The idea of an asteroid smashing into Earth has been a source of fascination for much of humanity, inspiring everything from blockbuster movies to existential dread. But when there are headlines about a “city-killer” asteroid heading straight for our planet, how seriously should we take those threats? These asteroids — space rocks big enough to wipe out a metro region — represent a real but frequently misunderstood threat. This article examines the science of asteroid impacts, how prepared humans are for such an event and if it’s worth losing sleep over a cosmic collision.
What Is a “City-Killer” Asteroid?
Defining the Threat
“City-killer” asteroid usually refers to a near-Earth object (NEO) between 50 and 140 meters (164–460 feet) across. Though not as big as the 10-kilometer-wide asteroid that killed the dinosaurs 66 million years ago, these mid-tier rocks still deliver enough explosive energy to obliterate entire regions. For perspective, a 50-meter-wide asteroid could produce an explosion equal to 10 megatons of TNT — about 600 times the strength of the atomic bomb dropped on Hiroshima. The impact would flatten buildings, ignite fires and create shock waves strong enough to kill people hundreds of miles away.
Historical Precedents
Earth has survived several close calls with city-killer-sized asteroids. The best-known example is the Tunguska event of 1908 in Siberia, when an asteroid with a 60-meter diameter detonated in the atmosphere, leveling 2,000 square kilometers of forest. Most recently, the 7- to 20-meter Chelyabinsk meteor of 2013 injured more than 1,500 people in Russia when its airburst windows shattered and damaged infrastructure. Neither of these asteroids hit populated areas, but their impacts show the potential danger.
Asteroids of the size that destroyed Tunguska are estimated to hit Earth every 100–200 years. Smaller objects like Chelyabinsk, meanwhile, enter the atmosphere much more often — once every 10–50 years or so. Most, of course, burn up harmlessly, but their variability illustrates that care is needed.
Detection Challenges
The detection of city-killer asteroids remains a challenge, despite advances in technology. Their still-diminutive size makes them especially difficult to detect compared with larger “planet-killer” asteroids, and their orbits can change on account of gravitational interactions with planets or solar radiation. Current surveys, notably NASA’s Near-Earth Object Observations Program, have mapped about 40 percent of asteroids 140 meters (about 460 feet) and larger — leaving thousands left unaccounted for.
How Common Is an Impact — and What Can We Do to Prevent It?
Calculating the Odds
Statistically, the chances of a calamitous asteroid impact occurring in any particular year is inordinately low. According to NASA’s Planetary Defense Coordination Office (PDCO), asteroids measuring 1/10 of a kilometer or larger hit Earth once every 10,000 years, whereas city-killer sized impacts take place roughly every 1,000 years. For some perspective, your risk of dying in a car crash is about 250,000 times greater than that of dying in an asteroid strike.
But low probability does not mean no risk. In 2019, a 100-meter asteroid, dubbed 2019 OK, flew past the Earth yet closer than 65,000 kilometers away—one-fifth the distance to the Moon—and with only 24 hours’ warning. Its detection pointed to weaknesses in our systems of monitoring.
Planetary Defense Strategies
If scientists did find an asteroid that could be hazardous years or decades in advance, humanity has a few tools for knocking it off course:
Kinetic Impactors: NASA’s Double Asteroid Redirection Test (DART) mission changed the orbit of a small asteroid by smashing a spacecraft into it in 2022. That “kinetic impactor” approach might, over time, push an asteroid off its collision course.
Gravity tractors: A spacecraft can fly next to an asteroid and with its gravitational attraction can gently tow the asteroid’s position.
Nuclear Devices: As a last resort, detonating a nuclear explosive close to an asteroid could either fragment it or alter its path. But this risky strategy carries political and ethical implications.
For shorter warning times (from months to years out), options are limited. The only realistic action to take would be to evacuate the expected impact area.
Current Detection Efforts
There are global initiatives seeking to improve tracking of these asteroids. NASA’s NEOWISE telescope, followed by its future NEO Surveyor mission, will target 90% of near-Earth objects larger than 140 meters by 2030. Ground-based projects such as the Asteroid Terrestrial-impact Last Alert System in Hawaii scan the sky every night for incoming threats. International collaborations like the International Asteroid Warning Network (IAWN) allow scientists to share information quickly.
Funding and political will continue, however, to be obstacles. Global planetary defense efforts are funded to the tune of under $200 million a year — a small slice of many national military budgets.
Evaluating the Risk: Where Should Our Concern Lie?
Contextualizing the Threat
Asteroid impacts are much further down the list of existential risks in the short term, compared to climate change, pandemics or nuclear war. Even if something like a city-killer struck, the effects would probably be regional, not global. Still, the psychological and economic toll may be severe. A 2018 U.S. government report cautioned that a lack of preparedness for an asteroid impact could induce panic, social and civic unrest and geopolitical instability.
The Role of Public Perception
Asteroid risks are often exaggerated in the media. Creepy headlines about “close shaves” rarely point out that “close” in astronomic terms means millions of miles. Films like Armageddon also pervert public understanding by framing asteroid deflection as a Hollywood-style race against time. In fact, mitigation takes decades of planning and international cooperation.
The Torino Scale — rated 0 to 10 — is used by scientists to convey impact risks. Most detected asteroids score 0 (“no hazard”), whereas a 1 indicates a “routine” discovery worth monitoring. No asteroid has ever reached level 4 (“close encounter with 1% or greater chance of collision”).
Preparing for the Inevitable
While a city-killer impact is not likely to occur in our lifetimes, the time to be prepared is now. Steps include:
Increasing detection capabilities: Funding for space-based telescopes to watch for blind spots — asteroids coming from the side of the Sun.
Running Simulations: NASA and ESA do “tabletop exercises” simulating response scenarios on a regular basis.
Strengthening Global Partnerships: We need interstate cooperation on asteroid defense, like the Paris Agreement or International Space Station partnerships.
Public education also has a role to play. Fear-mongering and evidence-based risk are indeed different; accepting that difference is the first step toward having a rational discussion.
Conclusion: No Panic, But Vigilance
The Earth is a place of a cosmic shooting gallery and asteroid impacts are inevitable over geologic timescales. But man is not impotent anymore before this threat: the advanced detection systems and emerging deflection technologies should make this clear. City-killer asteroids warrant study and investment, but not existential dread.
The key takeaway? We gol to scientists and governments, to stay ahead of the curve; and to the public, to be informed and not scared.” As Lindley Johnson, NASA’s Planetary Defense Officer, eloquently put it: “The best time to stop an asteroid is before it’s headed toward us.” With persistence and vigilance, even a city-killer asteroid can be converted from disaster into a victory of human ingenuity.