Last updated September 22, 2025

The Invisible Threat of Asteroids: From Pebbles to Flying Mountains

Vredefort Crater (South Africa): the largest in the world

Bigger, Rarer, but More Devastating: The Unyielding Law of Near-Earth Objects

Asteroids pose a constant threat to life on Earth, but not all are equally dangerous. The relationship between their size, impact frequency, and destructive potential follows an exponential curve that defies our intuition. While small objects (less than 20 meters) strike our planet several times a year without major consequences, near-Earth objects (NEOs) larger than 1 km in diameter—capable of causing global catastrophes—hit us only once every 500,000 years on average. This inverse relationship between size and frequency is explained by complex celestial mechanisms that scientists are only beginning to fully understand.

As demonstrated by Eugene Shoemaker (1928–1997), a pioneer in the study of cosmic impacts, "the probability of an asteroid striking 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 event (17 m, 500 kt of TNT) occurs roughly every 50 years, while an impact like Chicxulub (10–15 km) happens only once every 100 million years on average.

N.B.:
1 kt of TNT is the energy released by a small 5-meter asteroid entering the atmosphere at 20 km/s (~4.184 × 10¹² J) = 1.16 billion watt-hours (Wh). In other words, this energy could:
• Power a city of 10,000 inhabitants for 3 days,
• Completely destroy a reinforced concrete building within a 50-meter radius,
• Shatter all windows within a 500-meter radius,
• Cause structural damage (roofs, load-bearing walls) up to 1 km away,
• Create a crater ~20 meters in diameter in rocky soil,
• Produce a fireball 60 meters in diameter (temperature > 3,000°C).

What Asteroid Sizes Should We Really Fear?

Recent studies by NASA and ESA have established a precise risk classification:

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), audible shockwave up to 100 km, micrometeorites	2018 LA (3 m, Botswana)
10 - 20	1 every 5-10 years	1-20 kt	Shockwave (1-5 psi at 10 km), shattered windows, injuries from debris 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-level), crater < 1 km Shockwave > 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 wide, oceanic tsunami (waves > 100 m) if impact occurs in water Regional climate disruption (1-2 years)	Meteor Crater (50 m, 50,000 years ago)
140 - 300	1 every 10,000-20,000 years	50-500 Mt	Regional destruction, crater > 5 km Mild asteroid winter (2-5 years, 2-5°C temperature drop)	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, 5-8°C temperature drop)	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, 8-12°C temperature drop) 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)

Updated sources (2023–2025):
• Frequencies: Bottke et al. (2023), Nature Astronomy 7(5)
• Energies: iSALE-3D models (Collins et al., 2024)
• Climate consequences: Bardeen et al. (2024), JGR Atmospheres 129(5)
• Historical examples: EID database (2025)

The 140-Meter Rule: When an Asteroid Becomes a Global Catastrophe

Understanding this distribution allows space agencies to prioritize their efforts. As Lindley Johnson (1956–), head of NASA’s Planetary Defense program, explains: "We particularly track objects larger than 140 meters, as they account for 90% of the total risk while being rare enough that we can hope to catalog all of them before an impact." The NEO Surveyor, scheduled for launch in 2026, should discover 90% of asteroids larger than 140 meters by 2035.

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

A 140-meter asteroid striking the ocean would generate a tsunami whose height depends on depth and distance, according to iSALE-3D models (2024):

Tsunami Heights for a 140 m Impactor (20 km/s velocity, 45° angle)
Distance from Impact Point	Ocean Depth	Initial Height (m)	Height at Coast (m)	Arrival Time
Epicenter	4,000 m	~1,200	N/A	0 min
10 km	4,000 m	~800	~300-400	2-3 min
100 km	4,000 m	~200	~50-80	20-30 min
1,000 km	4,000 m	~50	~10-20	2-3 h
5,000 km	4,000 m	~10	~3-5	6-8 h

Manicouagan Crater (80 km in Diameter)

What Can We Do About This Threat?

Several strategies are under study:

Deflection: The DART mission (2022) proved we can alter an asteroid’s trajectory
Fragmentation: Using nuclear explosives for very large objects
Mitigation: Emergency plans to evacuate threatened areas
Prevention: Better detection via dedicated telescopes like Vera Rubin (2025)

While our ability to detect these objects improves (over 30,000 NEOs known in 2025 vs. 10,000 in 2010), the real challenge remains international coordination. As Detlef Koschny (1963–), head of ESA’s SSA segment, notes: "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."