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Top left: near-Earth asteroid 2006 DP14 imaged by a DSN radar antenna Top right: faint near-Earth asteroid 2009 FD (marked by circle) as seen by the VLT telescope Middle: near-Earth comet 103P/Hartley as seen by NASA's Deep Impact probe Bottom: there are 19,229 known NEOs, divided into several orbital subgroups as of November 25, 2018[update][1]   Comets: 107 (0.6%)   Atiras/Apohele: 31 (0.2%)   Atens: 1,411 (7.3%)   Apollos: 9,559 (49.7%)   Amors: 8,120 (42.2%)

A near-Earth object (NEO) is any small Solar System body whose orbit brings it to proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU).^[2] If a NEO's orbit crosses the Earth's and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO).^[3] Most known PHOs and NEOs are asteroids, but a small fraction are comets.^[1]

There are over 19,000 known near-Earth asteroids (NEAs), over a hundred short-period near-Earth comets (NECs),^[1] and a number of solar-orbiting spacecraft and meteoroids large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth.^[4] NEOs have become of increased interest since the 1980s because of greater awareness of the potential danger some of the asteroids or comets pose. When impacting the Earth, asteroids as small as 20 m cause sufficiently strong shock waves and heat to damage the local environment and populations.^[5] Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they hit ground and tsunamis if water bodies are hit. It is in principle possible to deflect asteroids, and methods of mitigation are being researched.^[6]

Based on the orbit calculations of identified NEOs, their risk of future impact is assessed on two scales, the Torino scale and the more complex Palermo scale, both of which rate a risk of any significance with values above 0. Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery, but as of March 2018^[update], more precise calculations based on subsequent observations led to a reduction of the rating to or below 0 in all cases.^[7]

Since 1998, the United States, the European Union, and other nations are scanning for NEOs in an effort called Spaceguard.^[8] The initial US Congress mandate to NASA of cataloging at least 90% of NEOs that are at least 1 kilometre (0.62 mi) in diameter, which would cause a global catastrophe in case of an impact with Earth, had been met by 2011.^[9] In later years, the survey effort has been expanded^[10] to objects as small as about 140 m (460 ft) across,^[11] which still have the potential for large-scale, though not global, damage.

Due to their Earth-like orbits and low surface gravity, NEOs are easy targets for spacecraft.^[12][13] As of August 2018^[update], five near-Earth comets^[14][15][16] and three near-Earth asteroids have been visited by spacecraft,^[17][18][19] and probes are en route to two more NEAs.^[20][21] Plans to mine NEAs commercially have been drafted by private companies.

Definitions

Plot of orbits of known potentially hazardous asteroids (size over 460 feet (140 m) and passing within 4.7 million miles (7.6×10^⁶ km) of Earth's orbit) as of early 2013 (alternate image)

The major technical astronomical definition for Near-Earth objects (NEOs) are small Solar System bodies with orbits around the Sun that by definition lie partly between 0.983 (perihelion) and 1.3 (aphelion) astronomical units (AU; Sun–Earth distance) away from the Sun.^[22][23] Thus, NEOs are not necessarily currently near the Earth, but they can potentially approach the Earth relatively closely. However, the term is also used more flexibly sometimes, for example for objects in orbit around the Earth or for quasi-satellites,^[24] which have a more complex orbital relationship with the Earth.

When a NEO is detected, like all other small Solar System bodies, it is submitted to the International Astronomical Union's (IAU's) Minor Planet Center (MPC) for cataloging. MPC maintains separate lists of confirmed NEOs and potential NEOs.^[25] The orbits of some NEOs intersect that of the Earth, so they pose a collision danger.^[3] These are considered potentially hazardous objects (PHOs). For the asteroids among PHOs, the potentially hazardous asteroids (PHAs), MPC maintains a separate list.^[26] NEOs are also catalogued by two separate units of the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA): the Center for Near Earth Object Studies (CNEOS)^[27] and the Solar System Dynamics Group.^[28]

PHAs are currently defined based on parameters relating to their potential to approach the Earth dangerously closely.^[2] Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude of 22.0 or brighter (a rough indicator of large size) are considered PHAs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU (7,500,000 km; 4,600,000 mi), or are smaller than about 140 m (460 ft) in diameter (i.e. H = 22.0 with assumed albedo of 14%), are not considered PHAs.^[2] NASA's catalog of near-Earth objects also includes the approach distances of asteroids and comets (expressed in lunar distances).^[29]

History of human awareness of NEOs

1910 drawing of the path of Halley's Comet

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 periodicity of some comets was recognised in 1705, when Edmond Halley first published his orbit calculations for the returning object now known as Halley's Comet.^[30] The 1758–1759 return of Halley's Comet was the first comet appearance predicted in advance.^[31]

The first near-Earth asteroid to be discovered was 433 Eros in 1898.^[32] The asteroid was subject to several observation campaigns, because measurements of its orbit enabled a precise determination of the distance of the Earth from the Sun.^[33] In has been said that Lexell's comet of 1770 was the first discovered Near-Earth object.^[34]

In 1937, asteroid 69230 Hermes was discovered when it passed the Earth at twice the distance of the Moon.^[35] 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.^[36] Hermes was re-discovered in 2003, and is now known to be no threat for at least the next century.^[35]

On June 14, 1968, the 1.4 km diameter asteroid 1566 Icarus passed Earth at a distance of 0.042482 AU (6,355,200 km), or 16 times the distance of the Moon.^[37] During this approach, Icarus became the first minor planet to be observed using radar, with measurements obtained at the Haystack Observatory^[38] and the Goldstone Tracking Station.^[39] This was the first close approach predicted years in advance (Icarus had been discovered in 1949), and also earned significant public attention, due to alarmist news reports.^[36] A year before the approach, MIT students launched Project Icarus, devising a plan to deflect the asteroid with rockets in case it was found to be on a collision course with Earth.^[40] 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.^[41]

On March 23, 1989, the 300 m (980 ft) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 km (430,000 mi). 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.^[42]

In March 1998, early orbit calculations for recently discovered asteroid (35396) 1997 XF11 showed a potential 2028 close approach 0.00031 AU (46,000 km) from the Earth, well within the orbit of the Moon, but with a large error margin allowing for a direct hit. Further data allowed a revision of the 2028 approach distance to 0.0064 AU (960,000 km), with no chance of collision. By that time, inaccurate reports of a potential impact had caused a media storm.^[36]

Known Near-Earth objects – as of January 2018
Video (0:55; July 23, 2018)

Risk

Asteroid 4179 Toutatis is a potentially hazardous object that has passed within 2.3 lunar distances

From the late 1990s, a typical frame of reference in searches for NEOs has been the scientific concept of risk. The risk that any near-Earth object poses is 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.^[6] Thus, NEOs have been seen as omens of natural disasters or wars; harmless spectacles in an unchanging universe; the source of era-changing cataclysms^[6] or potentially poisonous fumes (during Earth's passage through the tail of Halley's Comet in 1910);^[43] and finally as a possible cause of a crater-forming impact that could even cause extinction of humans and other life on Earth.^[6]

The potential of catastrophic impacts by near-Earth comets was recognised as soon as the first orbit calculations provided an understanding of their orbits: in 1694, Edmond Halley presented a theory that Noah's flood in the Bible was caused by a comet impact.^[44] 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.^[13] Scientists have recognised the threat of impacts that create craters much bigger than the impacting bodies and have indirect effects on an even wider area since the 1980s, after the confirmation of a theory that the Cretaceous–Paleogene extinction event (in which dinosaurs died out) 65 million years ago was caused by a large asteroid impact.^[6][45]

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.^[6][45] In 1998, the movies Deep Impact and Armageddon popularised the notion that near-Earth objects could cause catastrophic impacts.^[45] Also at that time, a conspiracy theory arose about the supposed 2003 impact of the fictitious planet Nibiru, which persisted on the internet as the predicted impact date was moved to 2012 and then 2017.^[46]

Risk scales

There are two schemes for the scientific classification of impact hazards from NEOs:

• the simple Torino scale, which rates the risks of impacts in the next 100 years according to impact energy and impact probability, using integer numbers between 0 and 10;^[47] and


• the more complex Palermo Technical Impact Hazard Scale, which ascribes ratings that can be any positive or negative real number; these ratings depend on the background impact frequency, impact probability and time until possible impact.^[48]
  

On both scales, risks of any concern are indicated by values above zero.^[47][48]

Magnitude of risk

The annual background frequency used in the Palermo scale for impacts of energy greater than E megatonnes is estimated as:^[48]

fB=0.03E−0.8{displaystyle f_{B}=0.03E^{-0.8};}

For instance, this formula implies that the expected value of the time from now until the next impact greater than 1 megatonne is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of E.

However, another paper^[49] published in 2002 – the same year as the paper on that the Palermo scale is based – found a power law with different constants:

fB=0.00737E−0.9{displaystyle f_{B}=0.00737E^{-0.9};}

This formula gives considerably lower rates for a given E. For instance, it gives the rate for bolides of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.

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.^[7] All or nearly all of the objects are highly likely to drop off the list eventually as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions.^[7][50]

In March 2002, (163132) 2002 CU11 became the first asteroid with a positive rating on the Torino Scale, with about a 1 in 9,300 chance of an impact in 2049.^[51] After more observations, the estimated risk was reduced to zero, and the asteroid was removed from the Sentry Risk Table in April 2002.^[52] It is now estimated with some certainty that in the next two centuries, 2002 CU₁₁ will pass the Earth safely at a closest distance (perigee) of 0.00425 AU (636,000 km; 395,000 mi) on August 31, 2080.^[53]

Radar image of asteroid 1950 DA

Asteroid 1950 DA was lost after its 1950 discovery, since its observations over just 17 days were insufficient to determine its orbit; it was rediscovered on December 31, 2000. It has a diameter of about a kilometer (0.6 miles). It was also observed by radar during its close approach in 2001, allowing much more precise orbit calculations. 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 because it was the first object with a Palermo scale value greater than zero.^[54][55] 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.^[56] Uncertainties in the orbit calculations were further reduced using radar observations in 2012, and this decreased the odds of an impact.^[57] Taking all radar and optical observations until 2015 into account, the probability of impact is, as of March 2018^[update], assessed at 1 in 8,300.^[7] The corresponding Palermo scale value of −1.42 is still the highest for all objects on the Sentry List Table.^[7] As of March 2018^[update], only one more object (2009 FD) has a Palermo scale value above −2 for a single impact date.^[7]

On December 24, 2004, 370 m (1,210 ft) asteroid 99942 Apophis (at the time known by its provisional designation 2004 MN₄) was assigned a 4 on the Torino scale, the highest rating ever given, as there was a 2.7% chance of Earth impact on April 13, 2029. By December 28, 2004, additional observations had produced a smaller uncertainty zone which no longer included the Earth during the 2029 approach. The 2029 risk of impact consequently dropped to zero, but later potential impact solutions were still rated 1 on the Torino scale. The 2036 risk was lowered to a Torino rating of 0 in August 2006. As of March 2018^[update], calculations show Apophis has no chance of impacting Earth before 2060.^[7]

In February 2006, (144898) 2004 VD17 has been assigned a Torino Scale rating of 2 due to a close encounter predicted for May 4, 2102.^[58] After more precise calculations, the rating was lowered to 1 in May 2006 and 0 in October 2006, and the asteroid was removed from the Sentry Risk Table entirely in February 2008.^[52]

As of March 2018^[update], 2010 RF12 is listed with the highest chance of impacting Earth, at 1 in 20 on September 5, 2095. At only 7 m (23 ft) across, the asteroid however does not pose a serious threat: the possible 2095 impact therefore rates only −3.32 on the Palermo Scale.^[7]

Projects to minimize the threat

Annual NEA discoveries by survey: all NEAs (top) and NEAs > 1 km (bottom)

NEOWISE – first four years of data starting in December 2013 (animated; April 20, 2018)

The first astronomical program dedicated to the discovery of near-Earth asteroids was the Palomar Planet-Crossing Asteroid Survey, started in 1973 by astronomers Eugene Shoemaker and Eleanor Helin.^[13] 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.^[45] Plans for a more comprehensive survey, named the Spaceguard Survey, were developed by NASA from 1992, under a mandate from the United States Congress.^[59][60] To promote the survey on an international level, the International Astronomical Union (IAU) organised a workshop at Vulcano, Italy in 1995,^[59] and set up the Spaceguard Foundation also in Italy a year later.^[8] In 1998, the United States Congress gave NASA a mandate to detect of 90% of near-earth asteroids over 1 km (0.62 mi) diameter (that threaten global devastation) by 2008.^[60][61]

Several surveys have undertaken "Spaceguard" activities (an umbrella term), including Lincoln Near-Earth Asteroid Research (LINEAR), Spacewatch, Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey (CSS), Campo Imperatore Near-Earth Object Survey (CINEOS), Japanese Spaceguard Association, Asiago-DLR Asteroid Survey (ADAS) and Near-Earth Object WISE (NEOWISE). As a result, the ratio of the known and the estimated total number of near-Earth asteroids larger than 1 km in diameter rose from about 20% in 1998 to 65% in 2004,^[8] 80% in 2006,^[61] and 93% in 2011. The original Spaceguard goal has thus been met, only three years late.^[9][62] As of June 12, 2018^[update], 893 NEAs larger than 1 km have been discovered,^[1] or 97% of an estimated total of about 920.^[63]

In 2005, the original USA Spaceguard mandate was extended by the George E. Brown, Jr. Near-Earth Object Survey Act, which calls for NASA to detect 90% of NEOs with diameters of 140 m (460 ft) or greater, by 2020.^[10] In January 2016, NASA announced the creation of the Planetary Defense Coordination Office (PDCO) to track NEOs larger than about 30–50 m (98–164 ft) in diameter and coordinate an effective threat response and mitigation effort.^[11][64]

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.^[66] Another project, the Zwicky Transient Facility (ZTF), which surveys for objects that change their brightness rapidly,^[67] also detects asteroids passing close to Earth.^[68]

Scientists involved in NEO research have also considered options for actively averting the threat from an object found to be on a collision course with Earth.^[45] All viable methods aim to deflect rather than destroy the threatening NEO, because the fragments would still cause widespread destruction.^[14] 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.^[14]

Number and classification

Cumulative discoveries of near-Earth asteroids known by size, 1980–2017

Near-Earth objects are classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets create meteoroid streams that can generate meteor showers.

As of January 8, 2019^[update], according to statistics maintained by CNEOS, 19,470 NEOs have been discovered. Only 107 (0.55%) of them are comets, whilst 19,363 (99.45%) are asteroids. There are 1,955 NEOs that are classified as potentially hazardous asteroids (PHAs).^[1] Discovery is heavily influenced by the opposition effect, see diagram below.

As of January 8, 2019^[update], there are 893 NEAs on the Sentry impact risk page at the NASA website.^[7] A significant number of these NEAs are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "green zone" (Torino Scale 1), meaning that none warrant the attention of general public.^[47]

Observational biases

One problem with understanding the true number of NEOs is that existing detection's are influenced by a number of factors.^[69] Observational biases need to be taken into account when trying to calculate the number of bodies in a population.^[69] There is a bias towards finding asteroids that are easier to detect.^[70]

For example, its has been easier to spot objects on the night-side of Earth, not only is there less noise from daylight, but its also the sunlit side of the asteroids whereas on in the daytime sky looking towards the sun, the backside of the object is seen (e.g. comparing a Full moon at night to a New Moon in daytime). In addition, because of opposition surge they become even brighter when the Earth is along the axis of sunlight. Finally, its hard make observations in daylight near the Sun.^[70] The light of sun hitting asteroids has been called "full asteroid" similar to a "full moon" and the greater amount of light, creates a bias that they are easier to detect in this case.^[70]

In the year 2000, it was predicted that there are 900 near earth asteroids of at least kilometer size taking into account known observational biases.^[69] Studies of asteroid populations try to take into account observational selection biases to make a more accurate assessment.^[71] Finding observational biases can help determine NEO populations, for example, one study noted that bodies in low-eccentricity Earth-crossing orbits were favored, making the detection of Aten more favorured then Apollos.^[72]

Two of the biggest Near-Earth objects 433 Eros and 1036 Ganymed, were also some the earliest to be detected.^[73] 1036 Ganymed is about 20 miles in diameter.^[73] As bigger asteroids they reflected more light.^[70] One way around simply finding the NEOs that reflect the most visible light, is to use infrared telescopes that observe the objects heat emissions.^[70]

Asteroids and other SSSB have been known to experience opposition effect (or surge) which means that when viewing closer to the axis of the direction of incoming light, more light is returned.^[74]

Over half (53%) of the discoveries of Near Earth objects were made in 3.8% of the sky, in a 22.5° cone facing directly away from the Sun, and the vast majority (87%) were made in 15% of the sky, in a 45° cone facing away from the Sun.^[75]

Near-Earth asteroids

Asteroid Toutatis from Paranal

These are objects in a near-Earth orbit without the tail or coma of a comet. As of January 6, 2019^[update], 19,363 near-Earth asteroids are known, only 1,955 of which are PHAs.^[1]

NEAs survive in their orbits for just a few million years.^[22] They are eventually eliminated by planetary perturbations, causing ejection from the Solar System or a collision with the Sun or a planet.^[22] With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that main-belt asteroids are moved into the inner Solar System through orbital resonances with Jupiter.^[22] The interaction with Jupiter through the resonance perturbs the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as Kirkwood gaps, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the Yarkovsky effect that provides a continuing supply of near-Earth asteroids.^[76] Compared to the entire mass of the asteroid belt, the mass loss necessary to sustain the NEA population is relatively small; totalling less than 6% over the past 3.5 billion years.^[22] The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.^[77]

A small number of NEAs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.^[22][78]

Many asteroids have natural satellites (minor-planet moons). As of March 15, 2017^[update], 66 NEAs were known to have at least one moon, including three known to have two moons.^[79] The asteroid 3122 Florence, one of the largest PHAs^[26] with a diameter of 4.5 km (2.8 mi), has two moons measuring 100–300 m (330–980 ft) across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.^[80]

Size distribution

Known near-Earth asteroids by size

While a small fraction of these asteroids has accurate direct diameter measurements, from radar observations, from images of the asteroid surface, or from stellar occultations, the size of the vast majority of near Earth asteroids can only be estimated on the basis of their brightness and a representative asteroid albedo or surface reflectivity, which is commonly assumed to be 14%.^[27] Such indirect size estimates have significant uncertainties for individual asteroids, since asteroid albedos can range as low as 0.05 and as high as 0.3. Using this method, an absolute magnitude of 17.75 roughly corresponds to a diameter of 1 km (0.62 mi)^[27] and an absolute magnitude of 22.0 corresponds to a diameter of 140 m (460 ft).^[2]
In May 2016, the precision of asteroid diameter estimates arising from the Wide-field Infrared Survey Explorer and NEOWISE missions was questioned by technologist Nathan Myhrvold,^[81][82][83] Although the early original criticism did not pass peer review^[82][84] and faced criticism for its methodology itself,^[85] a more recent peer-reviewed study was subsequently published.^[86][87]

As of January 4, 2019 and mostly using the crude estimate from an absolute magnitude, 897 NEAs listed by CNEOS, including 156 PHAs, measure at least 1 km in diameter, and 8,452 known NEAs are larger than 140 m in diameter.^[1]
The smallest known near-Earth asteroid is 2008 TS₂₆ with an absolute magnitude of 33.2,^[28] corresponding to a diameter of about 1 m (3.3 ft).^[88] The largest such object is 1036 Ganymed,^[28] with an absolute magnitude of 9.45 and a diameter of about 38 km (24 mi).^[89]

In 2000, NASA reduced its estimate of the number of existing near-Earth asteroids over one kilometer in diameter from 1,000–2,000 to 500–1,000.^[90][91] Shortly thereafter, the LINEAR survey provided an alternative estimate of 7003122700000000000♠1,227+170
−90.^[92] In 2011, on the basis of NEOWISE observations, the estimated number of one-kilometer NEAs was narrowed to 7002981000000000000♠981±19 (of which 93% had been discovered at the time), while the number of NEAs larger than 140 meters across was estimated at 7004132000000000000♠13,200±1,900.^[9][62] The NEOWISE estimate differed from other estimates in assuming a slightly lower average asteroid albedo, which produces larger estimated diameters for the same asteroid brightness. This resulted in 911 then known asteroids at least 1 km across, as opposed to the 830 then listed by CNEOS.^[93] In 2017, using an improved statistical method, two studies reduced the estimated number of NEAs brighter than absolute magnitude 17.75 (approximately over one kilometer in diameter) to 7002921000000000000♠921±20.^[63][94] The estimated number of asteroids brighter than absolute magnitude of 22.0 (approximately over 140 m across) rose to 7004271000000000000♠27,100±2,200, double the WISE estimate,^[94] of which about a third are known as of 2018.

The number of asteroids brighter than H = 25, which corresponds to about 40 m (130 ft) in diameter, is estimated at about 7005840000000000000♠840,000±23,000—of which about 1.3 percent had been discovered by February 2016; the number of asteroids brighter than H = 30 (larger than 3.5 m (11 ft)) is estimated at about 7002400000000000000♠400±100 million—of which about 0.003 percent had been discovered by February 2016.^[94]

Orbital classification

Types of near-Earth asteroid orbits

Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q):^[2][22]

• The Atiras or Apoheles have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, Q < 0.983 AU. (This implies that the asteroid's semi-major axis is also less than 0.983 AU.)


• The Atens have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, a < 1.0 AU and Q > 0.983 AU.


• The Apollos have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, a > 1.0 AU and q < 1.017 AU. (1.017 AU is Earth's aphelion distance.)


• The Amors have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so q < 1.3 AU. In summary, 1.017 AU < q < 1.3 AU. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.

(Note: Some authors define Atens differently: they define it as being all the asteroids with a semi-major axis of less than 1 AU.^[95][96] That is, they consider the Atiras to be part of the Atens.^[96] Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.)

Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.^[22]

As of May 10, 2018^[update], 17 Atiras, 1,342 Atens, 9,964 Apollos and 6,813 Amors have been discovered and cataloged.^[1]

Co-orbital asteroids

The five Lagrangian points relative to Earth and possible orbits along gravitational contours

NEAs on a co-orbital configuration have the same orbital period as the Earth. All co-orbital asteroids have special orbits that are relatively stable and, paradoxically, can prevent them from getting close to Earth:

• 
  Trojans: Near the orbit of a planet, there are five gravitational equilibrium points, the Lagrangian points, in which an asteroid would orbit the Sun in fixed formation with the planet. Two of these, 60 degrees ahead and behind the planet along its orbit (designated L4 and L5 respectively) are stable; that is, an asteroid near these points would stay there for millions of years even if perturbed by other planets and non-gravitational forces. As of March 2018^[update], Earth's only confirmed Trojan is 2010 TK7, circling Earth's L4 point.^[97]
  


• 
  Horseshoe librators: The region of stability around L4 and L5 also includes orbits for co-orbital asteroids that run around both L4 and L5. Seen from Earth, the orbit can resemble the circumference of a horseshoe, or may consist of annual loops that wander back and forth (librate) in a horseshoe-shaped area. In both cases, the Sun is at the horseshoe's center of gravity, Earth is in the gap of the horseshoe, and L4 and L5 are inside the ends of the horseshoe. By 2016, 12 horseshoe librators of Earth have been discovered.^[98] The most-studied and, at about 5 km (3.1 mi), largest is 3753 Cruithne, which travels along bean-shaped annual loops and completes its horseshoe libration cycle every 770 to 780 years.^[99][100](419624) 2010 SO16 is an asteroid on a relatively stable circumference-of-a-horseshoe orbit, with a horseshoe libration period of about 350 years.^[101]
  


• 
  Quasi-satellites: Quasi-satellites are co-orbital asteroids on a normal elliptic orbit with a higher eccentricity than Earth's, which they travel in a way synchronised with Earth's motion. Since the asteroid orbits the Sun slower than Earth when further away and faster than Earth when closer to the Sun, when observed from Earth, the quasi-satellite appears to orbit Earth in a retrograde direction in one year, even though it is not bound gravitationally. By 2016, five asteroids were known to be a quasi-satellite of Earth. (469219) 2016 HO3 is Earth's closest quasi-satellite, in an orbit that has been stable for almost a century.^[102] Orbit calculations until 2016 showed that all quasi-satellites and four of the horseshoe librators then known repeatedly transfer between horseshoe and quasi-satellite orbits.^[102] One of these objects, 2003 YN107, was observed during its transition from a quasi-satellite orbit to a horseshoe orbit in 2006; it is expected to transfer back to a quasi-satellite orbit 60 years later.^[103]
  


• 
  Temporary satellites: NEAs can also transfer between solar orbits and distant Earth orbits, becoming gravitationally bound temporary satellites. According to simulations, temporary satellites are typically caught when they pass the L1 or L2 Lagrangian points, and Earth has at least one temporary satellite 1 m (3.3 ft) across at any given time, but they are too faint to detect by current surveys.^[104] As of March 2018^[update], the only observed transition was that of asteroid 2006 RH120, which was a temporary satellite from September 2006 to June 2007^[105][106] and has been on a solar orbit with a 1.003-year period ever since.^[107] According to orbital calculations, on its solar orbit, 2006 RH₁₂₀ passes Earth at low speed every 20 to 21 years,^[107] at which point it can become a temporary satellite again.

Meteoroids

In 1961, the IAU defined meteoroids as a class of solid interplanetary objects distinct from asteroids by their considerably smaller size.^[108] This definition was useful at the time because, with the exception of the Tunguska event, all historically observed meteors were produced by objects significantly smaller than the smallest asteroids observable by telescopes.^[108] As the distinction began to blur with the discovery of ever smaller asteroids and a greater variety of observed NEO impacts, revised definitions with size limits have been proposed from the 1990s.^[108] In April 2017, the IAU adopted a revised definition that generally limits meteoroids to a size between 30 µm and 1 m in diameter, but permits the use of the term for any object of any size that caused a meteor, thus leaving the distinction between asteroid and meteoroid blurred.^[109]

Near-Earth comets

Halley's Comet during its 0.10 AU^[110] approach of Earth in May 1910

Near-Earth comets (NECs) are objects in a near-Earth orbit with a tail or coma. Comet nuclei are typically less dense than asteroids but they pass Earth at higher relative speeds, thus the impact energy of comet nucleus is slightly larger than that of a similar-sized asteroid.^[111] NECs may pose an additional hazard due to fragmentation: the meteoroid streams which produce meteor showers may include large inactive fragments, effectively NEAs.^[112] Although no impact of a comet in Earth's history has been conclusively confirmed, the Tunguska event may have been caused by a fragment of Comet Encke.^[113]

Comets are commonly divided between short-period and long-period comets. Short-period comets, with an orbital period of less than 200 years, originated in the Kuiper belt, beyond the orbit of Neptune; while long-period comets originate in the Oort Cloud, in the outer reaches of the Solar System.^[14] The orbital period distinction is of importance in the evaluation of the risk from near-Earth comets because short-period NECs are likely to have been observed during multiple apparitions and thus their orbits can be determined with some precision, while long-period NECs can be assumed to have been seen for the first and last time when they appeared during the Age of Science, thus their approaches cannot be predicted well in advance.^[14] Since the threat from long-period NECs is estimated to be at most 1% of the threat from NEAs, and long-period comets are very faint and thus difficult to detect at large distances from the Sun, Spaceguard efforts consistently focused on asteroids and short-period comets.^[59][111] CNEOS even restricts its definition of NECs to short-period comets^[2]—as of May 10, 2018^[update], 107 such objects have been discovered.^[1]

As of March 2018^[update], only 20 comets have been observed to pass within 0.1 AU (15,000,000 km; 9,300,000 mi) of Earth, including 10 which are or have been short-period comets.^[114] Two of these comets, Halley's Comet and 73P/Schwassmann–Wachmann, have been observed during multiple close approaches.^[114] The closest observed approach was 0.0151 AU (5.88 LD) for Lexell's Comet on July 1, 1770.^[114] After an orbit change due to a close approach of Jupiter in 1779, this object is no longer a NEC. The closest approach ever observed for a current short-period NEC is 0.0229 AU (8.92 LD) for Comet Tempel–Tuttle in 1366.^[114] This comet is the parent body of the Leonid meteor shower, which also produced the Great Meteor Storm of 1833.^[115] Orbital calculations show that P/1999 J6 (SOHO), a faint sungrazing comet and confirmed short-period NEC observed only during its close approaches to the Sun,^[116] passed Earth undetected at a distance of 0.0121 AU (4.70 LD) on June 12, 1999.^[117]

Comet 109P/Swift–Tuttle, which is also the source of the Perseid meteor shower which hits Earth every year in August, has a roughly 130-year orbit which passes close to the Earth. After the comet's 1992 return, when only the two previous returns in 1862 and 1737 have been identified, orbital calculations showed that the comet would pass very close to Earth during its next return in 2126, with an impact within the range of uncertainty. By 1993, even earlier returns (back to at least 188 AD) have been identified, and the new orbital calculation eliminated the impact risk, predicting the comet to pass Earth in 2126 at a distance of 24 million kilometers. In 3044, the comet is expected to pass Earth at less than 1.6 million kilometers.^[118]

Artificial near-Earth objects

J002E3 discovery images taken on September 3, 2002. J002E3 is in the circle

Defunct space probes and final stages of rockets can end up in near-Earth orbits around the Sun, and be re-discovered by NEO surveys when they return to Earth's vicinity.

In September 2002, astronomers found an object designated J002E3. The object was on a temporary satellite orbit around Earth, leaving for a solar orbit in June 2003. Calculations showed that it was also on a solar orbit before 2002, but was close to Earth in 1971. J002E3 was identified as the third stage of the Saturn V rocket that carried Apollo 12 to the Moon.^[119][120] In 2006, two more apparent temporary satellites were discovered which were suspected of being artificial.^[120] One of them was eventually confirmed as an asteroid and classified as the temporary satellite 2006 RH₁₂₀.^[120] The other, 6Q0B44E, was confirmed as an artificial object, but its identity is unknown.^[120] Another confirmed artificial temporary satellite with unidentified origin is 2013 QW₁.^[120]

In some cases, active space probes on solar orbits have been observed by NEO surveys and erroneously catalogued as asteroids before identification. During its 2007 flyby of Earth on its route to a comet, ESA's space probe Rosetta was detected unidentified and classified as asteroid 2007 VN₈₄, with an alert issued due to its close approach.^[121] The designation 2015 HP₁₁₆ was similarly removed from asteroid catalogues when the observed object was identified with Gaia, ESA's space observatory for astrometry.^[122]

Impacts

When a near-Earth object impacts Earth, objects up to a few tens of metres across ordinarily explode in the upper atmosphere (usually harmlessly), with most or all of the solids vaporized, while larger objects hit the water surface, forming waves, or the solid surface, forming impact craters.^[123]

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.^[124][125] 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.^[22] 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 4 m (13 ft) at about one year; for asteroids 7 m (23 ft) across (which impacts with as much energy as the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT) at five years, for asteroids 60 m (200 ft) across (an impact energy of 10 megatons, comparable to the Tunguska event in 1908) at 1,300 years, for asteroids 1 km (0.62 mi) across at half a million years, and for asteroids 5 km (3.1 mi) across at 18 million years.^[126] Some other models estimate similar impact frequencies,^[22] while others calculate higher frequencies.^[125] For Tunguska-sized (10-megaton) impacts, there are estimates of one event every 2,000–3,000 years and one event every 300 years.^[125]

Location and impact energy of small asteroids impacting Earth's atmosphere

The second-largest observed impact after the Tunguska meteor was a 1.1-megaton air blast near the Prince Edward Islands off the coast of South Africa in 1963, which was detected only by infrasound sensors.^[127] The third-largest but most-observed impact was the Chelyabinsk meteor of February 15, 2013. A 20 m (66 ft) asteroid exploded with the equivalent blast yield of 400–500 kilotons.^[127] The calculated orbit of the pre-impact asteroid is similar to that of Apollo asteroid 2011 EO40, making the latter the meteor's possible parent body.^[128]

On October 7, 2008, 19 hours after it has been first observed, 4 m (13 ft) asteroid 2008 TC3 blew up 37 km (23 mi) above the Nubian Desert in Sudan. It was the first time that an asteroid was observed and its impact had been predicted prior to its entry into the atmosphere as a meteor.^[129] 10.7 kg of meteorites were recovered after the impact.^[130]

On January 2, 2014, just 21 hours after it was the first asteroid to be discovered in 2014, 2–4 m 2014 AA blew up in Earth's atmosphere above the Atlantic Ocean. Far from any land, the meteor explosion was only observed by three infrasound detectors of the Comprehensive Nuclear-Test-Ban Treaty Organization. This impact was the second to be predicted in advance.^[131]

Asteroid impact prediction is however in its infancy and successfully predicted asteroid impacts are rare. The vast majority of impacts recorded by infrasound sensors designed to detect detonation of nuclear devices:^[132] are not predicted in advance.

Observed impacts aren't restricted to the surface and atmosphere of Earth. Dust-sized NEOs have impacted man-made spacecraft, including NASA's Long Duration Exposure Facility, which collected interplanetary dust in low Earth orbit for six years from 1984.^[108] Impacts on the Moon can be observed as flashes of light with a typical duration of a fraction of a second.^[133] The first lunar impacts were recorded during the 1999 Leonid storm.^[134] Subsequently, several continuous monitoring programs were launched.^{[133][135][136]} As of March 2018^[update], the largest observed lunar impact occurred on September 11, 2013, lasted 8 seconds, and was likely caused by an object 0.6–1.4 m (2.0–4.6 ft) in diameter.^[135]

Close approaches

Flyby of asteroid 2004 FH (centre dot being followed by the sequence). The other object that flashes by is an artificial satellite

Each year, several mostly small NEOs pass Earth closer than the distance of the Moon.^[137]

On August 10, 1972, a meteor that became known as the 1972 Great Daylight Fireball was witnessed by many people; it moved north over the Rocky Mountains from the U.S. Southwest to Canada. It was an Earth-grazing meteoroid that passed within 57 km (35 mi) of the Earth's surface, and was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera.^[138]

On October 13, 1990, Earth-grazing meteoroid EN131090 was observed above Czechoslovakia and Poland, moving at 41.74 km/s (25.94 mi/s) along a 409 km (254 mi) trajectory from south to north. The closest approach to the Earth was 98.67 km (61.31 mi) 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.^[139]

On March 18, 2004, LINEAR announced that a 30 m (98 ft) asteroid, 2004 FH, would pass the Earth that day at only 42,600 km (26,500 mi), about one-tenth the distance to the Moon, and the closest miss ever noticed until then. They estimated that similar-sized asteroids come as close about every two years.^[140]

On March 31, 2004, two weeks after 2004 FH, 2004 FU162 set a new record for closest recorded approach above the atmosphere, passing Earth's surface only 6,500 km (4,000 mi) away (about one Earth radius or one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU₁₆₂ was detected only hours before its closest approach. If it had collided with Earth, it probably would have disintegrated harmlessly in the atmosphere.^[141]

On February 4, 2011, an asteroid designated 2011 CQ1, estimated at 0.8–2.6 m (2.6–8.5 ft) in diameter, passed within 5,500 km (3,400 mi) of the Earth, setting a new record for closest approach without impact,^[142] which still stands as of September 2018^[update].^[137]

On November 8, 2011, asteroid (308635) 2005 YU55, relatively large at about 360 m (1,180 ft) in diameter, passed within 324,600 km (201,700 mi) (0.85 lunar distances) of Earth.^[143]

On February 15, 2013, the 30 m (98 ft) asteroid 367943 Duende (2012 DA₁₄) passed approximately 27,700 km (17,200 mi) above the surface of Earth, closer than satellites in geosynchronous orbit.^[144] The asteroid was not visible to the unaided eye. This was the first close passage of an object discovered during a previous passage, and was thus the first to be predicted well in advance.^[145]

Exploratory missions

Some NEOs are of special interest because they can be physically explored with lower mission velocity than is necessary for even the Moon, due to their combination of low velocity with respect to Earth and weak gravity. They may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation.^[12] This makes them an attractive target for exploration.^[146]

Missions to NEAs

433 Eros as seen by NASA's NEAR probe

The IAU held a minor planets workshop in Tucson, Arizona, in March 1971. At that point, launching a spacecraft to asteroids was considered premature; the workshop only inspired the first astronomical survey specifically aiming for NEAs.^[13] Missions to asteroids were considered again during a workshop at the University of Chicago held by NASA's Office of Space Science in January 1978. Of all of the near-Earth asteroids (NEA) that had been discovered by mid-1977, it was estimated that spacecraft could rendezvous with and return from only about 1 in 10 using less propulsive energy than is necessary to reach Mars. It was recognised that due to the low surface gravity of all NEAs, moving around on the surface of a NEA would cost very little energy, and thus space probes could gather multiple samples.^[13] Overall, it was estimated that about one percent of all NEAs might provide opportunities for human-crewed missions, or no more than about ten NEAs known at the time. A five-fold increase in the NEA discovery rate was deemed necessary to make a manned mission within ten years worthwhile.^[13]

The first near-Earth asteroid to be visited by a spacecraft was 17 km (11 mi) asteroid 433 Eros when NASA's Near Earth Asteroid Rendezvous (NEAR) probe orbited it from February 2001, landing on the asteroid surface in February 2002.^[17] A second near-Earth asteroid, the 535 m (1,755 ft) long peanut-shaped 25143 Itokawa, was visited in September 2005 by JAXA's Hayabusa mission,^[18] which succeeded in taking material samples back to Earth. A third near-Earth asteroid, the 2.26 km (1.40 mi) long elongated 4179 Toutatis, was explored by CNSA's Chang'e 2 spacecraft during a flyby in December 2012.^[19][54]

Radar images of asteroid 101955 Bennu, target of NASA's OSIRIS-REx probe

The 980 m (3,220 ft) Apollo asteroid 162173 Ryugu is the target of JAXA's Hayabusa 2 mission. The space probe was launched in December 2014, is expected to arrive at the asteroid in June 2018, and to return a sample to Earth in December 2020.^[20] The 500 m (1,600 ft) Apollo asteroid 101955 Bennu, which, as of March 2018^[update], has the second-highest cumulative Palermo scale rating (−1.71 for several close encounters between 2175 and 2199),^[7] is the target of NASA's OSIRIS-REx probe. The New Frontiers program mission was launched in September 2016.^[21] On its two-year journey to Bennu, the probe is searching for Earth's Trojan asteroids,^[147] will rendezvous with Bennu in August 2018, and will return samples from the asteroid in September 2023.^[21]

In April 2012, the company Planetary Resources announced its plans to mine asteroids commercially. In a first phase, the company reviewed data and selected potential targets among NEAs. In a second phase, space probes would be sent to the selected NEAs; mining spacecraft would be sent in a third phase.^[148] Planetary Resources launched two testbed satellites in April 2015^[149] and January 2018,^[150] and the first prospecting satellite for the second phase is planned for a 2020 launch.^[149]

Missions to NECs

67P/Churyumov–Gerasimenko as seen by ESA's Rosetta probe

The first near-Earth comet visited by a space probe was 21P/Giacobini–Zinner in 1985, when the NASA/ESA probe International Cometary Explorer (ICE) passed through its coma. In March 1986, ICE, along with Soviet probes Vega 1 and Vega 2, ISAS probes Sakigake and Suisei and ESA probe Giotto flew by the nucleus of Halley's Comet. In 1992, Giotto also visited another NEC, 26P/Grigg–Skjellerup.^[14]

In November 2010, the NASA probe Deep Impact flew by the near-Earth comet 103P/Hartley. Earlier, in July 2005, this probe flew by the non-near-Earth comet Tempel 1, hitting it with a large copper mass.^[15]

In August 2014, ESA probe Rosetta began orbiting near-Earth comet 67P/Churyumov–Gerasimenko, while its lander Philae landed on its surface in November 2014. After the end of its mission, Rosetta was crashed into the comet's surface in 2016.^[16]

See also

References

External links

Wikimedia Commons has media related to Near-Earth objects.

• 
  Center for Near Earth Object Studies (CNEOS) – Jet Propulsion Laboratory, NASA
  


• 
  Table of Asteroids Next Closest Approaches to the Earth – Sormano Astronomical Observatory


• 
  Earth In The Cosmic Shooting – D.J. Asher, The Observatory, 2005


• 
  Catalogue of the Solar System Small Bodies Orbital Evolution – Samara State Technical University


• 
  Current Map Of The Solar System – Armagh Observatory
  

Minor Planet Center

• The NEO Confirmation Page


• 
  Minor Planet Center: Asteroid Hazards, Part 2: The Challenge of Detection on YouTube (min. 7:14)
  


• 
  Minor Planet Center: Asteroid Hazards, Part 3: Finding the Path on YouTube (min. 5:38)