Thousands of “city killing asteroids” that are small enough to evade detection and big enough to crush a metropolitan region are floating through the night like the plot of a late-night science fiction movie. This isn’t Hollywood, though. This discussion is taking place in conference rooms; most recently, NASA experts discussed it at a science symposium in Phoenix with a guarded candor that seems out of character for government organizations.
NASA planetary defense officer Kelly Fast was blunt. She claimed that the asteroids we are unaware of are what keep her up at night. The big, dinosaur-ending rocks, on the other hand, are mostly tracked. Not the pieces that burn up in the atmosphere every day, the size of pebbles. They are the mid-sized ones, about 140 meters across, big enough to destroy an entire region but still faint enough to evade even the most advanced observatories.
Near-Earth “City Killing” Asteroids – Key Facts
| Category | Details |
|---|---|
| Official Classification | Near-Earth Objects (NEOs) |
| “City Killer” Size Range | ~140–500 feet (40–150 meters) |
| Estimated Undetected Objects | ~15,000 mid-size NEOs |
| Responsible Agency | NASA |
| Planetary Defense Office | NASA Planetary Defense Coordination Office |
| Key Mission | Double Asteroid Redirection Test (DART) |
| Notable Test Target | Dimorphos (moonlet of Didymos) |
| Reference | https://www.nasa.gov/planetarydefense |
You can almost see the irony when you’re standing in Arizona under a clear desert sky. The stars seem innocuous and sharp. Planets have steady blinks. Nothing seems dangerous. Nevertheless, many of rocky objects traverse Earth’s orbital neighborhood beyond that silence, some of which follow us in sunlight that is difficult for ground telescopes to see.
Only around 40% of the approximately 25,000 near-Earth asteroids in this size category that NASA thinks exist have been found. That leaves about 15,000 people missing. Perhaps none will hit Earth within our lifetimes. According to statistics, there is little risk in any given year. However, history serves as a reminder that unlikely events do occur, therefore low probability does not equate to zero.
When a mid-sized asteroid exploded above Siberia in 1908, it caused the Tunguska catastrophe, which destroyed 80 million trees over 800 square miles. Skyscrapers weren’t present. No freeways. no electrical grids. Consider the same explosion over Los Angeles, Mumbai, or Tokyo. Broken glass wouldn’t be the only damage.
It’s difficult to ignore how this threat seems different from pandemics or climate change. They develop gradually. No, asteroids don’t bargain. Traveling at tens of thousands of miles per hour, they arrive abruptly and transform kinetic energy into shockwaves and fireballs.
NASA’s Double Asteroid Redirection Test, or DART, garnered media attention in 2022. The mission successfully nudged the orbit of the asteroid Didymos by purposefully crashing a spacecraft into Dimorphos, a tiny moonlet orbiting it. By most measures, it was a victory—evidence that humans can alter the course of an asteroid.
There is a catch, though.
DART co-creator Nancy Chabot has noted that we do not yet have deflection spacecraft on launchpads that are prepared for deployment. It takes years to build a mission and launch it. Our options would be severely constrained if a city-killer asteroid were discovered with only a few months’ notice. Political leaders’ willingness to finance long-term, perhaps useless planetary defense systems is still up in the air.
It seems that few policymakers want to focus on this silent vulnerability. Governments get ready for cyberattacks, economic downturns, and wars. Constant investment is not justified by a cosmic impact since it feels too theatrical. Space defense should continue to be mission-based rather than infrastructure-based, according to investors. They might be placing a probability wager.
Astronomers continue to search the sky every night, processing faint light streaks with rotating telescopes over the horizon. At night, the air outside observatories has a slight metallic odor, and red lights softly glimmer to maintain vision in the dark. Tracking moving dots against static star backgrounds while seated at monitors, scientists are aware that every discovery fills a tiny awareness gap.
By using thermal fingerprints rather than reflected sunlight to detect asteroids, the forthcoming Near-Earth Object Surveyor satellite telescope seeks to increase that awareness. This is significant because a large number of dark asteroids effectively hide in plain sight by absorbing light rather than reflecting it. Such a telescope is not a showy launch. It doesn’t create videos that become viral. However, it could lessen the unknowns.
Humanity seems to be at an uncomfortable stage of development as we watch this happen—advanced enough to measure galaxies in the distance, but not yet sufficiently prepared to protect a single city from a boulder moving faster than a bullet. That tension is humbling in an odd way.
Are asteroids that kill cities a real threat? Most likely not. No known asteroid constitutes a serious threat in the coming century, according to NASA themselves. However, “known” is a powerful word. Timelines for discovery are the true source of concern. The key is detection. Deflection may be achievable with years of warning. We would be figuring out evacuation zones in weeks.
And public psychology comes next. “City killer” is a headline-grabbing phrase. It makes you nervous. It might even exaggerate the sense of danger. However, it also compels a discussion about readiness versus complacency. It’s likely that the anxiety itself has a function, influencing financial choices that would otherwise lean toward more pressing emergencies.
Millions of people stroll along city streets every evening without seeing the orbital dance taking place above them. Towers of apartments have flickering lights. There is a hum of traffic. Aircraft fly through the sky. A tiny, icy asteroid might be orbiting in time with Earth somewhere outside of that well-known pattern, but it is now invisible.
The probabilities are in favor of clear sky. However, experts in planetary defense are not compensated to depend solely on odds. They are compensated to investigate, make calculations, and covertly become ready for the unexpected.
We are not helpless, but we are not yet completely prepared, and that may be the most disturbing reality of all.