More than half of the near-Earth objects large enough to destroy a city have never been spotted by any telescope on or off the planet. NASA estimates the total population of these objects at roughly 25,000, yet fewer than half have been cataloged, leaving thousands of potential threats hidden in the dark stretches between Earth and the Sun. The gap between what scientists know and what they cannot yet see is the central problem, and it is driving a new generation of planetary defense missions.
Thousands of Asteroids Still Missing From the Catalog
The scale of the detection shortfall is striking. NASA’s near‑Earth object program states that less than half of the estimated 25,000 near-Earth objects measuring 140 meters or larger have been found. Objects at that size threshold are frequently described as capable of regional devastation, a category sometimes called “city-killer” asteroids because a single impact could flatten an urban area and its surroundings.
A Science Definition Team convened by NASA pegged the total population of NEOs at or above 140 meters at approximately 25,000, a figure detailed in an agency assessment report on detection status. That same report noted that Congress has mandated the detection of 90 percent of these objects. Progress has been steady but slow. Ground-based surveys add hundreds of newly discovered NEOs each year, as tracked by JPL’s discovery statistics, yet the pace is not fast enough to close the gap within the mandate’s original timeline.
The reason so many remain hidden is partly physical. Smaller and darker asteroids reflect less sunlight, making them nearly invisible to optical telescopes that rely on reflected light to spot moving objects against the star field. An asteroid’s apparent brightness depends on both its size and its albedo, or surface reflectivity. Two objects of identical diameter can look dramatically different if one has a charcoal-dark surface and the other is lighter rock. That ambiguity means even size estimates for known objects carry uncertainty, and genuinely dark objects can slip past surveys entirely.
Geometry adds another layer of difficulty. Many ground-based surveys avoid looking too close to the Sun in the sky to protect their instruments, yet some NEOs spend much of their time in those sunward regions. Others approach from the daytime side of Earth, where optical telescopes on the night side cannot see them at all. These blind spots leave room for surprise encounters, including objects that might be discovered only days or weeks before a close pass.
Why 140 Meters Is the Critical Line
NASA’s planetary defense office defines a potentially hazardous object as one larger than 140 meters that also passes close enough to Earth’s orbit to pose a future collision risk. That 140-meter threshold is not arbitrary. At that size, an asteroid carries enough kinetic energy to cause severe regional damage on impact, destroying infrastructure across an area the size of a small state or a densely populated metropolitan zone.
Most public attention focuses on extinction-class asteroids, the kilometer-wide bodies that could trigger global climate disruption. Those larger objects are actually the easier detection problem. Their size makes them bright enough for existing surveys to spot, and NASA has already found more than 90 percent of NEOs above one kilometer. The harder challenge, and the one that carries the most practical risk for any given generation, involves the mid-range objects between 140 meters and one kilometer. They are large enough to be catastrophic on a regional scale but small enough to hide.
According to NASA’s overview of near‑Earth asteroids, impacts from smaller bodies are more frequent than civilization-ending strikes from giant objects, which means the cumulative risk to people and infrastructure is dominated by these mid-sized, harder-to-detect rocks. The 140-meter benchmark therefore represents a compromise between what is technically feasible to find and what poses a realistic danger to modern society.
NEO Surveyor and the Push to Close the Gap
To address the detection deficit, NASA is building NEO Surveyor, a space-based infrared telescope designed specifically to hunt for asteroids that ground observatories miss. The mission carries the congressionally directed goal of finding and characterizing 90 percent of NEOs at or above 140 meters.
The advantage of an infrared instrument in space is direct. Instead of relying on reflected sunlight, an infrared sensor detects the heat that asteroids radiate. Dark objects that are nearly invisible to optical telescopes glow in the infrared, which means NEO Surveyor can find the very population that current surveys struggle to catalog. By operating beyond Earth’s atmosphere, the spacecraft also avoids weather, atmospheric blurring, and the day and night cycle that limit ground-based observing time.
Another critical advantage is vantage point. NEO Surveyor is designed to operate in a solar orbit that allows it to look back toward Earth’s neighborhood and scan regions near the Sun that are currently difficult or impossible to monitor from the ground. Many of the most concerning discovery scenarios involve asteroids approaching from the direction of the Sun, where they are effectively hidden against the bright sky. A dedicated space telescope can watch those regions continuously, shrinking the window in which a hazardous object could remain undetected.
The practical question is timing. Every year that the full catalog remains incomplete is another year during which an undetected object could approach Earth without warning. Ground-based telescopes have done impressive work over the past two decades, but they face inherent limitations: weather, daylight, and the inability to scan regions of the sky near the Sun where some asteroids lurk. A space-based platform designed expressly for survey work offers the best chance to meet the 90 percent mandate within a realistic timeframe.
How NASA Tracks What It Has Found
For asteroids already in the catalog, NASA operates a layered monitoring system. The agency’s Sentry system continuously recalculates the collision probability for every known NEO based on the latest observations. When a new asteroid is discovered, its initial orbit solution is rough, sometimes producing an elevated impact probability that drops sharply as additional data refine the trajectory.
Each new position measurement helps pin down the object’s path, shrinking the uncertainty region that astronomers project forward in time. In the early days after a discovery, that uncertainty region can intersect Earth’s orbit at specific dates, which is why newly found asteroids occasionally appear on public risk lists with small but non-zero impact probabilities. As more observations come in, those potential impact solutions are usually ruled out, and the object is either removed from the risk list or left with a vanishingly small probability of collision.
A recent example illustrates the process. When a newly identified asteroid such as 2024 YR4 is first added to the catalog, its initial orbital solution may suggest a range of possible future paths, some of which graze Earth’s position. Follow-up observations quickly narrow that range, and the calculated impact probability typically falls to zero or near-zero as the orbit is refined. Early uncertainty is normal, not alarming, and the monitoring system is designed to converge on an accurate answer as more data arrive.
That system works well for known objects. The danger lies with the thousands that have never been observed at all. An asteroid that has not been detected cannot be placed on a risk list, cannot have its orbit refined, and cannot trigger a warning. The entire monitoring infrastructure, from discovery surveys to automated impact probability calculations, depends on first seeing the object against the background of space. Until missions like NEO Surveyor can close the discovery gap, planetary defense will remain a story of impressive tools applied to only part of the threat.
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*This article was researched with the help of AI, with human editors creating the final content.
