Near-Earth object

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

Asteroid Toutatis from Paranal.

A near-Earth object (NEO) is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU.^[1] They include a few thousand near-Earth asteroids (NEAs), near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the planet.^[2] 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.^[3]

Those NEOs that are asteroids (NEA) have orbits that lie partly between 0.983 and 1.3 astronomical units away from the Sun.^[4] When an NEA is detected it is submitted to the Harvard Minor Planet Center for cataloging. Some near-Earth asteroids' orbits intersect that of Earth's so they pose a collision danger.^[5] The United States, European Union and other nations are currently scanning for NEOs^[6] in an effort called Spaceguard.

In the United States, NASA has a congressional 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. As of October 2008^[update], 982 of these mandated NEOs have been detected.^[7] It was estimated in 2006 that 20% of the mandated objects have not yet been found.^[6] Efforts are under way to use an existing telescope in Australia to cover the ~30% of the sky that has not yet been surveyed.

Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.^[8] 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 (7,500,000 km; 4,600,000 mi), or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%), are not considered PHOs.^[9] The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in Lunar Distances,^[10] 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) 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.^[11] This makes them an attractive target for exploration.^[12] As of 2008, two near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe,^[13] and 25143 Itokawa, by the JAXA Hayabusa mission.^[14]

Risk scales

There are two schemes for classification of impact hazards:

• the simple Torino Scale, and
• the more complex Palermo Technical Impact Hazard Scale

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

$f_B = 0.03E^{-0.81} \;$

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^[15] published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:

$f_B = 0.00737 E^{-0.9} \;$

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

Highly rated risks

On 25 December 2004, minor planet 2004 MN₄ (later named 99942 Apophis) 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.^[16]

As of 12 September 2011, the only known NEO with a Palermo scale value greater than zero is (29075) 1950 DA, 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.^[17]

List of current threats

NASA maintains a continuously updated web page of the most significant NEO threats in the next 100 years.^[16] 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, 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 meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets can leave debris in their orbits.

As of April 2011^[update], 7,954 NEOs have been discovered: 87 near-Earth comets and 7,867 near-Earth Asteroids. Of those there are 647 Aten asteroids, 2,920 Amor asteroids, and 4,289 Apollo asteroids. There are 1,215 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 148 PHAs and 824 NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.^[18]

As of April 2011^[update], there are 368 NEAs on the impact risk page at the NASA website.^[16] A significant number of these NEAs – 215 as of May 2010^[update] – 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 2), meaning that none warrant the attention of general public.^[19] As of November 2011^[update], only asteroids 2007 VK₁₈₄ and 2011 AG₅ 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

There are significantly fewer near-Earth asteroids in the mid-size range than previously thought.

These are objects of 50 meters or more in diameter in a near-Earth orbit without the tail or coma of a comet. As of May 2010^[update], 7,075 near-Earth asteroids are known,^[18] ranging in size up to ~32 kilometers (1036 Ganymed).^[20] The number of near-Earth asteroids over one kilometer in diameter is estimated to be 500–1,000.^[21][22] The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.^[23]

NEAs survive in their orbits for just a few million years.^[4] They are eventually eliminated by planetary perturbations 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 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.^[24]

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.^[4][25]

The three families of near-Earth asteroids

There are three families of near-Earth asteroids:^[4]

• The Atens, which have average orbital radii less than one AU^[26] and aphelia of more than Earth's perihelion (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, 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 classification, for asteroids with extremely Earth-like orbits.^[27]

Near-Earth comets

As of May 2010^[update], 84 near-Earth comets have been discovered.^[18] 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.^[28] Cometary fragmenting may also be responsible for some impacts from near-Earth objects.

These near-Earth objects were probably derived from the Kuiper belt, beyond the orbit of Neptune.

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, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized.^[29] Every 2000–3000 years NEAs produce explosions comparable to the one observed at Tunguska in 1908.^[30] Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval.^[4] 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 and 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.^[31]

Historic impacts

Illustration of the impact of an asteroid a few kilometers across. Such impacts are expected to occur less often than every 100 million years.

The general acceptance of the Alvarez hypothesis, explaining the Cretaceous–Tertiary extinction event as the result of a large object impact event, raised the awareness of the possibility of future Earth impacts with other objects that cross the Earth's orbit.^[2]

1908 Tunguska event

Main article: 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.^[32]

1979 Vela Incident

Main article: 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, was identified by a U.S. Vela 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

Main article: 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, between Greece and Libya, 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.^[33]

2008 Sudan event

Main article: 2008 TC₃

On 6 October 2008, scientists calculated that a small near-Earth asteroid, 2008 TC₃, just sighted that night, would impact the Earth on 7 October over Sudan, at 0246 UTC, 5:46 local time.^[34][35] The asteroid arrived as predicted.^[36][37] 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.^[38]

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.^[39]

Close approaches

Flyby of asteroid 2004 FH. The other object that flashes by is an artificial satellite.

See also: Potentially hazardous object

On August 10, 1972 a meteor that became known as The Great Daylight 1972 Fireball was witnessed by many people moving north over the Rocky Mountains 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 in Wyoming with an 8-millimeter color movie camera.^[40]

On March 23, 1989 the 300-meter (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometers (430,000 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, the most powerful nuclear bomb ever exploded by man. It attracted widespread attention as early calculations had its passage being as close as 64,000 km (40,000 mi) from the Earth, with large uncertainties that allowed for the possibility of it striking the Earth.^[41]

On March 18, 2004, LINEAR announced a 30-meter asteroid, 2004 FH, which 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. They estimated that similar-sized asteroids come as close about every two years.^[42]

On March 31, 2004, two weeks after 2004 FH, meteoroid 2004 FU₁₆₂ set a new record for closest recorded approach, passing Earth only 6,500 km (4,000 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 flew by Earth at about 13:40 UT. The estimated distance from Earth was 72,000 km (45,000 mi), approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters (115 feet) wide.^[43]

On January 13, 2010 at 12:46 UT, near-Earth asteroid 2010 AL30^[44] passed at about 122,000 km (76,000 mi). It was approximately 10–15 m (33–49 ft) wide. If 2010 AL30 had entered the Earth's atmosphere, it would have created an air burst equivalent to between 50 kT and 100 kT (kilotons of TNT). The Hiroshima "Little Boy" atom bomb had a yield between 13-18kT.^[45]

On June 28, 2011 an asteroid designated 2011 MD, estimated at 5–20 m (16–66 ft) in diameter, passed within 20,000 km (12,000 mi) of the Earth, passing over the Atlantic Ocean.^[46]

On November 8, 2011 2005 YU55 (at about 400m diameter) passed within 324,600 km (201,700 mi) (0.85 lunar distances) from Earth.

Future impacts

Radar image of asteroid 1950 DA.

Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. (89959) 2002 NT7 was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale, with approximately one in a million on a potential impact date of February 1, 2019. It is now known that on January 13, 2019, 2002 NT₇ will safely pass 0.4078 AU (61,010,000 km; 37,910,000 mi) from the Earth.^[47]

Asteroid (29075) 1950 DA 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.^[48] It has a diameter of about a kilometer (0.6 miles).

Only the asteroids 99942 Apophis (provisionally known as 2004 MN4) and (144898) 2004 VD₁₇ have briefly had above-normal rankings on the Torino Scale.

Projects to minimize the threat

Main article: Asteroid-impact avoidance

Number of NEOs detected by various projects.

Several surveys have undertaken "Spaceguard" activities (an umbrella term), including Lincoln Near-Earth Asteroid Research (LINEAR), Spacewatch, Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey, Campo Imperatore Near-Earth Objects Survey (CINEOS), Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey. In 1998, the 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.^{[49][Need quotation to verify][Full citation needed]}

As of 2011, 911 of the largest (>1 km diameter) near-Earth asteroids have been found, with some 70 still missing.^[50]

See also

• List of Earth-crossing minor planets

• 6Q0B44E, in Earth orbit with a period of 80 days
• J002E3, probably the 3rd stage of Apollo 12
• BRAMS (Belgian RAdio Meteor Stations); study of meteor population ^[51]
• Bolide
• Co-orbital configuration
• Euronear
• Mission Marco Polo
• NEODyS
• Orbit@home
• Space debris
• 2010 SO16
• Asteroid mining
• Potentially hazardous object

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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 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)