Studies at JPL track the growing count of known objects. Using the report’s estimate, that leaves roughly 15,000 asteroids in the 140-meter hazard class still unaccounted for.
Independent modeling supports that picture. A 2024 preprint from asteroid-population researchers, including scientists affiliated with JPL, used debiased size-distribution methods and albedo data to estimate that there are on the order of tens of thousands of near-Earth objects larger than 140 meters. The overlap between that range and the 25,000 figure gives planetary scientists reasonable confidence in the order of magnitude, even as the precise count carries uncertainty. What is not uncertain is the shortfall: most of these objects have never been observed, their orbits remain unknown, and any risk they pose is effectively invisible until they are finally detected.
Ground Surveys and Their Limits
The bulk of asteroid detection work today falls to two NASA-funded ground-based programs. The Catalina and Pan-STARRS surveys have driven the known near-Earth asteroid catalog past the 15,000-object mark, a milestone that JPL highlighted as evidence of steady progress. Both surveys scan the night sky in visible light, picking up sunlight reflected off rocky surfaces, and together they account for the vast majority of new discoveries each year. Their wide-field cameras and automated software have transformed what used to be painstaking manual searches into a nightly pipeline of candidate objects.
Yet visible-light telescopes have a structural blind spot. Dark asteroids with low albedo reflect very little sunlight, making them nearly invisible to optical instruments. Objects that orbit close to the Sun’s glare are similarly difficult to observe from the ground because telescopes can only point away from the Sun at night. These observational blind spots are a key reason NASA is pursuing infrared detection from space, because the current ground-based survey architecture, no matter how productive, cannot close the gap on its own. The Planetary Defense Coordination Office publishes regular tallies of known near-Earth asteroids, and those numbers continue to climb, but finding the remaining 140-meter-class objects is harder because ground surveys tend to pick up the easier, brighter targets first.
NEO Surveyor and the Infrared Advantage
NASA’s planned answer to the detection shortfall is the Near-Earth Object Surveyor, an infrared space telescope designed specifically to find the asteroids that ground-based surveys miss. Because every asteroid radiates heat regardless of how reflective its surface is, an infrared sensor can spot dark objects that optical telescopes overlook. Placing the instrument in space also eliminates the atmospheric interference and daylight constraints that limit Earth-based observatories, allowing NEO Surveyor to scan regions near the Sun’s glare where hidden threats may lurk and to revisit patches of sky on a regular cadence.
The mission’s performance goals are tied directly to the 140-meter hazard class, the same threshold Congress set in its detection mandate. If the telescope operates as designed, it could dramatically accelerate the catalog of city-killer-size objects and push the detection fraction well beyond the current 40%. That matters because orbit determination requires multiple observations over time; the sooner an asteroid is spotted, the longer the warning window for any deflection effort. A late discovery, by contrast, leaves almost no time to act, forcing decision-makers to weigh evacuation and disaster-response planning against the slim possibility of a last-minute space mission.
What 2024 YR4 Revealed About Detection Gaps
A recent case study shows why faster detection matters. When asteroid 2024 YR4 was first discovered, initial calculations suggested a very small chance of impacting Earth on December 22, 2032. The object drew intense public attention precisely because it illustrated the kind of scenario planetary defense experts worry about most: an asteroid found with relatively little lead time and an orbit that could not be immediately ruled out as a threat. As follow-up observations refined its trajectory, the impact probability dropped and the object was eventually ruled out as an impact risk for that date, but not before the episode highlighted how unsettling even a low-probability impact can be.
The 2024 YR4 episode exposed a gap between detection capability and public expectation. Many people assume that any asteroid on a collision course would be spotted decades in advance, giving engineers plenty of time to mount a deflection mission. In reality, the current catalog covers less than half of the 140-meter population, and new objects regularly appear with only years of warning rather than decades. Each discovery like 2024 YR4 is a reminder that the detection infrastructure remains incomplete, and that the window between finding a threat and needing to respond to it can be uncomfortably narrow for policymakers, emergency managers, and the public alike.
Why the 90% Goal Still Matters
Congress set the 90% detection target for a reason: statistical coverage is the cheapest form of planetary insurance. Deflecting an asteroid, whether through a kinetic impactor like NASA’s DART mission or some future technique, requires knowing the object’s orbit years in advance. Reaching 90% completeness for city-killer-size asteroids does not guarantee safety, but it dramatically reduces the odds that a dangerous object will arrive without warning. It also allows engineers to focus on a manageable list of known risks instead of designing defenses around hypothetical, unseen threats scattered throughout near-Earth space.
Closing the remaining gap will demand sustained investment not only in NEO Surveyor but also in follow-up observations, orbit refinement, and public communication. NASA has increasingly used digital outreach, including series on platforms such as NASA Plus, to explain how asteroid detection works and what current capabilities can and cannot do. As more near-Earth objects are found, the number of entries on risk lists may temporarily rise, simply because previously unknown asteroids are finally being tracked. Paradoxically, that surge in apparent threats is a sign of progress: each new detection moves the world closer to the 90% goal and to a future in which a surprise city-killer is far less likely to slip through the cosmic net.
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*This article was researched with the help of AI, with human editors creating the final content.
