NASA has found fewer than a third of the near-Earth asteroids large enough to destroy a city, and the agency’s best tool for closing that gap will not reach space until at least 2027. The shortfall matters because Congress set a deadline two decades ago that has already expired, and the detection void left by a recently retired spacecraft means Earth currently lacks a dedicated infrared eye scanning for these threats. Thousands of these objects, each 140 meters or wider, remain invisible to existing surveys, raising practical questions about how quickly the United States can fulfill its own planetary defense mandate.
A Congressional Mandate That Missed Its Own Deadline
The legal foundation for finding these asteroids dates to 2005, when Congress passed a NASA authorization law that directed the agency to detect, track, catalog, and characterize near-Earth objects measuring 140 meters or larger. Lawmakers set a goal of reaching 90% completion within 15 years of enactment, effectively giving NASA until 2020 to identify the vast majority of these so‑called “city killers.” That 15-year window has closed, and the agency has acknowledged that it fell well short of the statutory benchmark.
How far short remains a sobering figure. Expert teams estimate that the total population of near-Earth asteroids at or above the 140-meter threshold is roughly 25,000, while a NASA status report found that discoveries at the time numbered just under 8,000 objects. That leaves on the order of 17,000 sizable bodies unaccounted for. The gap is not simply a bookkeeping failure: each unknown asteroid carries enough kinetic energy to devastate a metropolitan area, and current ground-based telescopes can only spot them when geometry, distance, and lighting line up. With no revised deadline written into law, the country’s legal benchmark for planetary safety remains unmet, and the timeline for achieving it is now effectively tied to the schedule of a single future mission.
NEOWISE’s Retirement Left a Detection Void
Until mid-2024, NASA had at least one space-based infrared asset dedicated to asteroid hunting. The NEOWISE spacecraft conducted repeated infrared scans of the sky that allowed scientists to estimate the size and reflectivity of near-Earth objects, rather than inferring those properties from brightness alone. Infrared detection is critical because many asteroids are dark, absorbing visible light instead of reflecting it efficiently. Without thermal measurements, surveys must rely on absolute magnitude and assumed albedo to estimate diameter, a method that the near‑Earth object statistics maintained by NASA’s Center for Near-Earth Object Studies show can introduce wide uncertainty in size estimates.
NEOWISE’s infrared survey ended on July 31, 2024, and the spacecraft was decommissioned after years of extended service. That retirement created a gap in capability that no existing instrument fully replaces. Ground-based optical telescopes continue to discover new asteroids, but they are far less effective at determining physical size, which is the property that matters most for threat assessment and impact modeling. A small, bright asteroid and a large, dark one can appear nearly identical in visible-light data. With NEOWISE gone, the global asteroid catalog is still growing in sheer object count, but progress has slowed in the one dimension (reliable size characterization) that determines whether an object qualifies as a genuine hazard under the congressional mandate.
NEO Surveyor: The Infrared Fix Still Years Away
NASA’s answer to the detection gap is the Near-Earth Object Surveyor, an infrared space telescope designed specifically to pick up where NEOWISE left off with far greater sensitivity. The agency has awarded a launch services contract to SpaceX valued at about $100 million, with a target launch date no earlier than September 2027. Reporting in a leading science journal notes that more than half of the “city killer” asteroids that might threaten Earth remain undiscovered, and NEO Surveyor’s infrared instruments are tuned to detect exactly these mid‑sized, potentially hazardous objects. Operating from a vantage point in space, the telescope is expected to scan regions of the sky that are difficult or impossible to observe from the ground, including areas near the Sun where some threatening orbits can hide.
Yet the September 2027 date is explicitly framed as “no earlier than,” meaning it should be read as the first possible launch window rather than a firm schedule. A recent assessment of major NASA projects by the Government Accountability Office highlights recurring cost growth and schedule slippage across the agency’s portfolio, with high‑risk acquisition challenges cited as a persistent concern. NEO Surveyor itself has already experienced budget and timeline adjustments during its formulation. If the telescope slips further, the detection void that opened with NEOWISE’s retirement will stretch well past three years, a period during which newly approaching asteroids could remain undetected until they are much closer to Earth. The mission is designed to accelerate discoveries dramatically once it flies, but its benefits cannot be realized before launch.
Why the Gap Matters Beyond the Numbers
For the average person, 17,000 unseen asteroids can sound like an abstract inventory problem. The stakes become clearer when framed against the 2023 National Preparedness Strategy and Action Plan for near‑Earth objects, published by the National Science and Technology Council. That document lays out U.S. planetary defense goals spanning detection and tracking, development of deflection and disruption technologies, emergency response procedures, international coordination, and governance. Every one of those pillars depends on finding hazardous objects early. Deflection missions like NASA’s DART experiment, which successfully altered the orbit of a small asteroid moon in 2022, require years of advance warning to meaningfully change a trajectory. Without early detection, deflection is not an option, and the remaining tools are evacuation, sheltering, and disaster response, approaches that can save lives but cannot prevent regional devastation.
The strategy also underscores that realistic impact modeling and coordinated emergency procedures demand accurate size, composition, and orbit data, the kind of information that infrared surveys provide and that ground-based visible‑light telescopes struggle to supply on their own. Underestimating an asteroid’s diameter by even a modest fraction can translate into a large error in predicted impact energy, which in turn affects everything from evacuation zones to infrastructure hardening plans. The current detection gap therefore has cascading consequences: it slows the march toward the congressional 90% goal, leaves civil authorities planning around incomplete threat catalogs, and forces the international community to make planetary defense decisions with less information than the available technology (if fully deployed) could offer.
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*This article was researched with the help of AI, with human editors creating the final content.
