Nearly two decades after the U.S. Congress ordered NASA to find 90% of the asteroids large enough to destroy a city, the agency has cataloged fewer than half of them. The gap between that mandate and the current detection rate has left planetary scientists increasingly vocal about the risk that a significant number of dangerous objects remain invisible to existing telescopes. With roughly 14,000 such asteroids still unaccounted for, the question is not whether another close call will happen but when, and whether anyone will see it coming.
A Congressional Mandate Falling Short
The legal foundation for America’s asteroid-hunting effort is the Near-Earth Object Survey Act, which established the policy requirement for a survey program targeting at least 90% of near-Earth objects 140 meters (460 feet) or larger. Objects at that size threshold can cause significant regional damage, according to NASA, making them the critical risk class for planetary defense planning. The law set an ambitious timeline for achieving that catalog completeness, but progress has not kept pace with the threat as estimates of the total asteroid population have become more refined.
During a hearing titled “From Detection to Deflection: Evaluating NASA’s Planetary Defense Strategy,” the chairman’s opening remarks noted that nearly 20 years after the 2005 act, only about 44% of the estimated near-Earth objects larger than 140 meters have been identified. NASA’s own Science Definition Team has estimated the total population of these objects at around 25,000, implying that more than 14,000 asteroids in this size class have never been observed, tracked, or characterized. For anyone living in a major metropolitan area, that number carries real weight: a single 140-meter asteroid striking near a population center could produce destruction on a scale that dwarfs any conventional disaster, yet the current survey architecture leaves thousands of such bodies effectively invisible.
Why Ground Telescopes Keep Missing Threats
The detection shortfall is not simply a matter of funding or willpower. It reflects a fundamental physical limitation in how ground-based telescopes see the sky. Most of the existing survey infrastructure relies on optical instruments that detect reflected sunlight, which makes it difficult to spot dark, carbon-rich asteroids and those that approach Earth from directions close to the Sun. Because observatories can only operate at night and must look through the atmosphere, large swaths of the sky near the solar glare remain poorly covered. A recent NASA assessment of near-Earth asteroid detection explicitly acknowledged that many objects at or above the 140-meter threshold remain undiscovered, and that ground-based surveys alone are unlikely to close the gap within the mandated timeframe.
These blind spots are not just theoretical. Past surprise events, such as the 2013 Chelyabinsk airburst over Russia, involved objects that were too small and came from directions that made them effectively undetectable with then-current systems. While Chelyabinsk was well below the 140-meter benchmark, it served as a vivid reminder that the combination of limited sky coverage, weather, and daylight constraints can allow hazardous bodies to slip through. As astronomers push existing facilities to their limits, they face diminishing returns: adding more nights of observation or slightly larger mirrors does not fundamentally change the geometry of what the atmosphere and the Sun’s glare will allow them to see.
NEO Surveyor and the Push into Space
Recognizing those limitations, NASA has turned to space-based assets as the only realistic way to approach the 90% detection goal. The agency’s planned NEO Surveyor mission is a dedicated infrared observatory designed specifically for planetary defense. From its vantage point in space, the spacecraft will scan for asteroids by picking up their heat signatures rather than depending on reflected visible light, a technique that makes it far more effective at spotting dark objects and those lurking near the Sun’s direction. NASA’s mission description emphasizes that infrared observations are particularly powerful for detecting the 140-meter class of near-Earth objects that Congress singled out as the highest priority.
Development work on the spacecraft and its instruments is already in progress, with NASA reporting that construction activities are underway to prepare NEO Surveyor for launch in the early years of the next decade. Once operational, the mission is expected to dramatically accelerate the discovery rate of potentially hazardous asteroids and provide more accurate size estimates by measuring thermal emission. However, even optimistic projections suggest that reaching 90% completeness for 140-meter objects will require several years of continuous surveying. That means the current period, before launch and during early operations, will remain a window of heightened vulnerability when a dangerous object could still emerge with little warning.
Deflection Works, but Only with Warning
While the detection picture remains incomplete, the prospects for active defense have improved. NASA’s Double Asteroid Redirection Test, or DART, provided the first real-world demonstration that a kinetic impactor can measurably alter an asteroid’s trajectory. By crashing a spacecraft into Dimorphos, a small moon of the asteroid Didymos, mission planners shortened Dimorphos’s orbital period by roughly half an hour, with the change measured as negative 33.0 plus or minus 1.0 minutes at the three-sigma confidence level and published in a peer-reviewed study. This result confirmed that, at least for objects of similar size and composition, a carefully targeted impact can shift an orbit enough to matter for planetary defense if sufficient lead time is available.
That caveat (lead time) is crucial. Designing, building, and launching a deflection mission requires years, and in some scenarios decades, of preparation. Without early detection, even the most sophisticated deflection technology is irrelevant, because there is no opportunity to intervene. NASA’s automated impact monitoring system known as Sentry continuously scans the catalog of known near-Earth objects, using updated observations to refine orbits and calculate impact probabilities. Objects enter and leave Sentry’s risk tables as new data arrive, and the system expresses risk using scales that combine probability and potential damage. However, Sentry can only analyze asteroids that have already been discovered; the thousands of 140-meter-class bodies still missing from the catalog remain outside any risk assessment, representing a reservoir of unknown threats that could appear too late for any realistic deflection effort.
Real Scares, Public Anxiety, and Policy Gaps
The asteroid designated 2024 YR4 offered a recent glimpse of how the current system behaves when a plausible impact scenario emerges. NASA’s analysis determined that the object initially had more than a 1% chance of impacting Earth on a specific December date, a probability high enough to trigger established international notification protocols and draw scrutiny from policymakers. As additional observations came in, the impact risk was eventually ruled out, but the episode exposed how quickly a routine tracking exercise can escalate into a global concern when probabilities cross certain thresholds. It also highlighted the ad hoc nature of decision-making about when and how to inform the public, and what kinds of contingency plans national governments actually have in place.
Events like 2024 YR4 do not occur in a vacuum, they intersect with broader cultural fears about space and existential risk. Social scientists have noted that narratives about cosmic threats can amplify feelings of helplessness and feed into conspiracy thinking, especially when official communication is opaque or delayed. One analysis argued that public unease about hazards beyond our control, including asteroid impacts, can intertwine with distrust of institutions and fuel speculative stories about hidden dangers in the sky, as discussed in research on space-related fears. That dynamic underscores the need for transparent, science-based communication strategies that can explain both the real level of risk and the concrete steps being taken to manage it.
Building a Coherent Planetary Defense Strategy
Behind the scenes, NASA and its partners have been working to turn scattered efforts in detection, modeling, and response into a more integrated planetary defense architecture. The agency’s Planetary Defense Coordination Office has supported a suite of technical studies, simulations, and international exercises aimed at improving preparedness for a range of impact scenarios. Many of these activities are documented in supporting materials that describe how detection campaigns, impact probability calculations, and emergency planning fit together. They show a growing recognition that planetary defense is not just an astronomical problem but a multidisciplinary challenge involving civil protection, diplomacy, and public communication.
Still, the central tension remains: technology for deflection is maturing faster than the survey systems needed to find targets in time. Until missions like NEO Surveyor are fully operational and the 140-meter population is largely cataloged, policymakers will be forced to make decisions in the face of significant uncertainty about what is actually out there. Bridging that gap will require sustained investment, clear policy priorities, and international cooperation, since an asteroid impact would not respect national borders. The unfinished search for thousands of potentially hazardous asteroids is therefore more than a scientific shortfall; it is a test of whether the global community can act on a long-term, low-probability, high-consequence threat before circumstances force its hand.
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*This article was researched with the help of AI, with human editors creating the final content.
