How Close Have Asteroids Come to Earth Without Us Knowing?

Asteroids pass near Earth far more often than most people realise. While large, civilisation-threatening impacts are rare, smaller space rocks frequently approach our planet at distances closer than the Moon. In several documented cases, scientists detected these objects only after they had already passed Earth. Advances in sky surveys and planetary defence programs have reduced the risk of surprise encounters, yet blind spots remain. Understanding how close asteroids have come without prior detection requires examining observational limits, documented near misses, and the science of impact risk.

The Scale of Near-Earth Objects

Near Earth Objects, known as NEOs, are asteroids and comets whose orbits bring them within 1.3 astronomical units of the Sun, placing them close to Earth’s orbital path. NASA’s Centre for Near-Earth Object Studies estimates that more than 32,000 NEOs have been discovered to date, with new objects identified every week.

Large asteroids greater than one kilometer in diameter are considered globally hazardous, but over 95 percent of those are believed to have already been cataloged. The greater uncertainty lies with smaller objects between 10 and 100 meters wide. These bodies can still produce regional damage if they enter Earth’s atmosphere. According to NASA planetary defence officer Lindley Johnson, “The biggest risk comes from objects we have not yet discovered,” emphasising that detection completeness decreases as size decreases.

Documented Close Passes Without Warning

One of the most striking examples occurred in 2013. Asteroid 2013 EC was discovered only days before passing Earth at a distance of about 230,000 kilometres, which is closer than the Moon. Another object, 2012 DA14, passed within 27,700 kilometres of Earth’s surface in February 2013. It was detected roughly a year in advance, but its approach highlighted how frequently objects can enter the geostationary satellite region.

Perhaps the most dramatic case was the Chelyabinsk meteor in February 2013. The object, estimated to be about 20 meters wide, exploded over Russia, releasing energy equivalent to roughly 500 kilotons of TNT. It was not detected prior to atmospheric entry because it approached from the direction of the Sun, making ground-based telescope detection extremely difficult. Research published in Nature in 2013 analyzed the Chelyabinsk event and concluded that similar-sized impacts occur on Earth roughly once every several decades. The event injured over 1,500 people, primarily from broken glass caused by the shockwave.

Why Some Asteroids Go Undetected

Asteroid detection relies heavily on ground-based optical telescopes that scan the night sky for moving points of light. These systems require darkness and clear atmospheric conditions. Objects approaching from the Sun’s direction are hidden by glare and cannot be observed by traditional ground surveys.

Dr. Amy Mainzer, principal investigator of NASA’s Near Earth Object Wide-field Infrared Survey Explorer mission, has explained that infrared detection from space can help identify dark asteroids that reflect little sunlight. Infrared instruments detect heat rather than reflected light, allowing for improved size estimates and discovery rates. However, until recently, no dedicated space-based infrared survey system operated continuously for planetary defence. This gap means that smaller objects can still approach without warning.

How Close Is “Close”

Several asteroids have passed within a few tens of thousands of kilometres of Earth. In astronomical terms, this is extremely close. Geostationary satellites orbit at about 36,000 kilometres above Earth. Some near passes occur inside that distance.

Asteroid 2020 QG passed just 2,950 kilometres above Earth’s surface in August 2020. It was discovered only after its closest approach. According to NASA’s Jet Propulsion Laboratory, this was the closest known non-impacting asteroid flyby on record. The object measured between three and six meters in diameter and posed no major hazard due to its small size. Such events demonstrate that very small asteroids can pass extremely close without detection, particularly if they are faint or approach from unfavourable angles.

Improvements in Planetary Defense

The scientific community has significantly expanded monitoring efforts over the past two decades. Programs such as the Catalina Sky Survey and Pan-STARRS have increased the discovery rates of near-Earth asteroids. In addition, NASA is developing the Near Earth Object Surveyor mission, a dedicated space-based infrared telescope scheduled for launch later this decade.

NASA’s Double Asteroid Redirection Test mission in 2022 successfully demonstrated the ability to alter an asteroid’s orbit by kinetic impact. While DART targeted a non-threatening asteroid, the experiment demonstrated that mitigation strategies are feasible when a hazardous object is identified early. Planetary defence researchers emphasise that early detection is the most critical factor in preventing impact damage. Even small velocity changes applied years in advance can significantly alter an asteroid’s trajectory.

Remaining Uncertainties

Although large objects are unlikely to strike Earth without warning, smaller asteroids remain difficult to track comprehensively. Estimates suggest that only a fraction of objects smaller than 30 meters have been catalogued. Many approach from solar glare regions, and detection sensitivity decreases for darker bodies.

Astronomer Paul Chodas of NASA’s Centre for Near-Earth Object Studies has noted that “We are not surprised by small objects passing close,” but he also emphasises that expanding survey capabilities is essential to reduce the unknown risks.

Conclusion

Asteroids have approached Earth at distances closer than many satellites, sometimes without prior detection. The Chelyabinsk meteor and the 2020 QG flyby illustrate the limits of current observation systems, particularly for small objects and sunward approaches. However, large civilisation-threatening asteroids are unlikely to arrive unannounced because most have already been identified.

Continued investment in space-based detection, infrared surveys, and impact mitigation research is steadily reducing the element of surprise. While Earth remains vulnerable to small, infrequent events, the scientific infrastructure for planetary defence is stronger than ever.