Near-Earth object
A near-Earth object is any small Solar System body orbiting the Sun whose closest approach to the Sun is less than 1.3 times the Earth–Sun distance. This definition applies to the object's orbit around the Sun, rather than its current position, thus an object with such an orbit is considered an NEO even at times when it is far from making a close approach of Earth. If an NEO's orbit crosses the Earth's orbit, and the object is larger than across, it is considered a potentially hazardous object. Most known PHOs and NEOs are asteroids, but about a third of a percent are comets.
There are over 37,000 known near-Earth asteroids and over 120 known short-period near-Earth comets. A number of solar-orbiting meteoroids were large enough to be tracked in space before striking Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of Earth. Asteroids as small as in diameter can cause significant damage to the local environment and human populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or tsunamis if they impact the sea. Interest in NEOs has increased since the 1980s because of greater awareness of this risk. Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched.
Two scales, the simple Torino scale and the more complex Palermo scale, rate the risk presented by an identified NEO based on the probability of it impacting the Earth and on how severe the consequences of such an impact would be. Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery. Since 1998, the United States, the European Union, and other nations have been scanning the sky for NEOs in an effort called Spaceguard. The initial US Congress mandate to NASA to catalog at least 90% of NEOs that are at least in diameter, sufficient to cause a global catastrophe, was met by 2011. In later years, the survey effort was expanded to include smaller objects which have the potential for large-scale, though not global, damage.
NEOs have low surface gravity, and many have Earth-like orbits that make them easy targets for spacecraft., five near-Earth comets and six near-Earth asteroids, one of them with a moon, have been visited by spacecraft. Samples of three have been returned to Earth, and one successful deflection test was conducted. Similar missions are in progress. Preliminary plans for commercial asteroid mining have been drafted by private startup companies, but few of these plans were pursued.
Definitions
Near-Earth objects are formally defined by the International Astronomical Union as all small Solar System bodies with orbits around the Sun that are at least partially closer than 1.3 astronomical units from the Sun. This definition excludes larger bodies such as planets, like Venus; natural satellites which orbit bodies other than the Sun, like Earth's Moon; and artificial bodies orbiting the Sun. A small Solar System body can be an asteroid or a comet, thus an NEO is either a near-Earth asteroid or a near-Earth comet. The organisations cataloging NEOs further limit their definition of NEO to objects with an orbital period under 200 years, a restriction that applies to comets in particular, but this approach is not universal. Some authors further restrict the definition to orbits that are at least partly further than 0.983 AU away from the Sun. NEOs are thus not necessarily currently near the Earth, but they can potentially approach the Earth relatively closely. Many NEOs have complex orbits due to constant perturbation by the Earth's gravity, and some of them can temporarily change from an orbit around the Sun to one around the Earth, but the term is applied flexibly for these objects, too.The orbits of some NEOs intersect that of the Earth, so they pose a collision danger. These are considered potentially hazardous objects if their estimated diameter is above 140 meters. PHOs include potentially hazardous asteroids. PHAs are defined based on two parameters relating to respectively their potential to approach the Earth dangerously closely and the estimated consequences that an impact would have if it occurs. Objects with both an Earth minimum orbit intersection distance of 0.05 AU or less and an absolute magnitude of 22.0 or brighter are considered PHAs. Objects that either cannot approach closer to the Earth than, or which are fainter than H = 22.0, are not considered PHAs.
History of human awareness of NEOs
The first near-Earth objects to be observed by humans were comets. Their extraterrestrial nature was recognised and confirmed only after Tycho Brahe tried to measure the distance of a comet through its parallax in 1577 and the lower limit he obtained was well above the Earth diameter; the periodicity of some comets was first recognised in 1705, when Edmond Halley published his orbit calculations for the returning object now known as Halley's Comet. The 1758–1759 return of Halley's Comet was the first comet appearance predicted.The extraterrestrial origin of meteors was only recognised on the basis of the analysis of the 1833 Leonid meteor shower by astronomer Denison Olmsted. The 33-year period of the Leonids led astronomers to suspect that they originate from a comet that would today be classified as an NEO, which was confirmed in 1867, when astronomers found that the newly discovered comet 55P/Tempel–Tuttle has the same orbit as the Leonids.
The first near-Earth asteroid to be discovered was 433 Eros in 1898. The asteroid was subject to several extensive observation campaigns, primarily because measurements of its orbit enabled a precise determination of the then imperfectly known distance of the Earth from the Sun.
Encounters with Earth
If a near-Earth object is near the part of its orbit closest to Earth's at the same time Earth is at the part of its orbit closest to the near-Earth object's orbit, the object has a close approach, or, if the orbits intersect, could even impact the Earth or its atmosphere.Close approaches
, only 23 comets have been observed to pass within of Earth, including 10 which are or have been short-period comets. Two of these near-Earth comets, Halley's Comet and 73P/Schwassmann–Wachmann, have been observed during multiple close approaches. The closest observed approach was 0.0151 AU for Lexell's Comet on July 1, 1770. After an orbit change due to a close approach of Jupiter in 1779, this object is no longer an NEC. The closest approach ever observed for a current short-period NEC is 0.0229 AU for Comet Tempel–Tuttle in 1366. Orbital calculations show that P/1999 J6, a faint sungrazing comet and confirmed short-period NEC observed only during its close approaches to the Sun, passed Earth undetected at a distance of 0.0120 AU on June 12, 1999.In 1937, asteroid 69230 Hermes was discovered when it passed the Earth at twice the distance of the Moon. On June 14, 1968, the diameter asteroid 1566 Icarus passed Earth at a distance of, or 16.5 times the distance of the Moon. During this approach, Icarus became the first minor planet to be observed using radar. This was the first close approach predicted years in advance, since Icarus had been discovered in 1949. The first near-Earth asteroid known to have passed Earth closer than the distance of the Moon was, a body which passed at a distance of. As NEA surveys were enhanced, at least one such object was observed each year from 2001, at least a dozen from 2005, and over a hundred from 2020.
As astronomers became able to discover ever smaller and fainter and ever more numerous near-Earth objects, they began to routinely observe and catalogue close approaches., the closest approach without atmospheric or ground impact ever detected was an encounter with asteroid on October 30, 2025, with a minimum distance of about from the Earth's centre, or about above its surface. has a mean diameter of 0.41 m - 0.93 m, which falls within the range required to classify it as a meteoroid. On November 8, 2011, asteroid, relatively large at about in diameter, passed within of Earth. On February 15, 2013, the asteroid 367943 Duende passed approximately above the surface of Earth, closer than satellites in geosynchronous orbit. The asteroid was not visible to the unaided eye. This was the first sub-lunar close passage of an object discovered during a previous passage, and was thus the first to be predicted well in advance. On October 8, 2025, asteroid 2025 TN2, approximately 87 feet in diameter, passed safely by Earth at a distance of 1.34 million km. On the same day, three additional small asteroids — 2025 SJ29, 2025 TF1, and 2020 QU5, measuring about 55 ft, 65 ft, and 81 ft respectively — also made close approaches, all without any risk of impact.
Earth-grazers
Some small asteroids that enter the upper atmosphere of Earth at a shallow angle remain intact and leave the atmosphere again, continuing on a solar orbit. During the passage through the atmosphere, due to the burning of its surface, such an object can be observed as an Earth-grazing fireball.On August 10, 1972, a meteor that became known as the 1972 Great Daylight Fireball was witnessed by many people and even filmed as it moved north over the Rocky Mountains from the U.S. Southwest to Canada. It passed within of the Earth's surface.
On October 13, 1990, Earth-grazing meteoroid EN131090 was observed above Czechoslovakia and Poland, moving at along a trajectory from south to north. The closest approach to the Earth was above the surface. It was captured by two all-sky cameras of the European Fireball Network, which for the first time enabled geometric calculations of the orbit of such a body.
Impacts
When a near-Earth object impacts Earth, objects up to a few tens of metres across ordinarily explode in the upper atmosphere, with most or all of the solids vaporized and only small amounts of meteorites arriving to the Earth surface. Larger objects, by contrast, hit the water surface, forming tsunami waves, or the solid surface, forming impact craters.The frequency of impacts of objects of various sizes is estimated on the basis of orbit simulations of NEO populations, the frequency of impact craters on the Earth and the Moon, and the frequency of close encounters. The study of impact craters indicates that impact frequency has been more or less steady for the past 3.5 billion years, which requires a steady replenishment of the NEO population from the asteroid main belt. One impact model based on widely accepted NEO population models estimates the average time between the impact of two stony asteroids with a diameter of at least at about one year; for asteroids across at five years, for asteroids across at 1,300 years, for asteroids across at 440 thousand years, and for asteroids across at 18 million years. Some other models estimate similar impact frequencies, while others calculate higher frequencies. For Tunguska-sized impacts, the estimates range from one event every 2,000–3,000 years to one event every 300 years.
The second-largest observed event after the Tunguska meteor was a 1.1 megaton air blast in 1963 near the Prince Edward Islands between South Africa and Antarctica. However, this event was detected only by infrasound sensors, which led to speculation that this may have been a nuclear test. The third-largest, but by far best-observed impact, was the Chelyabinsk meteor of 15 February 2013. A previously unknown asteroid exploded above this Russian city with an equivalent blast yield of 400–500 kilotons. The calculated orbit of the pre-impact asteroid is similar to that of Apollo asteroid, making the latter the meteor's possible parent body.
On October 7, 2008, 20 hours after it was first observed and 11 hours after its trajectory has been calculated and announced, asteroid blew up above the Nubian Desert in Sudan. It was the first time that an asteroid was observed and its impact was predicted prior to its entry into the atmosphere as a meteor. of meteorites were recovered after the impact., eleven impacts have been predicted, all of them small bodies that produced meteor explosions, with some impacts in remote areas only detected by the Comprehensive Nuclear-Test-Ban Treaty Organization's International Monitoring System, a network of infrasound sensors designed to detect the detonation of nuclear devices. Asteroid impact prediction remains in its infancy and successfully predicted asteroid impacts are rare. The vast majority of impacts recorded by IMS are not predicted.
Observed impacts aren't restricted to the surface and atmosphere of Earth. Dust-sized NEOs have impacted man-made spacecraft, including the space probe Long Duration Exposure Facility, which collected interplanetary dust in low Earth orbit for six years from 1984. Impacts on the Moon can be observed as flashes of light with a typical duration of a fraction of a second. The first lunar impacts were recorded during the 1999 Leonid storm. Subsequently, several continuous monitoring programs were launched. A lunar impact that was observed on September 11, 2013, lasted 8 seconds, was likely caused by an object in diameter, and created a new crater across, was the largest ever observed as of 2019.