Near-Earth object - AbsoluteAstronomy.com

A near-Earth object is a Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

. They include a few thousand near-Earth asteroids (NEAs), near-Earth comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, a number of solar-orbiting spacecraft, and meteoroid

Meteoroid

A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...

s 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 planet. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose to the Earth, and active mitigations are being researched. A study showed that the United States and China are the nations most vulnerable to a meteor strike.

Those NEOs that are asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s (NEA) have orbits that lie partly between 0.983 and 1.3 astronomical unit

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s away from the Sun. When an NEA is detected it is submitted to the Harvard Minor Planet Center

Minor Planet Center

The Minor Planet Center operates at the Smithsonian Astrophysical Observatory , which is part of the Center for Astrophysics along with the Harvard College Observatory ....

for cataloging. Some near-Earth asteroids' orbits intersect that of Earth's so they pose a collision danger. The United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, European Union

European Union

The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

and other nations are currently scanning for NEOs in an effort called Spaceguard

Spaceguard

The term Spaceguard loosely refers to a number of efforts to discover and study near-Earth objects . Asteroids are discovered by telescopes which repeatedly survey large areas of sky. Efforts which concentrate on discovering NEOs are considered part of the "Spaceguard Survey," regardless of which...

.

In the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

has a congressional

United States Congress

The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....

mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be expected to produce severe to catastrophic effects. , 982 of these mandated NEOs have been detected. It was estimated in 2006 that 20% of the mandated objects have not yet been found. Efforts are under way to use an existing telescope in Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

to cover the ~30% of the sky that has not yet been surveyed.

Potentially hazardous object

Potentially Hazardous Object

A potentially hazardous object is an asteroid or comet with an orbit such that it has the potential to make close approaches to the Earth and a size large enough to cause significant regional damage in the event of impact....

s (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth. Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU, or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

of 13%), are not considered PHOs. The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in Lunar Distances

Lunar distance (astronomy)

In astronomy, a lunar distance is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,400 kilometers...

, and this usage has become the more usual unit of measure used by the press and mainstream media in discussing these objects.

Some NEOs are of high interest because they can be physically explored with lower mission velocity even than the Moon, due to their combination of low velocity with respect to Earth (ΔV

Delta-v

In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

) and small gravity, so 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. As of 2008, two near-Earth objects have been visited by spacecraft: 433 Eros

433 Eros

433 Eros is a near-Earth asteroid discovered in 1898, and the first asteroid to be orbited by a probe . It is an S-type asteroid approximately 34.4×11.2×11.2 km in size, the second-largest NEA after 1036 Ganymed, and belongs to the Amor group.Eros is a Mars-crosser asteroid, the first known...

, by NASA

NASA

's Near Earth Asteroid Rendezvous probe, and 25143 Itokawa

25143 Itokawa

25143 Itokawa is an Apollo and Mars-crosser asteroid. It was the first asteroid to be the target of a sample return mission, the Japanese space probe Hayabusa.-Discovery and naming:...

, by the JAXA Hayabusa

Hayabusa

was an unmanned spacecraft developed by the Japan Aerospace Exploration Agency to return a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis....

mission.

Risk scales

There are two schemes for classification of impact hazards:

the simple Torino Scale
Torino Scale
The Torino Scale is a method for categorizing the impact hazard associated with near-Earth objects such as asteroids and comets.It is intended as a communication tool for astronomers and the public to assess the seriousness of collision predictions, by combining probability statistics and known...

, and
the more complex Palermo Technical Impact Hazard Scale
Palermo Technical Impact Hazard Scale
The Palermo Technical Impact Hazard Scale is a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-earth object . It combines two types of data—probability of impact, and estimated kinetic yield—into a single "hazard" value...

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

For instance, this formula implies that the expected value

Expected value

In probability theory, the expected value of a random variable is the weighted average of all possible values that this random variable can take on...

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 which 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

On 25 December 2004, minor planet 2004 MN₄ (later named 99942 Apophis

99942 Apophis

99942 Apophis is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability that it would strike the Earth in 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on...

) was assigned a 4 on the Torino scale, the highest rating so far. On 27 December 2004, there was a 2.7% chance of Earth impact on 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but, due to a resonant return possibility the Torino rating for an April 2036 impact rose to 4 in early 2005, but by October 2009 the Torino rating was 0 (zero). The Palermo rating (October 2009) is −3.08.

As of 12 September 2011, the only known NEO with a Palermo scale value greater than zero is (29075) 1950 DA

(29075) 1950 DA

-External links:* from JPL /...

, which is predicted to pass very close to or collide with the Earth (p ≤ 0.003) in the year 2880. Depending on the orientation of its axis of rotation, it will either miss the Earth by tens of millions of kilometers, or have a 1 in 300 chance of hitting the Earth. However, humanity has over 800 years to refine its estimates of the orbit of (29075) 1950 DA, and to deflect it, if necessary.

List of current threats

NASA maintains a continuously updated web page of the most significant NEO threats in the next 100 years. All or nearly all of the items on this page are highly likely to drop off the list eventually as more data comes in, enabling more accurate predictions. (The page does not include 1950 DA

(29075) 1950 DA

-External links:* from JPL /...

, because that will not strike for at least 800 years.)

Number and classification of near-Earth objects

While orbiting the Sun, most potential impactors can be classified as meteoroid

Meteoroid

s, asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, or comet

Comet

s depending on size and composition. Asteroids can also be members of an asteroid family

Asteroid family

An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions...

, and comets can leave debris in their orbits.

, 7,954 NEOs have been discovered: 87 near-Earth comets and 7,867 near-Earth Asteroids. Of those there are 647 Aten asteroid

Aten asteroid

The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...

s, 2,920 Amor asteroid

Amor asteroid

The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...

s, and 4,289 Apollo asteroid

Apollo asteroid

The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...

s. There are 1,215 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 148 PHAs and 824 NEAs have an absolute magnitude

Absolute magnitude

Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

of 17.75 or brighter, which roughly corresponds to at least 1 km in size.

, there are 368 NEAs on the impact risk page at the NASA

NASA

website. A significant number of these NEAs – 215 – are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "yellow zone" (Torino Scale

Torino Scale

The Torino Scale is a method for categorizing the impact hazard associated with near-Earth objects such as asteroids and comets.It is intended as a communication tool for astronomers and the public to assess the seriousness of collision predictions, by combining probability statistics and known...

2), meaning that none warrant the attention of general public. , only asteroids and are listed as having a Torino Scale of 1.

Near-Earth meteoroids

Near-Earth meteoroids are objects with orbits in the vicinity of Earth's orbit having a diameter less than 50 meters.

Near-Earth asteroids

These are objects of 50 meters or more in diameter in a near-Earth orbit without the tail or coma of a comet. , 7,075 near-Earth asteroids are known, ranging in size up to ~32 kilometers (1036 Ganymed

1036 Ganymed

1036 Ganymed is the largest Amor asteroid, at about 32 km in diameter. It was discovered by Walter Baade on October 23, 1924, and is named after Ganymede, the Trojan prince turned god whom Zeus designated the cupbearer to the Greek gods...

). The number of near-Earth asteroids over one kilometer in diameter is estimated to be 500–1,000. The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types

Asteroid spectral types

Asteroids are assigned a type based on spectral shape, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition...

.

NEAs survive in their orbits for just a few million years. They are eventually eliminated by planetary perturbations

Perturbation (astronomy)

Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

which cause ejection from the Solar System or a collision with the Sun or a planet. 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 asteroid-belt asteroids

Asteroid belt

The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

are moved into the inner Solar System through orbital resonance

Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

s with Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. The interaction with Jupiter through the resonance perturb

Perturbation (astronomy)

s the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as Kirkwood gap

Kirkwood gap

A Kirkwood gap is a gap or dip in the distribution of main-belt asteroids with semi-major axis , as seen in the histogram below...

s, 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

Yarkovsky effect

The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum...

that provides a continuing supply of near-Earth asteroids.

A small number of NEOs 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

Jupiter

.
There are three families of near-Earth asteroids:

The Atens
Aten asteroid
The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...

, which have average orbital radii less than one AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

and aphelia
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

of more than Earth's perihelion
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

(0.983 AU), placing them usually inside the orbit of Earth.
The Apollos, which have average orbital radii more than that of the Earth and perihelia less than Earth's aphelion (1.017 AU).
The Amors
Amor asteroid
The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...

, which have average orbital radii in between the orbits of Earth and Mars and perihelia slightly outside Earth's orbit (1.017–1.3 AU). Amors often cross the orbit of Mars, but they do not cross the orbit of Earth.

Many Atens and all Apollos have orbits that cross (though not necessarily intersect) that of the Earth, so they are a threat to impact the Earth on their current orbits. Amors do not cross the Earth's orbit and are not immediate impact threats. However, their orbits may evolve into Earth-crossing orbits in the future.

Also sometimes used is the Arjuna asteroid

Arjuna asteroid

The Arjuna asteroids are a class of near-Earth asteroids whose orbits are very Earth-like in character, having low inclination, orbital periods close to one Earth year, and low eccentricity. The class is named after Arjuna, a central hero in Hindu mythology. The definition is somewhat fuzzy and...

classification, for asteroids with extremely Earth-like orbits.

Near-Earth comets

, 84 near-Earth comets have been discovered. Although no impact of a comet in Earth´s history has been conclusively confirmed, the Tunguska event

Tunguska event

The Tunguska event, or Tunguska blast or Tunguska explosion, was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 7:14 a.m...

may have been caused by a fragment of Comet Encke

Comet Encke

Comet Encke or Encke's Comet is a periodic comet that completes an orbit of the Sun once every three years — the shortest period of any known comet...

. Cometary fragmenting may also be responsible for some impacts from near-Earth objects.

These near-Earth objects were probably derived from the Kuiper belt

Kuiper belt

The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

, beyond the orbit of Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

Impact rate

Objects with diameters of 5-10 m impact the Earth's atmosphere approximately once per year, with as much energy as the atomic bomb dropped on Hiroshima

Little Boy

"Little Boy" was the codename of the atomic bomb dropped on Hiroshima on August 6, 1945 by the Boeing B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets of the 393rd Bombardment Squadron, Heavy, of the United States Army Air Forces. It was the first atomic bomb to be used as a weapon...

, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere

Mesosphere

The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

, and most or all of the solids are vaporized

Vaporization

Vaporization of an element or compound is a phase transition from the liquid or solid phase to gas phase. There are three types of vaporization: evaporation, boiling and sublimation....

. Every 2000–3000 years NEAs produce explosions comparable to the one observed at Tunguska

Tunguska event

in 1908. Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval. Large collisions with five kilometer objects happen approximately once every ten million years.

The rate of impacts of objects of at least 1 km in diameter is estimated as 2 per million years. Assuming that this rate will continue for the next billion years, there exist at least 2,000 objects of diameter greater than 1 km that will eventually hit Earth. However, most of these are not yet considered potentially hazardous objects because they are currently orbiting between Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

and Jupiter

Jupiter

. Eventually they will change orbits and become NEOs. Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or (for a small proportion) hitting a planet.

Historic impacts

The general acceptance of the Alvarez hypothesis

Alvarez hypothesis

The Alvarez hypothesis claims that the mass extinction of the dinosaurs and many other living things was caused by the impact of a large asteroid on the Earth sixty-five million years ago, called the Cretaceous-Tertiary extinction event. Evidence indicates that the asteroid fell in the Yucatán...

, explaining the Cretaceous–Tertiary extinction event as the result of a large object impact event

Impact event

An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

, raised the awareness

Awareness

Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

of the possibility of future Earth impacts with other objects that cross the Earth's orbit.

1908 Tunguska event

It is now commonly believed that on 30 June 1908 a stony asteroid exploded over Tunguska with the energy of the explosion of 10 megatons of TNT. The explosion occurred at a height of 8.5 kilometers. The object that caused the explosion has been estimated to have had a diameter of 45–70 meters.

1979 Vela Incident

A 22 September 1979 event recorded as occurring near the junction of the South Atlantic and the Indian Ocean was possibly a low-yield nuclear test, but was also initially thought to have been caused by the possible impact of an extraterrestrial object. The event, which became known as the Vela Incident

Vela Incident

The Vela Incident was an unidentified "double flash" of light that was detected by an American Vela Hotel satellite on September 22, 1979....

, was identified by a U.S. Vela

Vela (satellite)

Vela was the name of a group of satellites developed as the Vela Hotel element of Project Vela by the United States to monitor compliance with the 1963 Partial Test Ban Treaty by the Soviet Union, and other nuclear-capable states. It means vigil or "watch" in Spanish.Vela started out as a small...

defence satellite in Earth orbit. The event alarm triggered multi-year investigations by several organizations which could not conclusively determine if the explosion was of nuclear or non-nuclear origin.

2002 Eastern Mediterranean event

On 6 June 2002 an object with an estimated diameter of 10 meters collided with Earth. The collision occurred over the Mediterranean Sea

Mediterranean Sea

The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

, between Greece

Greece

Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

and Libya

Libya

Libya is an African country in the Maghreb region of North Africa bordered by the Mediterranean Sea to the north, Egypt to the east, Sudan to the southeast, Chad and Niger to the south, and Algeria and Tunisia to the west....

, at approximately 34°N 21°E and the object exploded in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a small nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

2008 Sudan event

On 6 October 2008, scientists calculated that a small near-Earth asteroid, , just sighted that night, would impact the Earth on 7 October over Sudan, at 0246 UTC, 5:46 local time. The asteroid arrived as predicted. This is the first time that an asteroid impact on Earth has been accurately predicted. However, no reports of the actual impact have so far been published since it occurred in a very sparsely populated area.

2009 Indonesia event

A large fireball was observed in the skies near Bone, Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 meters in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice the Nagasaki atomic bomb. No injuries were reported.

Close approaches

On August 10, 1972 a meteor that became known as The Great Daylight 1972 Fireball

The Great Daylight 1972 Fireball

The Great Daylight 1972 Fireball was an Earth-grazing meteoroid which passed within of the surface of the Earth at 20:29 UTC on August 10, 1972. It entered the Earth's atmosphere in daylight over Utah, United States and passed northwards leaving the atmosphere over Alberta, Canada...

was witnessed by many people moving north over the Rocky Mountains

Rocky Mountains

The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

from the U.S. Southwest to Canada. It was an Earth-grazing meteoroid that passed within 57 kilometers (about 34 miles) of the Earth's surface. It was filmed by a tourist at the Grand Teton National Park

Grand Teton National Park

Grand Teton National Park is a United States National Park located in northwestern Wyoming, U.S. The Park consists of approximately and includes the major peaks of the long Teton Range as well as most of the northern sections of the valley known as Jackson Hole. Only south of Yellowstone...

in Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

with an 8-millimeter color movie camera.

On March 23, 1989 the 300-meter (1,000-foot) diameter Apollo asteroid 4581 Asclepius

4581 Asclepius

-External links:* from JPL /...

(1989 FC) missed the Earth by 700000 kilometres (434,960.9 mi) passing through the exact position where the Earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history, twelve times more powerful than the Tsar Bomba

Tsar Bomba

Tsar Bomba is the nickname for the AN602 hydrogen bomb, the most powerful nuclear weapon ever detonated. It was also referred to as Kuz'kina Mat , in this usage meaning "something that has not been seen before"....

, the most powerful nuclear bomb ever exploded by man. It attracted widespread attention as early calculations had its passage being as close as 64000 km (39,767.9 mi) from the Earth, with large uncertainties that allowed for the possibility of it striking the Earth.

On March 18, 2004, LINEAR

Linear

In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

announced a 30-meter asteroid, 2004 FH

2004 FH

2004 FH is a near-Earth asteroid that was discovered on March 15, 2004, by the NASA-funded LINEAR asteroid survey. The object is roughly 30 metres in diameter and passed just above the Earth's surface on March 18, 2004, at 22:08 UTC; making it the 11th closest approach to Earth recorded...

, which would pass the Earth that day at only 42600 km (26,470.5 mi), about one-tenth the distance to the Moon, and the closest miss ever noticed. They estimated that similar-sized asteroids come as close about every two years.

On March 31, 2004, two weeks after 2004 FH, meteoroid

Meteoroid

2004 FU₁₆₂

2004 FU162

' is a meteoroid which passed within about one Earth radius of the surface of the Earth at 15:35 UTC on March 31, 2004 this is the third or fourth closest approach...

set a new record for closest recorded approach, passing Earth only 6500 km (4,038.9 mi) away (about 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 harmlessly disintegrated in the atmosphere.

On March 2, 2009, near-Earth asteroid 2009 DD45

2009 DD45

2009 DD45 is a small Apollo asteroid that passed near Earth at an altitude of on 2 March 2009 at 13:44 UTC. It was discovered by Australian astronomers at the Siding Spring Observatory on 27 February 2009, only three days before its closest approach to the Earth. Its estimated diameter is between...

flew by Earth at about 13:40 UT. The estimated distance from Earth was 72000 km (44,738.8 mi), approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters (115 feet) wide.

On January 13, 2010 at 12:46 UT, near-Earth asteroid 2010 AL30

2010 AL30

' is a near-Earth asteroid that was discovered on 10 January 2010.Italian scientists Ernesto Guido and Giovanni Sostero told RIA Novosti that it had an orbital period of almost exactly one year and might be a spent rocket booster...

passed at about 122000 km (75,807.5 mi). It was approximately 10–15 m (32.8–49.2 ft) wide. If 2010 AL30 had entered the Earth's atmosphere, it would have created an air burst

Air burst

An air burst is the detonation of an explosive device such as an anti-personnel artillery shell or a nuclear weapon in the air instead of on contact with the ground or target or a delayed armor piercing explosion....

equivalent to between 50 kT and 100 kT (kilotons of TNT

TNT equivalent

TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

). The Hiroshima "Little Boy

Little Boy

" atom bomb had a yield between 13-18kT.

On June 28, 2011 an asteroid designated 2011 MD, estimated at 5–20 m (16.4–65.6 ft) in diameter, passed within 20000 km (12,427.5 mi) of the Earth, passing over the Atlantic Ocean.

On November 8, 2011 2005 YU55

2005 YU55

', also written as 2005 YU55, is a potentially hazardous asteroid 310 meters or about 400 m in diameter. It was discovered on 28 December 2005 by Robert S. McMillan at Steward Observatory, Kitt Peak. On 8 November 2011 it passed 0.85 lunar distances from the Earth...

(at about 400m diameter) passed within 324600 km (201,697.6 mi) (0.85 lunar distances) from Earth.

Future impacts

Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. (89959) 2002 NT7

(89959) 2002 NT7

' is a near-Earth object with a diameter of 1.2 miles that became the first object observed by NASA's NEO program to be assigned a positive rating on the Palermo Technical Impact Hazard Scale, for a potential impact on February 1, 2019...

was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale

Palermo Technical Impact Hazard Scale

The Palermo Technical Impact Hazard Scale is a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-earth object . It combines two types of data—probability of impact, and estimated kinetic yield—into a single "hazard" value...

, with approximately one in a million on a potential impact date of February 1, 2019. It is now known that on January 13, 2019, will safely pass 0.4078 AU from the Earth.

Asteroid (29075) 1950 DA

(29075) 1950 DA

-External links:* from JPL /...

was lost after its discovery in 1950 since not enough observations were made to allow plotting of its orbit, and then rediscovered on December 31, 2000. The chance it will impact Earth on March 16, 2880 during its close approach has been estimated as 1 in 300. This chance of impact for such a large object is roughly 50% greater than that for all other such objects combined between now and 2880. It has a diameter of about a kilometer (0.6 miles).

Only the asteroids 99942 Apophis

99942 Apophis

(provisionally known as 2004 MN4) and have briefly had above-normal rankings on the Torino Scale

Torino Scale

Projects to minimize the threat

Several surveys have undertaken "Spaceguard

Spaceguard

" activities (an umbrella term), including Lincoln Near-Earth Asteroid Research

Lincoln Near-Earth Asteroid Research

The Lincoln Near-Earth Asteroid Research project is a cooperative project between the United States Air Force, NASA, and MIT's Lincoln Laboratory for the systematic discovery and tracking of near-Earth asteroids. LINEAR was responsible for the majority of asteroid detections since 1998 until...

(LINEAR), Spacewatch

Spacewatch

Spacewatch is a project at the University of Arizona led by Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets...

, Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search

Lowell Observatory Near-Earth-Object Search

Lowell Observatory Near-Earth-Object Search ' was a project designed to discover asteroids and comets that orbit near the Earth. The project, funded by NASA, was directed by Dr. Ted Bowell of Lowell Observatory in Flagstaff, Arizona...

(LONEOS), Catalina Sky Survey

Catalina Sky Survey

Catalina Sky Survey is a project to discover comets and asteroids, and to search for Near-Earth objects. More specifically, to search for potentially hazardous asteroids , that may pose a threat of impact.-Mission:...

, Campo Imperatore Near-Earth Objects Survey (CINEOS), Japanese Spaceguard Association

Japanese Spaceguard Association

The is a not-for-profit organization based in Tokyo, Japan. Its aim echoes that of The Spaceguard Foundation and other spaceguard movements: to protect the Earth's environment from a disastrous near-Earth object collision by studying and observing the NEOs. Its formal status under the Japanese...

, and Asiago-DLR Asteroid Survey

Asiago-DLR Asteroid Survey

The Asiago-DLR Asteroid Survey is a project to search for comets and asteroids, with special emphasis on near-Earth objects.It is a joint venture between the Department of Astronomy of the University of Padua and the DLR - Institute of Space Sensor Technology and Planetary Exploration at...

. In 1998, the United States Congress

United States Congress

mandated the Spaceguard Survey – detection of 90% of near-earth asteroids over 1 km diameter (which threaten global devastation) by 2008. This could be extended by the George E. Brown, Jr. Near-Earth Object Survey Act, which calls for NASA to detect 90 percent of NEOs with diameters of 140 meters or greater by 2020.

As of 2011, 911 of the largest (>1 km diameter) near-Earth asteroids have been found, with some 70 still missing.

External links

Interactive Earth damage impact calculator from NEO
The Jet Propulsion Laboratory's NEO website / NEO Earth Close Approaches
TECA Table of next close approaches to the Earth
SAEL Small Asteroids Encounter List
MBPL Minor Body Priority List
Spaceguard UK
House Holds Hearing On NEO Threat
The UK NEO Information Centre
Earth in the Cosmic Shooting Gallery Paper published 2005
Near Earth Asteroids Astronomy Cast
Astronomy Cast
Astronomy Cast is an educational nonprofit podcast discussing various topics in the field of astronomy. The specific subject matter of each episode shifts from week to week, ranging from planets and stars to cosmology and mythbusting...

episode #29, includes full transcript.
Upcoming Close Approaches (<0.10 A.U.) of Near-Earth Objects to Earth
Catalogue of the Solar System Small Bodies Orbital Evolution
NASA Space Telescope Finds Fewer Asteroids Near Earth (09.29.11)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Risk scales

Highly rated risks

List of current threats

Number and classification of near-Earth objects

Near-Earth meteoroids

Near-Earth asteroids

Near-Earth comets

Impact rate

Historic impacts

1908 Tunguska event

1979 Vela Incident

2002 Eastern Mediterranean event

2008 Sudan event

2009 Indonesia event

Close approaches

Future impacts

Projects to minimize the threat

See also

External links