Near-Earth Asteroids (NEAs) are a population of small Solar System bodies, specifically asteroids, whose orbits bring them into close proximity with Earth's orbit. They are a subset of Near-Earth Objects (NEOs), a broader category that includes both asteroids and comets that approach our planet's vicinity. NEAs are of great interest to scientists, offering clues about the early Solar System, and they are the subject of intensive global tracking due to the potential, albeit small, risk of an impact event.
Defining Near-Earth Asteroids
A Near-Earth Object (NEO) is formally defined as an asteroid or comet that orbits the Sun and has a closest approach distance to the Sun (perihelion) of less than 1.3 Astronomical Units (AU). (One AU is the average distance from the Earth to the Sun, about 150 million kilometers or 93 million miles).
Near-Earth Asteroids (NEAs) specifically are the rocky, non-cometary bodies within this group. As of early 2025, astronomers have cataloged over 37,000 NEAs.
Classification by Orbit: NEAs are further subdivided into groups based on their orbital characteristics:
Aten Asteroids: Have a semi-major axis (average distance from the Sun) less than 1 AU and their aphelion (farthest point from the Sun) is greater than 0.983 AU (Earth's perihelion). They cross Earth's orbit.
Apollo Asteroids: Have a semi-major axis greater than 1 AU and their perihelion (closest point to the Sun) is less than 1.017 AU (Earth's aphelion). They are Earth-crossing asteroids and represent the majority of the discovered population.
Amor Asteroids: Have orbits that are strictly outside Earth's orbit but come very close, with their perihelion falling between 1.017 AU and 1.3 AU. They do not cross Earth's orbit but can cross Mars's orbit.
Atira Asteroids (or Apohele): These asteroids have orbits entirely contained within Earth's orbit (their aphelion is less than Earth's perihelion). They do not cross the Earth's orbit but are still technically near-Earth objects.
Composition and Origin
NEAs are thought to be primitive leftovers from the formation of the Solar System about 4.6 billion years ago. Studying their composition provides direct insight into the chemical mixture from which the terrestrial planets formed.
Origin: Most NEAs originate in the main asteroid belt located between Mars and Jupiter. Their orbits are perturbed over millions of years, often by the gravitational influence of Jupiter or through collisions, gradually pushing them inward toward the inner Solar System.
Composition: Like other asteroids, NEAs are generally classified by their composition:
C-type (Carbonaceous): The most common type, dark in color, consisting of rock and clay with carbon compounds. These may be more porous.
S-type (Stony/Silicaceous): Made up of silicates (rocky material) and nickel-iron.
M-type (Metallic): Consist mostly of nickel-iron.
Missions like NASA's OSIRIS-REx to asteroid Bennu and JAXA's Hayabusa2 to asteroid Ryugu have provided invaluable data on the composition, density, and physical properties of specific NEAs, revealing that some are essentially "rubble piles" held together loosely by gravity.
The Impact Risk and Planetary Defense
The primary concern regarding NEAs is the potential for a catastrophic impact with Earth. Even relatively small objects can cause significant damage, as evidenced by the 2013 Chelyabinsk event, where a small, previously undetected meteoroid (estimated 20 meters in diameter) exploded over Russia, generating a powerful shockwave.
Potentially Hazardous Asteroids (PHAs)
A subset of NEAs is designated as Potentially Hazardous Asteroids (PHAs). These are defined as objects that:
Have a Minimum Orbit Intersection Distance (MOID) with Earth of 0.05 AU or less (about 7.5 million kilometers).
Have an estimated size (based on absolute magnitude) of larger than 140 meters in diameter.
An impact from an object larger than about 1 kilometer could cause a global catastrophe, while one over 140 meters could cause significant regional damage.
Tracking and Mitigation
Global efforts, often led by NASA's Center for Near-Earth Object Studies (CNEOS) and ESA's Near-Earth Object Coordination Centre (NEOCC), focus on Planetary Defense through a five-step process:
Find: Continuously discovering new objects using ground- and space-based telescopes.
Track: Obtaining enough observations to accurately determine an object's orbit and predict its path for decades, or even centuries, into the future.
Characterize: Determining the asteroid's size, composition, and physical properties.
Deflect: Developing and testing technologies to alter a threatening asteroid's trajectory.
Mitigate: Planning for the consequences of an impact, should it occur.
Deflection Strategies being researched and tested include:
Kinetic Impactor: Hitting the asteroid with a spacecraft to slightly change its velocity and trajectory. This was successfully demonstrated by NASA's Double Asteroid Redirection Test (DART) mission in 2022 against the moonlet Dimorphos.
Gravity Tractor: Using the subtle gravitational pull of a nearby hovering spacecraft to gently "tow" the asteroid into a safer orbit over time.
Nuclear Explosives (Last Resort): For very large objects with short warning times, a nuclear device could be detonated near or on the surface to push it off course or break it into smaller, less harmful fragments.
While the probability of a major impact in the foreseeable future remains low, the ongoing monitoring and development of mitigation strategies are crucial for safeguarding our planet.