The invisible threat of asteroids: from pebbles to flying mountains

Scientific summary

Astronoo's article analyses the statistical relationship between the size of near-Earth asteroids and their hazard to Earth. It presents a precise risk classification ranging from bolides of a few metres, very frequent, up to kilometre-sized impactors responsible for mass extinctions. The study is based on the latest data from NASA, ESA and numerical simulation models (iSALE-3D) to quantify the energies released, tsunami heights and potential climatic consequences. It highlights the effectiveness of detection programmes (NEO Surveyor) and deflection missions (DART) for planetary defence.

What is the real probability that a devastating asteroid will hit Earth?

This article is devoted to the threat that asteroids pose to life, exploring precisely the question of their impact frequency and destructive power. The answer to this question is counter-intuitive: small objects, the size of a car, hit Earth several times a year, but they disintegrate in the atmosphere without consequence. In contrast, asteroids capable of causing a global catastrophe, larger than 140 metres, are extremely rare (about one impact every 10,000 years). The probability of an impact is therefore inversely proportional to size, but the energy released increases exponentially. An asteroid of 10 km, like the one that wiped out the dinosaurs, releases energy equivalent to several million atomic bombs, and such an event occurs only once every 100 million years. Space agencies, through programmes such as NEO Surveyor, are methodically mapping these objects to anticipate any threat. The issue is not whether a major impact will occur, but when, and whether we will be ready to deflect its trajectory.

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Vredefort crater (South Africa): the largest in the world — The Vredefort crater (South Africa) is the largest known crater (300 km in diameter) and the oldest (2 billion years old). Inscribed as a UNESCO World Heritage Site, the Vredefort crater is so eroded that only the western half remains visible.
Image source: NASA - Júlio Reis (new window) — public domain.

Bigger, rarer but more devastating: the implacable law of near-Earth objects

Shoemaker's law and its implications

Asteroids pose a constant threat to life on Earth, but not all present the same danger. The relationship between their size, impact frequency and destructive potential follows an exponential curve that defies our intuition. While small objects (less than 20 metres) hit our planet several times a year without major consequences, near-Earth objects (near-Earth object: asteroid or comet whose orbit crosses that of Earth) larger than 1 km in diameter, capable of causing global catastrophes, only hit us on average every 500,000 years. This inverse relationship between size and frequency is explained by complex celestial mechanisms that scientists are only just beginning to understand.

Shoemaker's inverse-square rule

As demonstrated by Eugene Shoemaker (1928-1997), a pioneer in the study of cosmic impacts, "the probability of an asteroid hitting Earth is inversely proportional to the square of its diameter". This empirical rule, confirmed by modern observations, means that an asteroid 10 times larger will have an impact 100 times less frequent, but with kinetic energy \(E = \frac{1}{2}mv^2\) (where \(m\) is mass and \(v\) is velocity) millions of times greater. The famous Chelyabinsk (impact of a 17-metre asteroid in the Chelyabinsk region in 2013) event (17 m, 500 kt of TNT) occurs about every 50 years, while an impact like Chicxulub (10-15 km asteroid responsible for the extinction of the dinosaurs 66 million years ago) (10-15 km) only happens on average once every 100 million years.

What 1 kiloton of TNT represents

1 kt of TNT is the energy released by a small 5-metre asteroid entering the atmosphere at 20 km/s, i.e. (4.184 × 10¹² J) = 1.16 billion watt-hours (Wh). In other words, it is the energy needed to:
• Power a town of 10,000 inhabitants for 3 days,
• Completely destroy a reinforced concrete building within a 50-metre radius,
• Break all windows within a 500-metre radius,
• Cause structural damage (roofs, load-bearing walls) up to 1 km,
• Create a crater about 20 metres in diameter in rocky ground,
• Produce a fireball 60 metres in diameter (temperature >3,000 °C).

Which asteroid sizes should we really fear?

Risk table by diameter category

The fear of asteroids is legitimate, but not all deserve the same attention. To prioritise detection and defence efforts, astronomers have classified near-Earth objects according to their diameter. If a 5-metre asteroid gives us a nice fireball in the night sky, a 140-metre block can raze an entire region. Between these two extremes, what is the probability of an impact and what would the consequences be? Studies by NASA (national Aeronautics and Space Administration) and ESA (european Space Agency) now allow us to answer this question with unprecedented precision:

Frequency and consequences of asteroid impacts by size (2025 data)
Diameter (m)	Average frequency	Energy (TNT)	Typical consequences	Historical example
<5	~10 per year	<0.1 kt	Visible fireball (magnitude -5 to -10), complete fragmentation in upper atmosphere	2014 AA (3 m, 2014)
5 - 10	1-2 per year	0.1-1 kt	Superbolide (magnitude -15), shock wave audible at 100 km, micrometeorites	2018 LA (3 m, Botswana)
10 - 20	1 every 5-10 years	1-20 kt	Shock wave (1-5 psi at 10 km), broken windows, shrapnel injuries Ex: Chelyabinsk (17 m, 500 kt, 2013)	Chelyabinsk (17 m, 2013)
20 - 50	1 every 50-100 years	20 kt - 1 Mt	Local destruction (city), crater <1 km Shock wave >10 psi at 5 km, secondary fires	Tunguska (~50 m, 1908)
50 - 140	1 every 1,000-2,000 years	1-50 Mt	Crater 1-3 km, tsunami if oceanic (waves >100 m) Regional climate disruption (1-2 years)	Meteor Crater (50 m, 50,000 years)
140 - 300	1 every 10,000-20,000 years	50-500 Mt	Regional destruction, crater >5 km Light asteroid winter (2-5 years, drop of 2-5°C)	Ries (150 m, 14.8 Ma)
300 - 1,000	1 every 100,000-200,000 years	500 Mt - 10 Gt	Continental catastrophe, crater >20 km Moderate asteroid winter (5-10 years, drop of 5-8°C)	Popigai (5-8 km, 35.7 Ma)
1,000 - 5,000	1 every 1-10 Ma	10-100 Gt	Regional mass extinction Severe asteroid winter (10-15 years, drop of 8-12°C) Ocean acidification (10,000 years)	Chesapeake Bay (3-5 km, 35 Ma)
>10,000	1 every 100-200 Ma	>10⁵ Gt	Mass extinction (>75% of species) Catastrophic asteroid winter (15-20 years, -10 to -15°C) Ecosystem recovery: 300,000-1M years	Chicxulub (12±2 km, 66.021 Ma)

The 140-metre rule: the threshold where an asteroid becomes a global catastrophe

Why is this diameter critical for planetary defence?

Understanding this distribution allows space agencies to prioritise their efforts. As Lindley Johnson (1956-), head of NASA's Planetary Defence programme, explains: "We particularly track objects larger than 140 metres, because they represent 90% of the total risk while being rare enough that we can hope to catalogue them all before an impact." The NEO Surveyor (near-Earth Object Surveyor: space telescope dedicated to detecting near-Earth objects), scheduled for launch in 2026, should enable the discovery of 90% of asteroids larger than 140 metres by 2035.

The impact of a 140-metre asteroid: between rarity and devastation

The good news is that major impacts are extremely rare on a human life scale. The bad news is that even an asteroid of "only" 140 metres could cause a catastrophe comparable to the 1815 eruption of Tambora ("the year without a summer"), with global economic and humanitarian consequences. Simulations show that an impact in the ocean could generate devastating tsunamis on coasts thousands of kilometres away.

Modelling tsunamis generated by a 140-metre impactor

A 140-metre asteroid falling into the ocean would generate a tsunami whose height would depend on depth and distance according to iSALE-3D (hydrocode for asteroid impacts) models (2024):

Tsunami heights for a 140 m impactor (velocity 20 km/s, angle 45°) — maximum heights at epicentre, attenuating strongly as they approach coasts
Distance from impact point	Ocean depth	Initial height (m)	Arrival time
Epicentre	4,000 m	~1,200	0 min
10 km	4,000 m	~800	2-3 min
100 km	4,000 m	~200	20-30 min
1,000 km	4,000 m	~50	2-3 h
5,000 km	4,000 m	~10	6-8 h

Manicouagan impact crater

asteroid crater of manicouagan Quebec (Canada) — Manicouagan impact crater (province of Quebec, Canada), photographed in 1983. 210 million years old, its average diameter is about 80 km, corresponding to the impact of a meteorite about 3.5 km in diameter.
Source: NASA/JSC.

What to do in the face of an impact threat?

In the face of the threat from near-Earth objects, international space agencies have developed a range of complementary strategies. These approaches fall into four main categories: deflection, fragmentation, mitigation and prevention. Each responds to specific scenarios, depending on the warning time and the size of the threatening object.

Kinetic impact deflection: the DART mission

The most promising and already tested technique is kinetic impactor deflection. NASA's DART (Double Asteroid Redirection Test) mission proved its feasibility on 26 September 2022 by deliberately crashing into the asteroid Dimorphos, which orbits Didymos. The impact changed Dimorphos' orbital period by 32 minutes, a deflection far greater than scientists expected. This success validates the technique for small to medium-sized asteroids (50 to 500 metres), with several years' warning.

A complementary mission, ESA's Hera, is scheduled for launch in October 2024 to observe the consequences of the DART impact up close and refine energy transfer models. These data will help calibrate future deflection missions.

Nuclear fragmentation: last resort for giants

For asteroids over one kilometre in diameter, kinetic impact deflection becomes ineffective. Nuclear fragmentation is then considered as a last-resort solution. The idea is to place a nuclear charge near the asteroid to vaporise part of its surface, creating thrust that alters its trajectory (deflection) or, in the worst case, fragments it into smaller pieces that would burn up in the atmosphere.

This approach, popularised by science-fiction films, is the subject of serious study. Los Alamos National Laboratory is working on simulation models of nuclear explosions in zero gravity. However, its use raises major ethical and political questions, particularly because of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Nuclear deflection is therefore reserved for existential threats, with very short warning times.

Emergency plans and consequence mitigation

Even with a successful deflection, a regional impact remains possible. Space agencies and governments are therefore working on emergency plans to protect populations:

Preventive evacuation of threatened areas, with protocols inspired by natural disaster management plans (hurricanes, tsunamis).
Protection of critical infrastructure (nuclear power plants, dams, hospitals) within a potential impact radius.
Strategic stockpiling of food, water and medicines to cope with an "asteroid winter" that could disrupt global agriculture for several years.

Simulation exercises are regularly organised, such as those conducted under the IAWN (International Asteroid Warning Network) and SMPAG (Space Mission Planning Advisory Group) under the auspices of the United Nations.

Prevention: tracking near-Earth objects

The best defence remains early detection. Surveillance programmes have progressed considerably in recent years. By 2025, more than 34,000 near-Earth objects have been catalogued, compared with only 10,000 in 2010. The main dedicated observatories are:

Vera C. Rubin Observatory (Chile, operational in 2025): equipped with the world's largest digital camera (3.2 gigapixels), it should discover 5 million new asteroids in ten years.
NEO Surveyor (NASA, launch scheduled for 2026): an infrared space telescope designed specifically to detect near-Earth asteroids, including those hidden in the Sun's glare.
ATLAS (Asteroid Terrestrial-impact Last Alert System): a network of telescopes that scans the sky every night to spot dangerous objects.

The goal is to detect 90% of asteroids larger than 140 metres by 2035, in accordance with the US Congressional mandate (George E. Brown Jr. Near-Earth Object Survey Act).

International coordination: the political and legal challenge

While our ability to detect these objects is improving, the real challenge remains international coordination. As Detlef Koschny (1963-), head of the SSA (space Safety Programme) segment at ESA, points out: "We now know how to find dangerous asteroids. The problem is deciding who should act and how, when we find one on a collision course."

The current framework is based on two main structures:

IAWN (International Asteroid Warning Network): coordinates observations, shares data and issues alerts.
SMPAG (Space Mission Planning Advisory Group): develops recommendations on deflection and defence missions.

But crucial questions remain unanswered: who authorises the launch of a deflection mission? On what legal basis? Who funds the operation? Discussions are underway within the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) to establish a clear international protocol.

Future innovations: laser, gravity and solar sail

Beyond the techniques already tested, more exotic approaches are being studied for the coming decades:

Gravity tractor: a spacecraft flying close to an asteroid would use its own mass to alter its trajectory by gravitational attraction. Slow but very precise.
Ion cannon: ion propulsion directed at the asteroid's surface to create continuous thrust (technology developed by ESA).
Solar sail or laser: using photon pressure to gently push the asteroid. An attractive option for very long warning times (several decades).

These technologies, still at the conceptual stage, could offer less destructive and more controllable alternatives to nuclear fragmentation.

References

FAQ: Everything you need to know about the asteroid threat

What is the probability that a killer asteroid will hit Earth in our lifetime?

An asteroid larger than 140 metres, capable of causing a regional catastrophe, hits Earth on average every 10,000 years. The probability of such an event occurring in a human lifetime (about 80 years) is therefore about 0.8% (1 in 125 chance). For a 1 km asteroid, capable of causing a global catastrophe, the probability is even lower, around 0.016% (1 in 6,250 chance) in a human lifetime.

What happened at Tunguska in 1908?

The Tunguska event was the explosion of a small asteroid (or comet fragment) about 50 metres in diameter, which disintegrated at an altitude of 5-10 km above Siberia. The explosion, with an estimated energy of between 10 and 15 megatons of TNT, flattened more than 2,000 km² of forest, but did not create an impact crater. It is the largest impact event in modern history, releasing energy comparable to the most powerful H-bomb ever tested.

How did the DART mission prove that we could deflect an asteroid?

NASA's DART (Double Asteroid Redirection Test) mission deliberately impacted the small asteroid Dimorphos, which orbits Didymos, on 26 September 2022. The impact changed Dimorphos' orbital period by 32 minutes, concrete proof that a kinetic impact can effectively deflect an asteroid. This success validates the kinetic impactor deflection technique, which is the preferred strategy for deflecting dangerous asteroids.

What to do if a giant asteroid is detected on a collision course?

If a giant asteroid (larger than 1 km) is detected with several years' warning, the most realistic strategy is kinetic impactor deflection (like DART). If warning time is short, the use of a nuclear charge to fragment the asteroid is considered, although this option is risky and controversial. Evacuation of threatened areas is also an option, but for a truly colossal asteroid (10 km), no defence would be effective, and humanity would have to prepare for a prolonged asteroid winter.

How do astronomers detect dangerous asteroids before they hit us?

Astronomers use a network of optical and infrared telescopes to scan the sky continuously. Programmes such as Pan-STARRS (Hawaii), the Catalina Sky Survey (Arizona) and the future Vera C. Rubin Observatory (Chile) take images of the sky every night to spot moving objects. Asteroids are detected by their movement between two successive images. Once spotted, their orbit is calculated to determine whether they pose a collision risk in the coming decades. The NEO Surveyor project, an infrared space telescope scheduled for launch in 2026, will detect near-Earth objects hidden in the Sun's glare, which escape ground-based observations.

Has there ever been an asteroid impact in recent history?

The Chelyabinsk event on 15 February 2013 is the most recent major impact. An asteroid about 17 metres in diameter entered the atmosphere over Russia, exploding at an altitude of 30 km with an energy of 500 kilotons of TNT. The shock wave broke windows within a radius of 100 km, injuring nearly 1,500 people (mainly from flying glass). In 2018, another 3-metre asteroid was detected a few hours before exploding over Botswana, producing a spectacular fireball but no ground damage. These events remind us that even small asteroids can have significant local consequences.

What are "hidden" asteroids and why are they dangerous?

"Hidden" asteroids are those whose orbit lies inside Earth's orbit, or which orbit near the Sun. They are particularly difficult to detect because they are eclipsed by sunlight. These objects, known as Vatira or Atira, remain invisible from Earth most of the time, except at twilight. Infrared space telescopes such as NEO Surveyor are specifically designed to track them, as they emit detectable thermal radiation in the infrared. An asteroid of this category could surprise planetary defence systems, hence the importance of cataloguing them as quickly as possible.

Could an asteroid hit a major city like Paris or New York?

The probability of an asteroid hitting a specific city is extremely low, as oceans cover 70% of the Earth's surface and urban areas represent only a tiny fraction of the land. However, a 50-metre asteroid hitting a large city would cause destruction comparable to a nuclear explosion of several megatons. Fortunately, such events are very rare (about one every 1,000 years). For small objects (less than 20 metres), they generally disintegrate in the atmosphere before reaching the ground, as was the case for Chelyabinsk.