Something about the term “city-killer asteroid” is unnerving. It has an almost exaggerated, cinematic sound. However, the atmosphere changes when NASA representatives use it coolly during scientific meetings.
During an American Association for the Advancement of Science meeting recently, NASA’s planetary defense leadership confirmed what has been known in private for years: approximately 25,000 near-Earth asteroids with a diameter of at least 140 meters most likely orbit in the cosmic neighborhood of Earth. Of them, only about 40 percent have been found.
NASA Asteroid Monitoring — Key Facts
| Category | Details |
|---|---|
| Agency | National Aeronautics and Space Administration (NASA) |
| Planetary Defense Office | Established 2016 |
| Estimated Mid-Sized Near-Earth Asteroids | ~25,000 (140m+) |
| Currently Tracked | ~40% |
| Potential Undetected Objects | ~15,000 |
| Demonstrated Deflection Test | DART Mission (2022) |
| Upcoming Detection Mission | Near-Earth Object Surveyor |
| Official Reference | https://science.nasa.gov/solar-system/asteroids |
Approximately 15,000 items remain unaccounted for.
NASA’s acting planetary defense officer, Kelly Fast, did not dramatize the situation while addressing a group of researchers in Phoenix. She didn’t have to. She claimed that the asteroids we are unaware of are what keep her awake at night. Understatement may be more powerful than any alarmist headline ever could be.
Most of the so-called extinction-level threats, or the largest asteroids, are listed. Scientists are aware of their location. They keep a close eye on them. Tension is caused by the in-between size, or middle class, of space rocks. Large enough to level a city and leave a lasting scar in the area, but not enough to wipe out civilization.
Asteroid defense is seen in Hollywood as heroic and decisive. In practice, it is incomplete and incremental.
DART, or the Double Asteroid Redirection Test, was hailed as a breakthrough in 2022. NASA purposefully altered the orbit of a spacecraft by crashing it into the small asteroid moonlet Dimorphos. There was a subdued sense of accomplishment as one watched the impact footage, which showed a grainy stream of debris floating outward. A celestial body had been pushed by humanity.
However, planetary scientists have since acknowledged that no other DART mission is in reserve. It’s still unclear if we could react in time if a recently discovered 140-meter asteroid were discovered on a short-notice collision course.
That’s the part that stays with you.
Geometry and light play a major role in the detection problem. From Earth’s perspective, many asteroids are close to the Sun due to their orbits. The night sky is difficult for ground-based telescopes to scan due to solar glare. Dark asteroids are almost invisible until they are close because they reflect very little sunlight.
It’s difficult to ignore how vulnerable that gap is.
NASA is getting ready to launch the Near-Earth Object Surveyor, a space-based telescope that will detect heat signatures instead of reflected light using infrared sensing. Theoretically, this ought to uncover a lot of hidden objects that optical telescopes overlook. It appears that thermal detection will fill in a large portion of the coverage gap, according to investors in aerospace tracking technologies.
However, the blind spots persist until that mission is operational.
When preliminary estimates indicated a slight but non-zero chance of impact in 2032, the asteroid YR4 momentarily sparked concern last year. An Earth strike was ruled out by additional refinements. There was no longer any risk. For a few weeks, however, analysts watched, journalists updated dashboards, and orbital models were repeatedly recalculated.
As that happened, there was a sense that humanity was becoming more aware of its own vulnerability, which was real but not catastrophic.
The striking thing is how commonplace small asteroid impacts are. Almost every day, tiny rocks burn up in the atmosphere. Most are overlooked. They are dismissed by the planet. However, the mid-sized ones are common enough to have a statistical impact but rare enough to avoid regular rehearsal.
And this is where science and policy diverge.
In many respects, the only natural disaster that humans could completely avoid is planetary defense. Hurricanes cannot be prevented, but they can be predicted. Earthquakes did not redirect, but they resisted. It is theoretically possible to deflect an asteroid if it is discovered early enough.
It’s possible that prioritization, rather than just technological limitations, is the problem. Money is allocated to urgent crises. Until they don’t, asteroids function on longer timescales.
An economic undercurrent is also becoming apparent. In recent months, insurance analysts have been more serious about low-probability, high-impact scenarios. Defense contractors that focus on tracking systems see gaps in detection as an opportunity. Markets often reprice tail risks gradually before doing so all at once.
However, it would be incorrect to portray this as an impending disaster. In any given year, a city-level impact is statistically unlikely. The odds are slim. Not zero, though.
And the unique tension in planetary defense comes from that non-zero margin.
After the conference remarks, one can picture the unseen traffic above—silent, icy, ancient pieces of the solar system that are tracing paths with Earth’s orbit—standing beneath the expansive Arizona sky. Most will go unnoticed. Some have been accurately mapped. Others drift undetected; only after they come into view can their trajectories be calculated.
Whether the Near-Earth Object Surveyor will bridge the gap quickly enough to meet planetary defense objectives is still up in the air. Additionally, it’s uncertain if political support for ongoing funding will endure after the headlines fade.
The topic of discussion has changed. NASA is not issuing apocalyptic warnings. It’s admitting that there is uncertainty. There is a quietly sobering truth to that acknowledgement: despite all of our satellites, supercomputers, and deep-space probes, there are still rocks out there that we haven’t seen.
They are not villains in movies. They are artifacts that have been drifting along ancient paths for 4.6 billion years. And our search for them is just getting started.