In this 19th-century piece by Edward Burne-Jones, the human woman Psyche receives affection from the hybrid deity Pan.
A human–animal hybrid and animal–human hybrid is an organism that incorporates elements from both humans and non-human animals. Technically, in a human–animal hybrid,
each cell has both human and non-human genetic material. It is in
contrast to an individual where some cells are human and some are
derived from a different organism, called a human-animal chimera.
Examples of human–animal hybrids mainly include humanized mice that have been genetically modified by xenotransplantation of human genes. Humanized mice are commonly used as small animal models in biological and medical research for human therapeutics.
Human-animal hybrids are the subject of legal, moral, and technological debate in the context of recent advances in genetic engineering.
Defined by the magazine H+
as "genetic alterations that are blendings [sic] of animal and human
forms", such hybrids may be referred by other names occasionally such as
"para-humans".They may additionally may be called "humanized animals". Technically speaking, they are also related to "cybrids" (cytoplasmic hybrids), with "cybrid" cells featuring foreign human nuclei inside of them being a topic of interest. Possibly, a real-world human-animal hybrid may be an entity formed from either a human egg fertilized by a nonhuman sperm or a nonhuman egg fertilized by a human sperm.
Artificially created human-animal hybrids include humanized mice that have been xenotransplanted with human gene products, so as to be utilized for gaining relevant insights in the in vivo context for understanding of human-specific physiology and pathologies.
Humanized mice are commonly used as small animal models in biological
and medical research for human therapeutics including infectious
diseases and cancer. For example, genetically modified mice may be born with human leukocyte antigen genes in order to provide a more realistic environment when introducing human white blood cells into them in order to study immune system responses.
Advances in genetic engineering have generally caused a large number of debates and discussions in the fields related to bioethics,
including research relating to the creation of human-animal hybrids.
Although the two topics are not strictly related, the debates involving
the creation of human-animal hybrids have paralleled that of the debates
around the stem-cell research controversy.
The question of what line exists between a "human" being and a
"non-human" being has been a difficult one for many researchers to
answer. While animals having one percent or less of their cells
originally coming from humans may clearly appear to be in the same boat
as other animals, no consensus exists on how to think about beings in a
genetic middle ground that have something like an even mix. "I don't
think anyone knows in terms of crude percentages how to differentiate
between humans and nonhumans," U.S. patent office official John Doll has stated.
Critics of increased government restrictions include scientists such as
Dr. Douglas Kniss, head of the Laboratory of Perinatal Research at Ohio State University,
who has remarked that formal laws aren't the best option since the
"notion of animal-human hybrids is very complex." He's also argued that
their creation is inherent "not the kind of thing we support" in his
kind of research since scientists should "want to respect human life".
In contrast, notable socio-economic theorist Jeremy Rifkin
has expressed opposition to research that creates beings crossing
species boundaries, arguing that it interferes with the fundamental
'right to exist' possessed by each animal species. "One doesn't have to
be religious or into animal rights to think this doesn't make sense," he
has argued when expressing support for anti-chimera and anti-hybrid
legislation. As well, William Cheshire, associate professor of neurology
at the Mayo Clinic's
Florida branch, has called the issue "unexplored biologic territory"
and advocated for a "moral threshold of human neural development" to
restrict the destroying a human embryo to obtain cell material and/or
the creation of an organism that's partly human and partly animal." He
has said, "We must be cautious not to violate the integrity of humanity
or of animal life over which we have a stewardship responsibility".
Legality
While laws against the creation of hybrid beings have been proposed in U.S. states and in the U.S. Congress,
several scientists have argued that legal barriers might go too far and
prohibit medically beneficial studies into human modification.
In terms of scientific ethics,
restrictions on the creation of human–animal hybrids have proved a
controversial matter in multiple countries. While the state of Arizona banned the practice altogether in 2010, a proposal on the subject that sparked some interest in the United States Senate
from 2011 to 2012 ended up going nowhere. Although the two concepts are
not strictly related, discussions of experimentation into blended human
and animal creatures has paralleled the discussions around embryonic stem-cell research (the 'stem cell controversy'). The creation of genetically modified organisms
for a multitude of purposes has taken place in the modern world for
decades, examples being specifically designed foodstuffs made to have
features such as higher crop yields through better disease resistance.
President George W. Bush brought up the topic in his 2006 State of the Union Address,
in which he called for the prohibition of "human cloning in all its
forms", "creating or implanting embryos for experiments", "creating human-animal hybrids",
and also "buying, selling, or patenting human embryos". He argued, "A
hopeful society has institutions of science and medicine that do not cut
ethical corners and that recognize the matchless value of every life."
He also stated that humanity "should never be discarded, devalued or put
up for sale."
A 2005 appropriations bill passed by the U.S. Congress and signed
into law by President Bush contained specific wording forbidding any
patents on humans or human embryos. In terms of outright bans on hybrid research in the first place, a measure came up in the 110th Congress entitled the Human-Animal Hybrid Prohibition Act of 2008. Congressman Chris Smith (R, NJ-4)
introduced it on April 24, 2008. The text of the proposed act stated
that "human dignity and the integrity of the human species are
compromised" if such hybrids exist and set up the punishment of
imprisonment for up to ten years as well as a fine of over one million
dollars. Though attracting support from many co-sponsors such as then RepresentativesMary Fallin, Duncan Hunter, Joseph R. Pitts, and Rick Renzi among others, the Act failed to get through Congress.
A related proposal had come up in the U.S. Senate the prior year, the Human-Animal Hybrid Prohibition Act of 2007, and it also had failed. That effort was proposed by then-Senator Sam Brownback (R, KS)
on November 15, 2007. Featuring the same language as the later measure
in the House, its bipartisan group of cosponsors included then Senators Tom Coburn, Jim DeMint, and Mary Landrieu.
A localized measure designed to ban the creation of hybrid entities came up in the state of Arizona in 2010. The proposal was signed into law by then GovernorJan Brewer. Its sponsor stated that it was needed to clarify important "ethical boundaries" in research.
For thousands of years, these hybrids have been one of the most common themes in storytelling
about animals throughout the world. The lack of a strong divide between
humanity and animal nature in multiple traditional and ancient cultures
has provided the underlying historical context for the popularity of
tales where humans and animals have mingling relationships, such as in
which one turns into the other or in which some mixed being goes through
a journey. Interspecies friendships
within the animal kingdom, as well as between humans and their pets,
additionally provides an underlying root for the popularity of such
beings.
In various mythologies throughout history, many particularly
famous hybrids have existed, including as a part of Egyptian and Indian
spirituality. The entities have also been characters in fictional media more recently in history such as in H. G. Wells' work The Island of Doctor Moreau, adapted into the popular 1932 filmIsland of Lost Souls.
In legendary terms, the hybrids have played varying roles from that of
trickster and/or villain to serving as divine heroes in very different
contexts, depending on the given culture.
For example, Pan is a deity in Greek mythology
that rules over and symbolizes the untamed wild, being worshiped by
hunters, fishermen, and shepherds in particular. The mischievous yet
cheerful character is a Satyr
who has the hindquarters, legs, and horns of a goat while otherwise
being essentially human in appearance, with stories of his encounters
with different gods, humans, and others being retold for centuries on
after the days of early Greece by groups such as the Delphian Society. Specifically, the human-animal hybrid has appeared in acclaimed works of art by figures such as Francis Bacon. Additional famous mythological hybrids include the Egyptian god of death, named Anubis, and the fox-like Japanese beings that are called Kitsune.
Legendary historical and mythological human-animal hybrids
Beings displaying a mixture of human and animal traits while also
having a similarly blended appearance have played a vast and varied role
in multiple traditions around the world.
Artist and scholar Pietro Gaietto has written that "representations of
human-animal hybrids always have their origins in religion". In
"successive traditions they may change in meaning but they still remain
within spiritual culture", Gaietto has argued, when looking back in an evolution-minded point of view. The beings show up in both Greek and Roman mythology, with various elements of ancient Egyptian society ebbing and flowing into those cultures in particular. Prominent examples in ancient Egyptian religion, featuring some of the earliest such hybrid beings, include the canine-like god of death known as Anubis and the lion-like Sphinx. Other instances of these types of characters include figures within both Chinese and Japanese mythology. The observation of interspecies friendships
within the animal kingdom, as well as the bonds existing between humans
and their pets, have been a source of the appeal in such stories.
A prominent hybrid figure that's internationally known is the mythological Greek
figure of Pan. A deity that rules over and symbolizes the untamed wild,
he helps express the inherent beauty of the natural world as the Greeks
saw things. He specifically received reverence by ancient hunters,
fishermen, shepherds, and other groups with a close connection to
nature. Pan is a Satyr who possesses the hindquarters, legs, and horns
of a goat while otherwise being essentially human in appearance; stories
of his encounters with different gods, humans, and others have been a part of popular culture in several different cultures for many years. The human-animal hybrid has appeared in acclaimed works of art by figures such as Francis Bacon, also being mentioned in poetic pieces such as in John Fletcher's writings.
In Chinese mythology, the figure of Chu Pa-chieh
undergoes a personal journey in which he gives up wickedness for
virtue. After causing a disturbance in heaven from his licentious
actions, he is exiled to Earth. By mistake, he enters the womb of a sow
and ends up being born as a half-man/half-pig entity. With the head and
ears of a pig coupled with a human body, his already animal-like sense
of selfishness from his past life remains. Killing and eating his mother as well as devouring his brothers,
he makes his way to a mountain hideout, spending his days preying on
unwary travelers unlucky enough to cross his path. However, the
exhortations of the kind goddessKuan Yin,
journeying in China, persuade him to seek a nobler path, and his life's
journey and the side of goodness proceeds on such that he even is
ordained a priest by the goddess herself. Remarking on the character's role in the religious novel Journey to the West, where the being first appears, professor Victor H. Mair
has commented that "[p]ig-human hybrids represent descent and the
grotesque, a capitulation to the basest appetites" rather than
"self-improvement".
This image depicts a set of Tanuki statues on the side of a Japanese road.
Several hybrid entities have long played a major role in Japanese
media and in traditional beliefs within the country. For example, a
warrior god known as Amida received worship as a part of Japanese mythology
for many years; he possessed a generally humanoid appearance while
having a canine-like head. However, the god's devotional popularity fell
in about the middle of the 19th century. A Tanuki resembles a raccoon dog, but its shape-shifting talents allow it to turn into humans for the purposes of trickery, such as impersonating Buddhistmonks. The fox-like creatures known as Kitsune also possess similar powers, and stories abound of them tricking human men into marriage by turning into seductive women.
"Theriocephaly" (from Greek θηρίον therion 'beast' and κεφαλή kefalí 'head') is the anthropomorphic
condition or quality of having the head of an animal with a body either
mostly or entirely looking human – the term being commonly used to
refer the depiction of deities or otherwise specially able individuals. An entity with such qualities is said to be "theriomorphous". Many of the gods and goddesses worshipped by the ancient Egyptians,
for example, were commonly depicted as being theriocephalic. This
phenomenon partly represented an intermediate step in a longer process
of anthropomorphization of former animal deities. Fe. the goddess Hathor
in her earliest form was depicted as a cow and in her latest
manifestation as a woman with cows ears and sometimes a hairstyle
resembling cows horns. But the form of depiction sometimes depended also
on the aspects of a deity an artist wanted to accentuate (fe. Ba
the aspect of personality of a human soul was depicted as a bird with a
humans head). This can also be seen in the different hieroglyphes that
could be used to write the name of a single deity.
Other notable examples include:
Many prominent pieces of children's literature over the past two centuries have featured humanized animal characters, often as protagonists in the stores. In the opinion of popular educator Lucy Sprague Mitchell,
the appeal of such mythical and fantastic beings comes from how
children desire "direct" language "told in terms of images— visual,
auditory, tactile, muscle images". Another author has remarked that an
"animal costume" provides "a way to emphasize or even exaggerate a
particular characteristic".
The anthropomorphic characters in the seminal works by English writer Beatrix Potter
in particular live an ambiguous situation, having human dress yet
displaying many instinctive animal traits. Writing on the popularity of Peter Rabbit,
a later author commented that in "balancing humanized domesticity
against wild rabbit foraging, Potter subverted parental authority and
its built in hypocrisy" in Potter's child-centered books. Writer Lisa
Fraustino has cited on the subject R.M. Lockley's tongue-in-cheek observation: "Rabbits are so human. Or is it the other way around— humans are so rabbit?"
Writer H. G. Wells created his famous work The Island of Doctor Moreau, featuring a mixture of horror and science fiction elements, to promote the anti-vivisection cause as a part of his long-time advocacy for animal rights.
Wells' story describes a man stuck on an island ruled over by the
titular Dr. Moreau, a morally depraved scientist who has created several
human-animal hybrids referred to as 'Beast Folk' through vivisection
and even by combining parts of other animals for some of the 'Beast
Folk'. The story has been adapted into film several times, with varying
success. The most acclaimed version is the 1932 black-and-white treatment called Island of Lost Souls.
Wells himself wrote that "this story was the response of an imaginative
mind to the reminder that humanity is but animal rough-hewn to a
reasonable shape and in perpetual internal conflict between instinct and
injunction," with the scandals surrounding Oscar Wilde being the impetus for the English writer's treatment of themes such as ethics and psychology. Challenging the Victorian era viewpoints of its time, the 1896 work
presents a complex situation in which enhancing animals into hybrids
involves both terrifying violence and pain as well as appears
essentially futile, given the power of raw instinct. A pessimistic view
towards the ability of human civilization to live by law-abiding, moral standards for long thus follows.
The 1986 horror filmThe Fly features a deformed and monstrous human-animal hybrid, played by actor Jeff Goldblum. His character, scientist Seth Brundle, undergoes a teleportation experiment that goes awry and fuses him at a fundamental genetic level with a common fly
caught besides him. Brundle experiences drastic mutations as a result
that horrify him. Movie critic Gerardo Valero has written that the
famous horror work, "released at the dawn of the AIDS epidemic", "was seen by many as a metaphor for the disease" while also playing on bodily fears about dismemberment and coming apart that human beings inherently share.
The science fiction filmSplice, released 2009, shows scientists mixing together human and animal DNA
in the hopes of advancing medical research at the pharmaceutical
company that they work at. Calamitous results occur when the hybrid
named Dren is born.
The H. P. Lovecraft–inspired movie Dagon, released in 2001, additionally features grotesque hybrid beings. In terms of comic books, examples of fictional human-animal hybrids include the characters in Charles Burns' Black Hole series. In those comics, a set of teenagers in a 1970s era town become afflicted by a bizarre disease; the sexually transmitted affliction mutates them into monstrous forms.
Multiple video games have featured human-animal hybrids as enemies for the protagonist(s) to defeat, including powerful boss characters. For instance, the 2014 survival horror release The Evil Within includes grotesque hybrid beings, looking like the undead,
attacking main character Detective Sebastian Castellanos. With partners
Joseph Oda and Julie Kidman, the protagonist attempts investigate a
multiple homicide at a mental hospital yet discovers a mysterious figure who turns the world around them into a living nightmare, Castellanos having to find the truth about the criminal psychopath.
Heroic character examples of human-animal anthropomorphic characters include the two protagonists of the 2002 movieThe Cat Returns (Japanese title: 猫の恩返し), with the animated film featuring a young girl (named "Haru") being transformed against her will into a feline-human hybrid and fighting a villainous king of the cats with the help of a dashing male cat companion (known as the "Baron") at her side.
With general U.S. popular culture and its various subcultures, the furry fandom consists of individuals interested in a variety of artistic materials, this often featuring "furry art... [that] depicts a human-animal hybrid in everyday life". Specific people involved in creative media will frequently come up with a "fursona"
depicting a version or versions of themselves as a hybrid creature.
This practice functions as an outlet based on "on personal ideas of
self-expression" (self-realization).
The kemonomimi
art style, widely popularized since the latter part of the 20th
century, involves humanoid characters with stylized animal features,
such as this anthropomorphic mouse girl.
Many synthetic organic compounds such as plasticpolymers, and a few natural ones, contain halogen atoms; they are known as halogenated compounds or organohalogens.
Organochlorides are the most common industrially used organohalides,
although the other organohalides are used commonly in organic synthesis.
Except for extremely rare cases, organohalides are not produced
biologically, but many pharmaceuticals are organohalides. Notably, many
pharmaceuticals such as Prozac have trifluoromethyl groups.
For information on inorganic halide chemistry, see halide.
The halogenatoms in halocarbon molecules are often called "substituents," as though those atoms had been substituted for hydrogen atoms. However halocarbons are prepared in many ways that do not involve direct substitution of halogens for hydrogens.
History and context
A few halocarbons are produced in massive amounts by microorganisms. For example, several million tons of methyl bromide
are estimated to be produced by marine organisms annually. Most of the
halocarbons encountered in everyday life – solvents, medicines, plastics
– are man-made. The first synthesis of halocarbons was achieved in the
early 1800s. Production began accelerating when their useful properties
as solvents and anesthetics were discovered. Development of plastics and
synthetic elastomers has led to greatly expanded scale of production. A
substantial percentage of drugs are halocarbons.
Natural halocarbons
A large amount of the naturally occurring halocarbons, such as dioxine, are created by wood fire and volcanic activity. A third major source is marine algae, which produce several chlorinated methane and ethane
containing compounds. Several thousand complex halocarbons are known to
be produced mainly by marine species. Although chlorine compounds are
the majority of the discovered compounds, bromides, iodides and
fluorides have also been found in nature. Tyrian purple is a bromide and is produced by certain sea snails. Thyroxine is secreted by the thyroid gland and is an iodide. The highly toxic fluoroacetate is one of the rare natural organofluorides and is produced by certain plants.
Organoiodine compounds, including biological derivatives
Organoiodine compounds, called organic iodides, are similar in
structure to organochlorine and organobromine compounds, but the C-I
bond is weaker. Many organic iodides are known, but few are of major
industrial importance. Iodide compounds are mainly produced as
nutritional supplements.
The thyroxin hormones are essential for human health, hence the usefulness of iodized salt.
Six mg of iodide a day can be used to treat patients with hyperthyroidism due to its ability to inhibit the organification process in thyroid hormone synthesis, the so-called Wolff–Chaikoff effect. Prior to 1940, iodides were the predominant antithyroid agents. In large doses, iodides inhibit proteolysis of thyroglobulin, which permits TH to be synthesized and stored in colloid, but not released into the bloodstream. This mechanism is referred to as Plummer effect.
This treatment is seldom used today as a stand-alone therapy
despite the rapid improvement of patients immediately following
administration. The major disadvantage of iodide treatment lies in the
fact that excessive stores of TH accumulate, slowing the onset of action
of thioamides
(TH synthesis blockers). In addition, the functionality of iodides
fades after the initial treatment period. An "escape from block" is also
a concern, as extra stored TH may spike following discontinuation of
treatment.
Uses
The first halocarbon commercially used was Tyrian purple, a natural organobromide of the Murex brandaris marine snail.
Before they became strictly regulated, the general public often encountered haloalkanes as paint and cleaning solvents such as trichloroethane (1,1,1-trichloroethane) and carbon tetrachloride (tetrachloromethane), pesticides like 1,2-dibromoethane (EDB, ethylene dibromide), and refrigerants like Freon-22 (duPont trademark for chlorodifluoromethane). Some haloalkanes are still widely used for industrial cleaning, such as methylene chloride (dichloromethane), and as refrigerants, such as R-134a (1,1,1,2-tetrafluoroethane).
Haloalkenes have also been used as solvents, including perchloroethylene (Perc, tetrachloroethene), widespread in dry cleaning, and trichloroethylene (TCE, 1,1,2-trichloroethene). Other haloalkenes have been chemical building blocks of plastics such as polyvinyl chloride ("vinyl" or PVC, polymerized chloroethene) and Teflon (duPont trademark for polymerized tetrafluoroethene, PTFE).
A few halocarbons, including acid halides like acetyl chloride, are highly reactive;
these are rarely found outside chemical processing. The widespread uses
of halocarbons were often driven by observations that most of them were
more stable than other substances. They may be less affected by acids
or alkalis; they may not burn as readily; they may not be attacked by bacteria or molds; or they may not be affected as much by sun exposure.
Since the 1970s there have been longstanding, unresolved controversies over potential health hazards of trichloroethylene (TCE) and other halocarbon solvents that had been widely used for industrial cleaning (Anderson v. Grace 1986) (Scott & Cogliano 2000) (U.S. National Academies of Science 2004) (United States 2004). More recently perfluorooctanoic acid
(PFOA), a precursor in the most common manufacturing process for Teflon
and also used to make coatings for fabrics and food packaging, became a
health and environmental concern starting in 2006 (United States 2010), suggesting that halocarbons, though thought to be among the most inert, may also present hazards.
Halocarbons, including those that might not be hazards in themselves, can present waste disposal issues. Because they do not readily degrade in natural environments, halocarbons tend to accumulate. Incineration and accidental fires can create corrosive byproducts such as hydrochloric acid and hydrofluoric acid, and poisons like halogenated dioxins and furans. Species of Desulfitobacterium are being investigated for their potential in the bioremediation of halogenic organic compounds.
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into 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). If a NEO's orbit crosses the Earth's orbit, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction are comets.
There are over 32,000 known near-Earth asteroids (NEAs) and over 120 known short-period near-Earth comets (NECs). 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 20 metres (66 ft) 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 potential danger. 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 are 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 1 kilometre
(3,300 ft) 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.As of January 2019, five near-Earth comets and five near-Earth asteroids have been visited by spacecraft. A small sample of one NEO was returned to Earth in 2010, and similar missions are in progress.Preliminary plans for commercial asteroid mining
have been drafted by private startup companies, either through the use
of robots or even by sending private commercial astronauts to act as
space miners.
Definitions
Plot of orbits of known potentially hazardous asteroids (size over 140 m (460 ft) and passing within 7.6×106 km (4.7×106 mi) of Earth's orbit) as of early 2013 (alternate image)
Near-Earth objects (NEOs) are by convention technically defined as
all small Solar System bodies with orbits around the Sun that lie partly
between 0.983 and 1.3 astronomical units (AU; Sun–Earth distance) away from the Sun.
NEOs are thus not necessarily currently near the Earth, but they can
potentially approach the Earth relatively closely. The term is also
sometimes used more flexibly, for example for objects in orbit around
the Earth or for quasi-satellites, which have a more complex orbital relationship with the Earth.
When a NEO is detected, like all other small Solar System bodies, its positions and brightness are submitted to the International Astronomical Union's (IAU's) Minor Planet Center (MPC) for cataloging. The MPC maintains separate lists of confirmed NEOs and potential NEOs. The orbits of some NEOs intersect that of the Earth, so they pose a collision danger. These are considered potentially hazardous objects
(PHOs) if their estimated diameter is above 140 meters. The MPC
maintains a separate list for the asteroids among PHOs, the potentially
hazardous asteroids (PHAs). 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) and the Solar System Dynamics Group.
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 (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 either cannot approach closer to the Earth i.e. MOID
greater than 0.05 AU (7,500,000 km; 4,600,000 mi), or which are fainter than H = 22.0 (about 140 m (460 ft) in diameter with assumed albedo of 14%), are not considered PHAs. NASA's catalog of near-Earth objects includes the approach distances of asteroids and comets (expressed in lunar distances).
History of human awareness of NEOs
1910 drawing of the path of Halley's CometThe near Earth asteroid 433 Eros was visited by a probe in the 1990s
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. It has been said that Lexell's comet of 1770 was the first discovered Near-Earth object.
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.
In 1937, asteroid 69230 Hermes was discovered when it passed the Earth at twice the distance of the Moon.
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. Hermes was only re-discovered in 2003, and it is now known to be no threat for at least the next century.
On June 14, 1968, the 1.4 km diameter asteroid 1566 Icarus passed Earth at a distance of 0.042 AU (6,300,000 km), or 16 times the distance of the Moon. During this approach, Icarus became the first minor planet to be observed using radar, with measurements obtained at the Haystack Observatory and the Goldstone Tracking Station.
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.
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. 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.
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.
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.
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. Thus, NEOs have been seen as omens of natural disasters or wars; harmless spectacles in an unchanging universe; the source of era-changing cataclysms or potentially poisonous fumes (during Earth's passage through the tail of Halley's Comet in 1910); and finally as a possible cause of a crater-forming impact that could even cause extinction of humans and other life on Earth.
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. 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.
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 the non-avian dinosaurs died out) 65 million years ago was caused by a large asteroid impact.
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. 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.
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; 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.
On both scales, risks of any concern are indicated by values above zero.
Magnitude of risk
The annual background frequency used in the Palermo scale for impacts of energy greater than Emegatonnes is estimated as:
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 published in 2002 – the same year as the paper on that the Palermo scale is based – found a power law with different constants:
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.
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.
In March 2002, (163132) 2002 CU11
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, 2002 CU11 will pass the Earth at a safe closest distance (perigee) of 0.00425 AU (636,000 km; 395,000 mi) on August 31, 2080.
Asteroid 1950 DA
was lost after its 1950 discovery, since its observations over just 17
days were insufficient to precisely determine its orbit; it was
rediscovered on December 31, 2000. It has a diameter of about a
kilometer (0.6 miles), and an impact would therefore be globally
catastrophic. It was observed by radar during its close 2001 approach,
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 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.
Uncertainties in the orbit calculations were further reduced using
additional radar observations in 2012, and this decreased the odds of an
impact.
Taking all radar and optical observations through 2021 into account,
the probability of impact in March 2880 is, as of June 2022, assessed at 1 in 34,000. The corresponding Palermo scale value of −2.05 is still among the highest for all objects on the Sentry List Table.
On December 24, 2004, 370 m (1,210 ft) asteroid 99942 Apophis (at the time known only by its provisional designation 2004 MN4)
was assigned a 4 on the Torino scale, the highest rating given to date,
as the information available at the time translated to a 2.7% chance of
Earth impact on Friday, April 13, 2029. By December 28, 2004,
additional observations had significantly reduced the uncertainty zone
for the 2029 approach and it no longer included the Earth. The 2029 risk
of impact consequently dropped to zero, but later potential impact
dates were still rated 1 on the Torino scale. Further observations
lowered the 2036 risk to a Torino rating of 0 in August 2006. In 2021
Apophis was removed from the Sentry Risk Table.
In February 2006, (144898) 2004 VD17 was assigned a Torino Scale rating of 2 due to a close encounter predicted for May 4, 2102.
After additional observations allowed increasingly precise predictions,
the Torino rating was lowered first to 1 in May 2006, then to 0 in
October 2006, and the asteroid was removed from the Sentry Risk Table
entirely in February 2008.
As of 2021, 2010 RF12
is listed with the highest chance of impacting Earth, at 1 in 22 on
September 5, 2095. At only 7 m (23 ft) across, the asteroid however is
much too small to be considered a potentially hazardous asteroid and it poses no serious threat: the possible 2095 impact therefore rates only −3.32 on the Palermo Scale.
Observations during the August 2022 close approach are expected to
ascertain whether the asteroid will impact or miss Earth in 2095.
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.
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. Plans for a more comprehensive survey, named the Spaceguard Survey, were developed by NASA from 1992, under a mandate from the United States Congress. To promote the survey on an international level, the International Astronomical Union (IAU) organised a workshop at Vulcano, Italy in 1995, and set up the Spaceguard Foundation also in Italy a year later. In 1998, the United States Congress
gave NASA a mandate to detect 90% of near-earth asteroids over 1 km
(0.62 mi) diameter (that threaten global devastation) by 2008.
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. As of January 2020,
it is estimated that less than half of these have been found, but
objects of this size hit the earth only about once in 2000 years. 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.
Survey programs aim to identify threats years in advance, giving humanity time to prepare a space mission to avert the threat.
REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ...
DR. A'HEARN: No. If we had spacecraft plans on the books already, that
would take a year ... I mean a typical small mission ... takes four
years from approval to start to launch ...
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
Cumulative discoveries of near-Earth asteroids known by size, 1980–2023
Near-Earth objects are classified as meteoroids, asteroids, or comets depending on size, composition, and orbit. Those which are asteroids can additionally be members of an asteroid family, and comets create meteoroid streams that can generate meteor showers.
As of October 1, 2022 and according to statistics maintained by
CNEOS, 33,078 NEOs have been discovered. Only 121 (0.37%) of them are
comets, whilst 32,957 (99.63%) are asteroids. 2,366 of those NEOs are
classified as potentially hazardous asteroids (PHAs).
As of November 2021, over 1,200 NEAs appear on the Sentry impact risk page at the NASA website.
Over 1,000 of these NEAs are less 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 the general public.
Observational biases
The
main problem with estimating the number of NEOs is that the probability
of detecting one is influenced by a number of aspects of the NEO,
starting naturally with its size but also including the characteristics
of its orbit and the reflectivity of its surface. What is easily detected will be more counted,
and these observational biases need to be compensated when trying to calculate the number of bodies in a population from the list of its detected members.
Artist's impression of an asteroid that orbits closer to the Sun than Earth's orbit
Bigger asteroids reflect more light, and the two biggest Near-Earth objects, 433 Eros and 1036 Ganymed, were naturally also among the first to be detected. 1036 Ganymed is about 35 km (22 mi) in diameter and 433 Eros is about 17 km (11 mi) in diameter.
The other major detection bias is that it is much easier to spot
objects on the night-side of Earth. The day sky near the Sun is much
brighter than the night sky, and there is therefore much better contrast
in the night sky.
The night-side searcher is also looking at the sunlit side of the
asteroids, while in the daytime sky a searcher looks towards the sun and
sees the unlit backside of the object. In addition, opposition surge
makes asteroids even brighter when the Earth is close to the axis of
sunlight. The combined effect is equivalent to the comparison of a Full moon at night to a New Moon in daytime, and the light of the Sun-lit asteroids has been called "full asteroid" similar to a "full moon".
Evidencing this bias and as depicted in the diagram below, over half
(53%) of the known Near Earth objects were discovered in just 3.8% of
the sky, in a 22.5° cone facing directly away from the Sun, and the vast majority (87%) were first found in only 15% of the sky, in the 45° cone facing away from the Sun. The most practical way around this opposition bias is to use thermal infrared
telescopes in space that observe their heat emissions instead of the
light they reflect, with a sensitivity that is almost independent of the
illumination.
Asteroids with orbits that make them spend more time on the
day-side of the Earth are therefore less likely to be discovered than
those that spend most of their time beyond the orbit of the Earth. For
example, one study noted that detection of bodies in low-eccentricity
Earth-crossing orbits is favored, making Atens more likely to be detected than Apollos.
Such observational biases must be identified and quantified to
determine NEO populations, as studies of asteroid populations then take
those known observational selection biases into account to make a more
accurate assessment.
In the year 2000 and taking into account all known observational
biases, it was estimated that there are approximately 900 near-Earth
asteroids of at least kilometer size, or technically and more
accurately, with an absolute magnitude brighter than 17.75.
These are asteroids in a near-Earth orbit without the tail or coma of a comet. As of October 2023, 32,957 near-Earth asteroids
are known, 2,366 of which are both sufficiently large and may come
sufficiently close to Earth to be classified as potentially hazardous.
NEAs survive in their orbits for just a few million years. They are eventually eliminated by planetary perturbations, causing ejection from the Solar System or a collision with the Sun, a planet, or other celestial body.
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. 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.
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. The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.
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.
Many asteroids have natural satellites (minor-planet moons). As of October 2021, 85 NEAs were known to have at least one moon, including three known to have two moons. The asteroid 3122 Florence, one of the largest PHAs
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.
In May 2022, an algorithm known as Tracklet-less Heliocentric
Orbit Recovery or THOR and developed by University of Washington
researchers to discover asteroids in the solar system was announced as a
success.
The International Astronomical Union's Minor Planet Center confirmed a
series of first candidate asteroids identified by the algorithm.
Size distribution
Known near-Earth asteroids by size
While the size of a very small fraction of these asteroids is known to better than 1%, from radar observations, from images of the asteroid surface, or from stellar occultations,
the diameter of the vast majority of near Earth asteroids has only been
estimated on the basis of their brightness and a representative
asteroid surface reflectivity or albedo, which is commonly assumed to be 14%.
Such indirect size estimates are uncertain by over a factor of 2 for
individual asteroids, since asteroid albedos can range at least as low
as 5% and as high as 30%. This makes the volume of those asteroids
uncertain by a factor of 8, and their mass by at least as much, since
their assumed density also has its own uncertainty. Using this crude
method, an absolute magnitude of 17.75 roughly corresponds to a diameter of 1 km (0.62 mi) and an absolute magnitude of 22.0 to a diameter of 140 m (460 ft).
Diameters of intermediate precision, better than from an assumed
albedo but not nearly as precise as good direct measurements, can be
obtained from the combination of reflected light and thermal infrared
emission, using a thermal model of the asteroid to estimate both its
diameter and its albedo. In May 2016, technologist Nathan Myhrvold questioned the precision of such asteroid diameter estimates arising from thermal modeling of measurements by the Wide-field Infrared Survey Explorer and NEOWISE missions. The original version of his criticism itself faced criticism for its methodology and did not pass peer review, but a revised version was subsequently published.
In 2000, NASA reduced from 1,000–2,000 to 500–1,000 its estimate
of the number of existing near-Earth asteroids over one kilometer in
diameter, or more exactly brighter than an absolute magnitude of 17.75.Shortly thereafter, the LINEAR survey provided an alternative estimate of 1,227+170 −90. In 2011, on the basis of NEOWISE observations, the estimated number of one-kilometer NEAs was narrowed to 981±19 (of which 93% had been discovered at the time), while the number of NEAs larger than 140 meters across was estimated at 13,200±1,900.
The NEOWISE estimate differed from other estimates primarily 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 from the same inputs but assuming a slightly higher
albedo.
In 2017, two studies using an improved statistical method reduced the
estimated number of NEAs brighter than absolute magnitude 17.75
(approximately over one kilometer in diameter) slightly to 921±20.
The estimated number of near-Earth asteroids brighter than absolute
magnitude of 22.0 (approximately over 140 m across) rose to 27,100±2,200, double the WISE estimate, of which about a third were 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 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 400±100 million—of which about 0.003 percent had been discovered by February 2016.
As of October 1, 2023, and using diameters mostly estimated
crudely from a measured absolute magnitude and an assumed albedo, 854
NEAs listed by CNEOS, including 152 PHAs, measure at least 1 km in
diameter, and 10,610 known NEAs, including 2,366 PHAs, are larger than
140 m in diameter.
The smallest known near-Earth asteroid is 2008 TS26 with an absolute magnitude of 33.2, corresponding to an estimated diameter of about 1 m (3.3 ft). The largest such object is 1036 Ganymed,
with an absolute magnitude of 9.45 and directly measured irregular
dimensions which are equivalent to a diameter of about 38 km (24 mi).
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):
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, which 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. (0.983 AU is Earth's perihelion distance.)
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. That is, they consider the Atiras to be part of the Atens. 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.
As of October 1, 2023, 32 Atiras, 2,590 Atens, 18,550 Apollos and 11,785 Amors have been discovered and cataloged.
Co-orbital asteroids
The five Lagrangian points relative to Earth and possible orbits along gravitational contoursOverview of the Inner Solar System with different co-orbital bodies.
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 lightly perturbed by other planets and by non-gravitational forces.
As of March 2018, Earth's only confirmed Trojan is 2010 TK7, circling Earth's L4 point.
Horseshoe librators:
The region of stability around L4 and L5 also includes orbits for
co-orbital asteroids that run around both L4 and L5. Relative to the
Earth and Sun, 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. 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 770–780 years. (419624) 2010 SO16 is an asteroid on a relatively stable circumference-of-a-horseshoe orbit, with a horseshoe libration period of about 350 years.
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 Kamoʻoalewa is Earth's closest quasi-satellite, in an orbit that has been stable for almost a century.
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. 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 sometime around year 2066.
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 typically 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. As of November 2021, the only observed transitions were those of asteroids 2006 RH120 and 2020 CD3, which were temporary satellites of Earth for at least a year since their capture dates.
Meteoroids
In 1961, the IAU defined meteoroids as a class of solid interplanetary objects distinct from asteroids by their considerably smaller size. 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 then observable by
telescopes.
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.
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.
Near-Earth comets
Halley's Comet during its 0.10 AU 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 a comet nucleus is slightly larger than that of a
similar-sized asteroid.
NECs may pose an additional hazard due to fragmentation: the meteoroid
streams which produce meteor showers may include large inactive
fragments, effectively NEAs. 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.
Comets are commonly divided between short-period and long-period
comets. Short-period comets, with an orbital period of less than 200
years, originate 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.
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.
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
have consistently focused on asteroids and short-period comets.CNEOS even restricts its definition of NECs to short-period comets—as of October 1, 2023, 121 such objects have been discovered.
As of November 2021, only 23 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. Two of these comets, Halley's Comet and 73P/Schwassmann–Wachmann, have been observed during multiple close approaches. The closest observed approach was 0.0151 AU (5.88 LD) 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 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. This comet is the parent body of the Leonid meteor shower, which also produced the Great Meteor Storm of 1833. 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, passed Earth undetected at a distance of 0.0121 AU (4.70 LD) on June 12, 1999.
Comet 109P/Swift–Tuttle, which is also the source of the Perseid meteor shower
every year in August, has a roughly 130-year orbit that passes close to
the Earth. During the comet's September 1992 recovery, when only the
two previous returns in 1862 and 1737 had been identified, calculations
showed that the comet would pass 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
longer observation arc eliminated the impact risk. The comet will pass
Earth in 2126 at a distance of 23 million kilometers. In 3044, the comet
is expected to pass Earth at less than 1.6 million kilometers.
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. In 2006, two more apparent temporary satellites were discovered which were suspected of being artificial. One of them was eventually confirmed as an asteroid and classified as the temporary satellite 2006 RH120. The other, 6Q0B44E, was confirmed as an artificial object, but its identity is unknown. Another temporary satellite was discovered in 2013, and was designated 2013 QW1 as a suspected asteroid. It was later found to be an artificial object of unknown origin. 2013 QW1 is no longer listed as an asteroid by the Minor Planet Center.
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. The designation 2015 HP116 was similarly removed from asteroid catalogues when the observed object was identified with Gaia, ESA's space observatory for astrometry.
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 and only small amounts of meteorites arriving to the Earth surface, while larger objects 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 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. Some other models estimate similar impact frequencies, while others calculate higher frequencies.
For Tunguska-sized (10 megaton) impacts, the estimates range from one
event every 2,000–3,000 years to one event every 300 years.
Location and impact energy of small asteroids impacting Earth's atmosphere
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, which was detected only by infrasound sensors. However this may not have been a meteor. The third-largest, but by far best-observed impact, was the Chelyabinsk meteor
of 15 February 2013. A previously unknown 20 m (66 ft) 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 2011 EO40, making the latter the meteor's possible parent body.
On 7 October 2008, 19 hours after it was 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 was predicted prior to its entry into the atmosphere as a meteor. 10.7 kg of meteorites were recovered after the impact.
On 2 January 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.
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. 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. As of March 2018,
the largest observed lunar impact occurred on 11 September 2013, lasted
8 seconds, and was likely caused by an object 0.6–1.4 m (2.0–4.6 ft) in
diameter.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Diagram showing spacecraft and asteroids (past and future) between the Earth and the Moon.
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. This makes them an attractive target for exploration.
Missions to NEAs
433 Eros as seen by NASA's NEAR probeImage mosaic of asteroid 101955 Bennu, target of NASA's OSIRIS-REx 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. 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 an NEA would cost very little energy,
and thus space probes could gather multiple samples. 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
crewed mission within ten years worthwhile.
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. 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, 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.
The 980 m (3,220 ft) Apollo asteroid 162173 Ryugu is the target of JAXA's Hayabusa2
mission. The space probe was launched in December 2014, arrived at the
asteroid in June 2018, and returned a sample to Earth in December 2020. The 500 m (1,600 ft) Apollo asteroid 101955 Bennu, which, as of November 2021, has the highest cumulative Palermo scale rating (−1.41 for several close encounters between 2178 and 2290), is the target of NASA's OSIRIS-REx probe. The New Frontiers program mission was launched in September 2016. On its two-year journey to Bennu, the probe had searched for Earth's Trojan asteroids, rendezvoused with Bennu in August 2018, and had entered into orbit around the asteroid in December 2018. OSIRIS-REx will return samples from the asteroid in September 2023.
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. Planetary Resources launched two testbed satellites in April 2015 and January 2018,
and the first prospecting satellite for the second phase was planned
for a 2020 launch prior to the company closing and its assets purchased
by ConsenSys Space in 2018.
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.
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.
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