Last updated September 1, 2025

Near-Earth Asteroids: An Underestimated Threat to Our Planet?

Near-Earth Asteroids

Near-Earth asteroids (NEOs) are celestial bodies whose orbits intersect that of Earth. These objects, remnants of the solar system's formation, 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 Asteroids

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

Atiras: orbit inside Earth's orbit
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 Turin 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.

Table: Major Near-Earth Asteroids and Estimated Impact Energy

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

Sources: NASA – CNEOS Near-Earth Object Program, ESA – Planetary Defence Office,

How Many Dangerous Near-Earth Asteroids Are Known?

Potentially Hazardous Asteroids (PHAs) are defined by two criteria: a geocentric Minimum Orbit Intersection Distance (MOID) 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 (approximately \(H \leq 22\), depending on the assumed albedo). These thresholds are operational and serve to prioritize the monitoring and analysis of impact risks.

The quantification of the actual PHA population combines direct observations and incompleteness models. Optical and infrared observation campaigns (survey surfaces, proper motion detection, photometric and radiometric measurements) provide the observed catalogs.

The values reported in the catalogs evolve with the discovery of smaller objects and the improvement of models. For example (orders of magnitude extracted from catalogs and published estimates):

Estimation of the PHA population according to different sources
Source	Approximate Date	Number of Observed/Estimated 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 the increased sensitivity of future detection campaigns

References and resources: NASA CNEOS, Minor Planet Center (IAU), NEOWISE analyses, and technical documents from the Rubin Observatory and NEO Surveyor programs.

Near-Earth Asteroids: An Underestimated Threat to Our Planet?