Near-Earth Asteroids

​Near-Earth asteroids (NEAs) are asteroids whose orbits bring them into Earth’s cosmic neighbourhood, typically defined as having a perihelion distance less than 1.3 astronomical units from the Sun, close enough that they are monitored for potential impact risk. As of late 2024–2025, astronomers have catalogued on the order of a few tens of thousands of these objects. This means their paths can approach, cross, or come just outside Earth’s orbital distance, so they are an important subset of near‑Earth objects monitored for planetary defense. NEAs are usually divided into orbital classes (such as Atira, Aten, Apollo, and Amor) depending on how their orbits relate to Earth’s orbit, and many originate in the main asteroid belt before being perturbed inward by gravitational interactions.
 
Current catalogues list on the order of 37,000–40,000 known near‑Earth asteroids, and that number is rising steadily as surveys improve and discover thousands of new NEAs each year. Only a fraction of the total population has been found; models suggest hundreds of thousands of NEAs larger than a few tens of meters likely exist, most of them still undiscovered. Of the known NEAs, a smaller subset, on the order of a couple of thousand, are classified as potentially hazardous asteroids; meaning they are both sufficiently large and pass close enough to Earth’s orbit to merit special tracking.
 
NEAs are detected and tracked primarily with wide-field telescopes on Earth and in space that repeatedly image the sky and look for moving points of light, followed by detailed orbit calculations and continual monitoring by specialized centres. The process combines optical and infrared surveys, radar ranging, and automated impact‑risk software to refine trajectories over time. Automated systems such as Sentry and Scout continuously analyze known and candidate NEAs to assess impact probabilities over time scales from days to about a century. These algorithms propagate orbital uncertainties forward, searching for any possible Earth intersections and updating the risk assessments whenever new astrometric data are added. Planetary radar facilities, such as the Goldstone Solar System Radar, can transmit radio pulses to nearby NEAs and measure the echoes, yielding very precise distance and velocity data that sharply constrain the orbit and refine impact predictions. Radar observations also provide shape, rotation, and sometimes surface‑property information, which is important for planning any potential deflection or mitigation mission.
 
No currently known NEA poses a significant, confirmed impact threat in the next century, but a few objects stand out because they have non‑zero impact probabilities or unusually high long‑term risk metrics. These risks remain very small in absolute terms and are continually revised downward as new observations come in. Asteroid 2024 YR4 temporarily reached a Torino Scale rating of 3 in early 2025, the highest for any known object at that time, but further observations reduced its 2032 impact probability to a tiny fraction of a per cent and it is now considered to pose no significant risk. Its case illustrates how initial risk estimates for newly discovered objects often fluctuate and then drop to effectively zero as the orbit is refined. Asteroid 2023 TL4 was identified in 2023 with an estimated impact probability of about 1 in 181,000 for a possible encounter in 2119, making it one of the more prominent potentially hazardous asteroids, but still far below any level that would prompt active deflection planning. Even in the unlikely event of impact, its expected energy release, while large on human scales, would correspond to severe regional rather than global effects.
 
NASA’s Sentry monitoring system reports that all currently tracked NEAs have impact probabilities below the normal background level of risk that Earth faces from random undiscovered objects, and thus none qualifies as a confirmed, high‑risk impactor. Planetary defense efforts focus on further surveying the sky, refining orbits, and developing deflection technologies so that even these low‑probability, long‑term threats can be addressed if future data ever indicate a serious impact scenario.
 
References
European Space Agency. (2022, October 13). 30,000 Near-Earth Asteroids Discovered and Numbers Are Rising. Phy.Org.
 
Haynes, K. (2025, February 28). Asteroid 2024 YR4’s Odds of Hitting Earth in 2032 are Now Effectively Zero. Astronomy.
 
Hyman, R. (2025, December 3). How We Track Near-Earth Asteroids. Astronomy.
 
Lea, R. (2024, June 19). NASA’s Most Wanted: The 5 Most Dangerous Asteroids to Earth. LIVEScience.
 
Lea, R. (2024, February 2). Near-Earth Asteroids: Hunting and Tracking Upcoming Flybys. Space.Com.
 
Ralls, E. (2025, November 25). Astronomers Confirm 40,000 Near-Earth Asteroids Within Striking Distance. Earth.Com.
 
Tavernier, L. (2024, October 9). How NASA Studies and Tracks Asteroids Near and Far. Jet Propulsion Laboratory.
 
(2025, December 10). Near-Earth Objects. In Wikipedia, https://en.wikipedia.org/wiki/Near-Earth_object