The Planet-Killer
Earth’s Asteroid Threat: The Objects Most Likely to Hit Us and What Would Actually Happen
Could an Asteroid Destroy Civilisation? The Real Planet-Killer Risk — and the Objects Scientists Are Watching
There is a natural disaster capable of dwarfing almost anything humanity has experienced in recorded history. No government could stop its immediate effects, national borders would become irrelevant and, at sufficient scale, the climate itself could change. A large asteroid striking Earth could theoretically collapse modern civilisation — and an object several miles across could trigger a mass extinction.
But that does not mean a planet-killing asteroid is currently heading towards us. As of September 2026, astronomers know of no large asteroid with a significant probability of striking Earth within the next century, and more than 90 per cent of the near-Earth asteroid population roughly one kilometre or larger has already been discovered.
The more interesting question is therefore not whether asteroid impacts can destroy civilisation. They can. It is how big the object would need to be, how likely such an event really is, which known asteroids carry the greatest calculated risk, and whether the object humanity ultimately needs to fear could still be undiscovered.
How Big Does an Asteroid Need to Be?
Hollywood has blurred together several very different levels of asteroid disaster. A 50-metre asteroid, a one-kilometre asteroid and a ten-kilometre asteroid do not represent variations of the same event. They occupy radically different categories of catastrophe.
NASA estimates that objects around 50 metres across strike Earth roughly once every thousand years and can produce severe local devastation. At approximately 140 metres, an asteroid could cause destruction across a metropolitan or regional area, with objects of that scale estimated to hit about once every 20,000 years.
Move into the hundreds of metres and the consequences become much harder to contain geographically. National Academies assessments have associated roughly 300-to-500-metre impacts with destruction on a continental scale.
The real civilisational threshold starts somewhere around the kilometre range. NASA says a roughly one-kilometre asteroid could cause global devastation and possibly the collapse of civilisation, while previous planetary-defence assessments classify an object around one kilometre as capable of producing a possible global catastrophe.
The reason is not simply the explosion at ground zero. A sufficiently powerful impact could inject enormous quantities of dust, vapour and debris into the atmosphere, disturb global temperatures, destroy agricultural production and create consequences thousands of miles from the crater. National Academies research has warned that an approximately one-kilometre impact could produce climate effects capable of causing global crop failures lasting several years.
That is where an asteroid becomes more than a disaster. Modern civilisation depends on international food production, power grids, communications, transport, fertiliser, finance, fuel and tightly interconnected supply chains. Destroy enough agricultural output simultaneously and the secondary crisis could prove far more dangerous than the initial blast.
Yet even a one-kilometre impact does not automatically mean human extinction.
What Would a True Planet-Killer Look Like?
For something approaching an extinction-level event, the asteroid needs to become substantially larger.
NASA's emergency planning framework places objects around five kilometres across above the approximate global-catastrophe threshold. At around ten kilometres, the description changes to mass extinction.
Earth already provides the most famous example.
Approximately 66 million years ago, an asteroid estimated at roughly 12 to 14 kilometres across struck near what is now Mexico's Yucatán Peninsula. The resulting Chicxulub impact was associated with drastic global environmental change and the extinction of more than 75 per cent of non-avian life.
An event of comparable scale happening today would not merely destroy a city or country. The principal danger would become planetary: enormous thermal effects, ejecta thrown through the atmosphere, fires, disruption of sunlight, collapsing ecosystems and a catastrophic shock to the global food chain.
Whether every human would die is much harder to establish. Humanity occupies every continent, possesses underground infrastructure, stored food, advanced technology and the ability to plan ahead if warning were available. "Planet-killer" therefore works better as shorthand for an impact capable of wrecking civilisation or causing mass extinction than as a scientific guarantee that Earth itself — or every human — would be destroyed.
The planet would survive. Human civilisation might not.
The Known Asteroids With the Most Interesting Risk
This is where the story becomes less terrifying.
The objects with the highest calculated impact probabilities are generally not enormous civilisation killers. Meanwhile, the known objects large enough to cause global damage have exceptionally small impact probabilities.
ESA's asteroid Risk List contained more than 2,000 objects with non-zero calculated impact probabilities at the beginning of September 2026. Most are tiny on planetary terms. The highest-ranked normal entry was 2023 VD3, estimated at only around 11 to 24 metres across — potentially dangerous locally, but nowhere near a civilisational threat.
Two much larger bodies sit on ESA's separate Special Risk List: Bennu and 1950 DA.
Bennu
Bennu is one of the best-studied asteroids in existence. It is approximately 484 metres across, meaning an impact would be an extraordinary catastrophe, particularly if it struck a populated landmass, but it is substantially below the approximate one-kilometre threshold normally associated with possible global civilisation collapse.
NASA's detailed orbital modelling found an overall chance of approximately one in 1,750 that Bennu could strike Earth between now and 2300. The most significant individual date identified was September 24, 2182, with a probability of roughly one in 2,700.
ESA's current Special Risk List similarly places the September 24, 2182 encounter at approximately one chance in 2,702.
Those numbers sound alarming until the other side of the probability is considered: the overwhelming likelihood is that Bennu misses Earth.
Its importance comes from the combination of relatively large size, unusually precise observations and a measurable far-future impact probability, rather than evidence that a collision is expected.
1950 DA
If there is one known asteroid that fits the civilisational-threat discussion more closely, it is 29075 1950 DA.
ESA currently estimates it at approximately 1.3 kilometres across — large enough to fall within the territory where worldwide consequences become plausible. Its relevant potential encounter does not occur until March 16, 2880. ESA's September 2026 assessment puts the probability at roughly one in 51,020.
NASA has previously described 1950 DA as a kilometre-sized asteroid and calculated an impact probability in the same extremely low general range, although estimates have changed as additional observations and improved orbital modelling have been introduced.
Of the named known asteroids, this makes 1950 DA one of the most interesting genuine global-hazard candidates.
It does not make it a predicted apocalypse. The possible encounter is more than eight centuries away, the probability remains tiny and enormous advances in observation and planetary-defence technology would be expected long before 2880.
What About 1979 XB?
Another object deserves attention because of its size rather than because an impact is likely.
ESA's current Risk List estimates 1979 XB at approximately 400 to 900 metres in diameter, although that size is indirectly estimated and therefore uncertain. Its highest listed individual impact scenario occurs in December 2056 at roughly one chance in 4.27 million, with cumulative calculated probability across possible encounters through 2113 of about one in 1.36 million.
At the upper end of its estimated size range, 1979 XB could produce extraordinary consequences. But probabilities measured in the million-to-one range are radically different from saying scientists expect it to hit.
It is worth monitoring precisely because planetary defence is supposed to identify extremely unlikely catastrophic events before they become emergencies.
Apophis Is Not Coming to Destroy Earth
No asteroid better illustrates how public fear can outlive scientific evidence than Apophis.
The approximately 340-metre asteroid became famous after its discovery in 2004 because early orbital calculations briefly produced a worrying probability of a future Earth collision. Further observations progressively eliminated those possibilities.
NASA now says there is no risk of Apophis striking Earth for at least the next 100 years.
It will nevertheless produce a spectacular close approach on April 13, 2029, passing roughly 32,000 kilometres above Earth's surface — close enough to be visible without a telescope from some locations.
That encounter is scientifically extraordinary. It is not a collision warning.
2024 YR4 Shows How Asteroid Scares Develop
Asteroid 2024 YR4 provided another useful demonstration.
After its discovery in December 2024, preliminary calculations indicated a small possibility that the roughly 53-to-67-metre asteroid could strike Earth in December 2032. As more observations became available, the Earth impact possibility effectively disappeared.
Attention then shifted to a possible lunar collision, with estimates temporarily rising above four per cent. Further observations by the James Webb Space Telescope in February 2026 eliminated that possibility as well; NASA now expects YR4 to miss the Moon by around 13,200 miles.
Nothing physically pushed the asteroid away.
Scientists simply became much more certain about where it was going.
That distinction explains why newly discovered asteroids can briefly appear frightening. Early observations produce a broad corridor of possible future positions. Earth may initially sit somewhere inside that uncertainty region. Additional measurements then narrow the projected orbit until Earth usually falls outside it.
A rising impact probability during that process does not necessarily mean the asteroid has suddenly become more dangerous.
The Most Dangerous Candidate May Be One We Haven't Found
The biggest uncertainty is therefore not Bennu, Apophis or even 1950 DA.
It is the unknown object.
Humanity has already discovered more than 90 per cent of estimated near-Earth asteroids roughly one kilometre or larger, dramatically reducing the possibility of a hidden giant asteroid arriving without warning.
But the survey is not complete, particularly for smaller objects. NASA has estimated a population of roughly 25,000 near-Earth objects at least 140 metres across, and historically fewer than half had been identified.
Objects arriving from near the direction of the Sun present another difficulty because sunlight hampers traditional ground-based optical searches.
That blind spot is one reason NASA is developing NEO Surveyor, the first space telescope designed specifically to seek potentially hazardous asteroids and comets. The infrared observatory is currently scheduled for launch no earlier than September 2027 and is designed to detect dark objects and bodies approaching from directions that conventional ground surveys struggle to observe.
The uncomfortable truth is that an undiscovered asteroid is harder to defend against than a known one with a tiny impact probability centuries in the future.
Warning time may be the most valuable planetary-defence resource humanity possesses.
Could We Actually Stop One?
For the first time in human history, the answer is no longer entirely theoretical.
In September 2022, NASA's Double Asteroid Redirection Test spacecraft deliberately slammed into Dimorphos, a roughly 160-metre moonlet orbiting the larger asteroid Didymos.
The collision altered Dimorphos's orbit around its parent asteroid by approximately 32 minutes. Humanity had deliberately changed the motion of a celestial body for the first time.
That does not mean NASA can casually destroy a ten-kilometre asteroid heading towards Earth.
DART demonstrated one particular method: a kinetic impactor. Its usefulness against a real threat would depend on the object's mass, structure, speed and orbit — and, above all, how early it was discovered.
With decades of warning, only a minute change in velocity may eventually cause an asteroid to miss Earth by thousands of kilometres. With months of warning, the problem becomes vastly more difficult.
A civilisation-threatening impact discovered generations in advance is therefore an engineering challenge. The same asteroid discovered shortly before arrival could become an emergency humanity has almost no ability to prevent.
So How Worried Should We Actually Be?
Asteroids deserve serious attention precisely because the underlying threat is real rather than because catastrophe is imminent.
NASA estimates that kilometre-class asteroids capable of global devastation strike Earth on timescales of roughly hundreds of thousands of years, while objects around ten kilometres across arrive on approximately hundred-million-year timescales.
Those are incredibly low annual probabilities.
They are not zero.
And unlike earthquakes, supervolcanoes or hurricanes, an asteroid impact occupies an unusual category of natural disaster: humanity may eventually possess enough warning and technology to prevent one completely.
The current known candidates offer considerable reassurance. Apophis is cleared for at least a century. YR4 is no longer an Earth threat. Bennu's measurable risk lies centuries away and overwhelmingly favours a miss. The kilometre-class 1950 DA carries only a tiny calculated possibility of impact in 2880.
The asteroid most likely to pose a genuinely civilisation-threatening surprise may consequently be one that has not yet been catalogued.
That is why the asteroid threat should neither be dismissed nor sensationalised into an impending apocalypse. Earth will be struck by large objects again if enough geological time passes. The extraordinary difference today is that one species on this planet has begun searching for them — and has already demonstrated that, given enough warning, it may be able to move one out of the way.
