Near-Earth Asteroids: An Underestimated Threat?

Scientific summary

Astronoo's article presents an overview of near-Earth asteroids, objects whose orbit crosses that of Earth. It synthesizes current data on their classification, detection, and impact risk. The study relies on catalogs from the Minor Planet Center and estimates from space missions such as NEOWISE, which estimate several thousand potentially hazardous objects. The article highlights that while most large near-Earth asteroids have been identified, a significant fraction of medium-sized objects (140 meters and larger) remains to be discovered, motivating the development of new observation programs like the Vera Rubin Observatory or the NEO Surveyor mission.

What makes near-Earth asteroids so threatening to our planet?

This article is dedicated to near-Earth asteroids, these celestial bodies that graze Earth's orbit. But why do they represent a serious threat? The answer lies in their number and their potential energy. Over 40,000 have already been cataloged, and among them, about 10,000 exceed 140 meters in diameter, a size sufficient to devastate an entire region in case of impact. Yet these numbers are only the tip of the iceberg: models estimate that there could be nearly a million smaller objects, whose effects, though local, remain destructive. The major difficulty lies in detecting these objects, often dark and moving rapidly on complex orbits. Monitoring programs, such as those of NASA and ESA, have made considerable progress, but they cover only a fraction of the sky. Moreover, the threat is not limited to impacts: Earth's gravity can modify their trajectories, making some predictions uncertain. The article explores these issues and presents the tools and missions shaping our understanding of this cosmic risk.

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Near-Earth Asteroids

Near-Earth asteroids (Near-Earth Objects or NEOs) are celestial bodies whose orbit crosses that of Earth. These objects, remnants of the formation of the solar system, represent both a remarkable scientific opportunity and a potential threat to our planet. It is estimated that there are about 25,000 near-Earth asteroids larger than 140 meters in diameter, and nearly a million smaller objects.

Classification of near-Earth objects

Near-Earth asteroids are classified according to their orbital parameters:

Atiras: orbit inside that of Earth
Atens: semi-major axis less than 1 AU
Apollos: semi-major axis greater than 1 AU but perihelion distance less than 1.017 AU
Amors: orbit between Earth and Mars

Impact risk assessment

The Torino scale and the Palermo scale are used to categorize the impact risk of an asteroid. The energy released during an impact depends on the mass and velocity of the object according to the formula: \(E = \frac{1}{2}mv^2\) where \(m\) is the mass and \(v\) is the relative velocity.

For example, a 1 km diameter asteroid traveling at 20 km/s would release enough energy to cause enormous damage on a continental scale.

The different types of near-Earth orbits

Near-Earth objects fall into four main orbital families, defined by their trajectory around the Sun. Atiras remain permanently inside Earth's orbit, while Atens cross it from the inside. Apollos, the most numerous, have an orbit that extends beyond Earth, and Amors orbit between Earth and Mars. This diversity makes their tracking particularly complex.

Table: Major near-Earth asteroids and estimated impact energy

This table presents a selection of remarkable near-Earth asteroids, sorted by decreasing size. It highlights the diversity of these objects, from giants several kilometers across to small bodies of a few tens of meters, and illustrates the variety of their orbits as well as the colossal energy they would release in case of impact.

Extended list of notable near-Earth objects, their parameters and potential kinetic energy
Name	Estimated diameter (m)	Orbital class	Minimum distance to Earth (km)	Next significant passage	Potential energy (MT TNT)	Comment
(433) Eros	~16,800	Amor	22,000,000	1975 (last passage)	~25,000,000	First asteroid visited by a probe (NEAR Shoemaker)
(3200) Phaethon	~5,100	Apollo	10,000,000	2093	~1,000,000	Parent of the Geminids, very eccentric orbit
(3122) Florence	~4,900	Apollo	7,000,000	2017 (last passage)	~850,000	One of the largest known near-Earth objects, observed by radar
(4179) Toutatis	~2,500	Apollo	1,600,000	2004 (last close passage)	~150,000	Massive object, extensively studied by planetary radar
(1566) Icarus	~1,400	Apollo	6,400,000	2015 (last passage)	~35,000	Near-Sun asteroid, very elongated orbit
(29075) 1950 DA	~1,300	Apollo	7,500,000	2880	~30,000	Considered one of the most dangerous in the long term
2001 FO32	~1,000	Apollo	2,000,000	2021 (last passage)	~12,000	Largest known near-Earth asteroid to have recently flown by Earth
(162173) Ryugu	~870	Apollo	5,800,000	2076	~11,000	Target of the Japanese Hayabusa2 mission (sample return)
(65803) Didymos	~780 (binary system)	Apollo	6,000,000	2123	~14,000	Target of the DART deflection test (NASA) and Hera mission (ESA)
(153814) 2001 WN5	~700	Apollo	250,000	2028	~8,000	Very close upcoming passage scheduled for 2028
(101955) Bennu	~490	Apollo	300,000	2135	~3,000	Target of the OSIRIS-REx mission, distant impact risk but intensively monitored
(99942) Apophis	~340	Aton	31,000	2029	~870	Very close passage, orbit modified by Earth's gravitational interaction
(3361) Orpheus	~300	Apollo	2,500,000	2021 (last passage)	~650	Small but closely monitored by radar and optics
(469219) Kamoʻoalewa	~40	Apollo (quasi-satellite)	3,800,000	Near continuously	~0.01	Small object in orbital resonance with Earth

Do we know the number of dangerous near-Earth objects?

Potentially hazardous asteroids (PHAs) are defined by two criteria (orbits and light): a geocentric MOID (minimum Orbit Intersection Distance) less than or equal to 0.05 astronomical units (≈ 7.48 × 10⁶ km) and an absolute magnitude \(H\) bright enough to indicate a typical diameter >~140 m (which approximately corresponds to \(H \leq 22\), depending on assumed albedo). These thresholds are operational; they serve to prioritize monitoring and impact risk analysis.

Quantifying the true population of PHAs combines direct observations and incompleteness models. Optical and infrared observation campaigns (sky surveys, proper motion detection, photometric and radiometric measurements) provide the observed catalogs. The values reported in catalogs evolve with the discovery of smaller objects and improvements to models.

Catalogs of hazardous near-Earth objects?

Estimate of the PHA population according to different sources
Source	Approximate date	Number observed / estimated of PHAs	Remarks
NASA - CNEOS (observed catalog)	2024–2025	~2,400–2,500 observed	Effective catalog, continuously updated
Minor Planet Center (IAU)	2025	~2,500	Consolidated inventory of astronomical discoveries
NEOWISE (radiometric estimate)	Recent analyses	~4,000–5,000 estimated	Correction for incompleteness, sensitive to albedo assumptions
Future projections (Rubin Observatory, NEO Surveyor)	Projections	~10,000–20,000 (D>~100 m) possible	Based on increased sensitivity of future detection campaigns

References

FAQ: Everything you need to know about near-Earth asteroids

What is a near-Earth asteroid?

A near-Earth asteroid is a celestial body whose orbit around the Sun brings it within 1.3 astronomical units (AU) of our star, and whose trajectory crosses that of Earth. These are rocky or metallic objects, remnants of the formation of the solar system, which can represent a threat in case of collision with our planet.

How many near-Earth asteroids are currently cataloged?

To date, over 40,000 near-Earth asteroids have been detected and cataloged by space agencies and observatories worldwide. This number continues to increase thanks to advances in telescopes and dedicated monitoring programs, such as the Catalina Sky Survey or Pan-STARRS.

What is the difference between a NEO and a PHA?

A NEO (Near-Earth Object) is any object (asteroid or comet) whose orbit approaches that of Earth. A PHA (Potentially Hazardous Asteroid) is a subset of NEOs that meet stricter criteria: their minimum orbit intersection distance with Earth is less than 0.05 AU (about 7.5 million kilometers) and their size is greater than about 140 meters. PHAs are subject to particularly close monitoring.

What would happen if a 140-meter asteroid struck Earth?

An impact from a 140-meter asteroid would release energy equivalent to several hundred megatons of TNT, far more than the most powerful atomic bomb ever tested. The local consequences would be catastrophic: total destruction within a radius of several tens of kilometers, fires, tsunamis if the impact occurs at sea, and regional climatic effects. This is why these objects are classified as hazardous.

What would happen if a 500-meter asteroid struck Earth?

A 500-meter diameter asteroid would release colossal energy, estimated between 5,000 and 10,000 megatons of TNT, equivalent to several hundred thousand Hiroshima bombs. The impact would create a crater nearly 6 kilometers in diameter and cause total destruction within a radius of 100 to 200 kilometers. The effects would be global: a dust cloud would rise into the stratosphere, blocking part of the sunlight and causing a temporary nuclear winter. Global temperatures would drop by several degrees for months, severely disrupting agriculture and ecosystems. Tsunamis generated by an ocean impact could devastate coastlines thousands of kilometers away. Such an event, which occurs on average once every 100,000 years, would endanger modern civilization and could result in the deaths of hundreds of millions of people.

What would happen if a 1000-meter asteroid struck Earth?

A 1-kilometer diameter asteroid (1,000 meters) striking Earth would cause a catastrophe of unprecedented scale in human history. The energy released would be on the order of 100,000 to 200,000 megatons of TNT, millions of times the power of the Hiroshima bomb. The impact would carve a crater over 15 kilometers in diameter, and the total destruction zone would extend over more than 500 kilometers. Ejecta and massive fires would engulf entire continents. The dust and gas cloud injected into the stratosphere would plunge the planet into a severe nuclear winter lasting several years, with temperatures dropping by 10 to 15 °C, leading to the collapse of global agriculture and widespread famine. Ocean acidification and destruction of the ozone layer would further worsen the situation. Such an event, occurring on average every 500,000 years, would be an extinction-level event (ELE) comparable to the one that caused the disappearance of the dinosaurs 66 million years ago, endangering the very survival of the human species.

How do scientists track near-Earth asteroids?

Tracking relies on a global network of optical and radar observatories. Programs like the Catalina Sky Survey or Pan-STARRS continuously scan the sky to detect new objects. Once identified, their orbit is calculated and refined through repeated observations. NASA and ESA coordinate these efforts through centers like CNEOS or NEOCC, which publish catalogs and impact risk assessments.

Are there space missions dedicated to asteroid deflection?

Yes, NASA's DART (Double Asteroid Redirection Test) mission successfully deflected the orbit of the asteroid Dimorphos in 2022, proving that a kinetic impact can alter the trajectory of a celestial body. ESA is preparing the Hera mission, which will study the consequences of this test in detail. Other concepts, such as using a nuclear explosion or a gravity tractor, are under study.

Where can I consult updated data on near-Earth objects?

The most comprehensive and up-to-date data are available on the websites of CNEOS (NASA) (new window) and the Minor Planet Center (IAU) (new window). These platforms provide lists of objects, their orbital parameters, and impact risk assessments.