as seen by the probe
NEAR Shoemaker 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 obtained a lower limit 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 in advance. The extraterrestrial origin of
meteors (shooting stars) 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. passed Earth undetected at a distance of 0.0120 AU (4.65 LD) 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. 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. has a mean diameter of 0.41 m - 0.93 m, which falls within the range required to classify it as a meteoroid. 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 (most of them harmlessly), 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. which at the time 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 (IMS), 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. 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 .
Risk , a
potentially hazardous object that passed within 4
lunar distances in September 2004 and currently has a minimum possible distance of 2.5 lunar distances Through human history, the
risk that any near-Earth object poses has been viewed having regard to both the
culture and the
technology of
human society. Through history, humans have associated NEOs with changing risks, based on religious, philosophical or scientific views, as well as humanity's technological or economical capability to deal with such risks. Thus, NEOs have been seen as
omens of natural disasters or wars; harmless spectacles in an unchanging universe; the source of era-changing cataclysms and finally as a possible cause of a crater-forming impact that could even cause
extinction of humans and other life on Earth. Human
perception of near-Earth asteroids as benign objects of fascination or killer objects with high risk to
human society has ebbed and flowed during the short time that NEAs have been scientifically observed. The 1937 close approach of Hermes and the 1968 close approach of Icarus first raised impact concerns among scientists. Icarus earned significant public attention due to alarmist news reports, while Hermes was considered a threat because it was lost after its discovery; thus its orbit and potential for collision with Earth were not known precisely. On March 23, 1989, the diameter Apollo asteroid
4581 Asclepius (1989 FC) missed the Earth by . If the asteroid had impacted it would have created the largest explosion in recorded history, equivalent to 20,000
megatons of TNT. It attracted widespread attention because it was discovered only after the closest approach. From the 1990s, a typical frame of reference in searches for NEOs has been the scientific concept of
risk. The awareness of the wider public of the impact risk rose after the observation of the impact of the fragments of
Comet Shoemaker–Levy 9 into Jupiter in July 1994. In 1998, the movies
Deep Impact and
Armageddon popularised the notion that near-Earth objects could cause catastrophic impacts.
Risk scales There are two schemes for the scientific classification of impact hazards from NEOs, as a way to communicate the risk of impacts to the general public. . The scale in metres is the approximate diameter of an asteroid with a typical collision velocity The simple
Torino scale was established at an IAU workshop in
Turin () in June 1999, in the wake of the public confusion about the impact risk of . It rates the risks of impacts in the next 100 years according to impact energy and impact probability, using integer numbers between 0 and 10: • ratings of 0 and 1 are of no concern to astronomers or the public, • ratings of 2 to 4 are used for events with increasing magnitude of concern to astronomers trying to make more precise orbit calculations, but not yet a concern for the public, • ratings of 5 to 7 are meant for impacts of increasing magnitude which are not certain but warrant public concern and governmental contingency planning over an increasing timescale, • 8 to 10 would be used for certain collisions of increasing severity. The more complex
Palermo scale, established in 2002, compares the likelihood of an impact at a certain date to the probable number of impacts of a similar energy or greater until the possible impact, and takes the
logarithm of this ratio. Thus, a Palermo scale rating can be any positive or negative real number, and risks of any concern are indicated by values above zero. Unlike the Torino scale, the Palermo scale is not sensitive to newly discovered small objects with an orbit known with low confidence.
Highly rated risks NASA maintains an automated system to evaluate the threat from known NEOs over the next 100 years, which generates the continuously updated
Sentry Risk Table. When the close approach of a newly discovered asteroid is first put on a risk list with a significant risk, it is normal for the risk to first increase, regardless of whether the potential impact will eventually be ruled out or confirmed with the help of additional observations. Similar tables are maintained by the
Near-Earth Object Coordination Centre (NEOCC) of the
European Space Agency (ESA) and on the
NEODyS (Near Earth Objects Dynamic Site) by the
University of Pisa spin-off company SpaceDyS. In March 2002, became the first asteroid with a temporarily positive rating on the Torino Scale, with about a 1 in 9,300 chance of an impact in 2049. Additional observations reduced the estimated risk to zero, and the asteroid was removed from the Sentry Risk Table in April 2002. It is now known that within the next two centuries, will pass the Earth at a safe closest distance (perigee) of on August 31, 2080. Asteroid has a diameter of about a kilometer (0.6 miles), and an impact would therefore be globally catastrophic. Although this asteroid will not strike for at least 800 years and thus has no Torino scale rating, it was added to the Sentry list in April 2002 as the first object with a Palermo scale value greater than zero. The then-calculated 1 in 300 maximum chance of impact and +0.17 Palermo scale value was roughly 50% greater than the background risk of impact by all similarly large objects until 2880. After additional radar and optical observations, , the probability of this impact is assessed at 1 in 2,600. As observations were collected over the next three days, the calculated chance of impact first increased to as high as 2.7%, then fell back to zero, as the shrinking uncertainty zone for this close approach no longer included the Earth. There was at that time still some uncertainty about potential impacts during later close approaches. However, as the precision of orbital calculations improved due to additional observations, the risk of impact at any date was eliminated and Apophis was removed from the Sentry Risk Table in February 2021. In February 2025, the impact risk peaked at 1 in 32, then dropped below 1 in 1000 and the Torino scale rating was reduced to 0. , the impact risk to Earth for the 2032 encounter was down to 1 in 120,000. Project Icarus received wide media coverage, and inspired the 1979 disaster movie
Meteor, in which the US and the USSR join forces to blow up an Earth-bound fragment of an asteroid hit by a comet. The first astronomical program dedicated to the discovery of near-Earth asteroids was the
Palomar Planet-Crossing Asteroid Survey. The link to impact hazard, the need for dedicated survey telescopes and options to head off an eventual impact were first discussed at a 1981
interdisciplinary conference in
Snowmass, Colorado. and set up
The Spaceguard Foundation also in Italy a year later. 80% in 2006, and 93% in 2011. The original Spaceguard goal has thus been met, only three years late. In January 2016, NASA announced the creation of the
Planetary Defense Coordination Office (PDCO) to coordinate an effective threat assessment, response and mitigation effort, which reinforced the goal to detect 90% of NEOs or greater, but without a deadline. In September 2020, it was estimated that about half of these have been found, but objects of this size hit the Earth only about once in 30,000 years. In December 2023, using a lower absolute brightness estimate for smaller asteroids, the ratio of discovered NEOs with diameters of or greater was estimated at 38%. while the
NEO Surveyor satellite, to be launched in 2027, is expected to push the ratio to 76% during its 5-year mission. Survey programs aim to identify threats years in advance, giving humanity time to prepare a space mission to avert the threat. The
ATLAS project, by contrast, aims to find impacting asteroids shortly before impact, much too late for deflection maneuvers but still in time to evacuate and otherwise prepare the affected Earth region. Another project, the
Zwicky Transient Facility (ZTF), which surveys for objects that change their brightness rapidly, also detects asteroids passing close to Earth. Scientists involved in NEO research have also considered options for actively averting the threat if an object is found to be on a collision course with Earth. All viable methods aim to deflect rather than destroy the threatening NEO, because the fragments would still cause widespread destruction. Deflection, which means a change in the object's orbit months to years prior to the
predicted impact, also requires orders of magnitude less energy. == Number and classification ==