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A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By definition, a solar system body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical unit (AU). NEOs include more than fourteen thousand near-Earth asteroids (NEAs), more than one hundred near-Earth comets (NECs), and a number of solar-orbiting spacecraft and meteoroids, large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose, and mitigations are being researched. In January 2016, NASA announced the Planetary Defense Coordination Office to track NEOs larger than 30 to 50 meters in diameter and coordinate an effective threat response and mitigation effort.
Contents
- History of human awareness of NEOs
- Risk
- Risk scales
- Highly rated risks
- History of NEO science and exploratory mission proposals
- Number and classification
- Near Earth asteroids
- Near Earth comets
- Impact rate
- Close approaches
- Future impacts
- Projects to minimize the threat
- Exploratory missions
- Notable objects
- References
NEAs have orbits that lie partly between 0.983 and 1.3 AU away from the Sun. When a NEA is detected it is submitted to the IAU's Minor Planet Center for cataloging. Some NEAs' orbits intersect that of Earth's so they pose a collision danger. The United States, European Union, and other nations are currently scanning for NEOs in an effort called Spaceguard.
In the United States and since 1998, 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 globally catastrophic. In 2006, it was estimated that 20% of the mandated objects had not yet been found. In 2011, largely as a result of NEOWISE, it was estimated that 93% of the NEAs larger than 1 km had been found and that only about 70 remained to be discovered. As of 5 February 2017, there have been 875 NEAs larger than 1 km discovered, of which 157 are potentially hazardous. The inventory is much less complete for smaller objects, which still have potential for large scale, though not global, damage.
Potentially hazardous objects (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 brighter (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. NASA's catalog for near-Earth objects also includes the approach distances of asteroids and comets measured in lunar distances, and this usage has become a common unit of measure used by the news media in discussing these objects.
Some NEOs are of special interest because they can be physically explored with lower mission velocity than for even the Moon, due to their combination of low velocity with respect to Earth (ΔV) and weak 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 2016, three near-Earth objects have been visited by spacecraft.
History of human awareness of NEOs
Human perception of near-Earth objects as benign objects of fascination or killer objects with high risk to human society has ebbed and flowed in the short period of human history that NEOs have been scientifically observed.
Risk
More recently, a typical frame of reference for looking at NEOs has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society. "NEOs have been understood differently throughout history." Each time an NEO is observed, "a different risk was posed, and throughout time that risk perception has evolved. It is not just a matter of scientific knowledge."
Such perception of risk is thus "a product of religious belief, philosophic principles, scientific understanding, technological capabilities, and even economical resourcefulness."
Risk scales
There are two schemes for the scientific classification of impact hazards from NEOs:
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 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 a continuously updated Sentry Risk Table of the known NEO threats in the next 100 years. All or nearly all of the objects are highly likely to eventually drop off the list as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions.
Although it will not strike for at least 800 years and thus has no Torino scale rating, (29075) 1950 DA was added to the list in 2002 because it was the first object with a Palermo scale value greater than zero. After further measurements, its impact risk was downgraded, but as of December 2016, its Palermo scale value of -1.42 is still the highest for all objects on the list. As of December 2016, only two more objects, both with potential strikes in the 22nd century only (101955 Bennu and (410777) 2009 FD), have cumulative Palermo scale values above -2.
On 24 December 2004, minor planet 99942 Apophis (at the time known by its provisional designation 2004 MN4) was assigned a 4 on the Torino scale, the highest rating ever achieved, as there was a 2.7% chance of Earth impact on 13 April 2029. By 28 December 2004, additional observations had produced a smaller uncertainty zone which no longer included the Earth during the 2029 approach. The 2029 risk of impact consequently dropped to zero, but later potential impact solutions were still rated 1 on the Torino scale. The 2036 risk was lowered to a Torino rating of 0 in August 2006. Apophis has no chance of impacting Earth before 2060.
History of NEO science and exploratory mission proposals
In a 2013 article in Wired Science, David Portree provides an overview of NEO science and proposed asteroidal missions, with an emphasis on the outcome of two conferences held in the 1970s. The International Astronomical Union minor planets workshop was held in Tucson, Arizona in March 1971 and a consensus "emerged that launching spacecraft to asteroids would be 'premature'." "In January 1978, NASA’s Office of Space Science held a workshop at the University of Chicago to "assess the state of asteroid studies and consider options for the future."
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 one in 10 using less propulsive energy than is necessary to reach Mars. "Because even the most massive NEA—35 kilometres (22 mi)-wide 1036 Ganymed, discovered in 1924, has a very weak surface gravity—landing and takeoff would need very little energy. This meant that a single spacecraft could sample multiple sites on any given NEA." Overall, it was estimated that about one percent of all NEAs might provide opportunities for human-crewed missions, or no more than about ten known NEAs. Therefore, unless the NEA discovery rate were "immediately increased five-fold, no opportunity to launch 'astronaut-scientists' to a NEA was likely to occur within a decade of the Chicago workshop."
Number and classification
Near-Earth objects are classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets create meteoroid streams that can generate meteor showers.
As of 5 February 2017, 15,727 NEOs have been discovered: 106 near-Earth comets and 15,621 near-Earth asteroids. Of those there are 1,138 Aten asteroids, 5,979 Amor asteroids, and 8,488 Apollo asteroids. There are 1,785 NEOs that are classified as potentially hazardous asteroids (PHAs). Currently, 157 known PHAs and 875 known NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size; and 7,539 known NEAs are larger than about 140 meters in size.
As of 13 December 2016, there are 673 NEAs on the Sentry impact risk page at the NASA website. A significant number of these NEAs are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "green zone" (Torino Scale 1), meaning that none warrant the attention of general public. As of 13 December 2016, the JPL Small-Body Database lists 3,117 near-Earth asteroids with an absolute magnitude (H) dimmer than 25 (roughly 50 meters in diameter).
Near-Earth asteroids smaller than ~1 meter are near-Earth meteoroids and are listed as asteroids on most asteroid tables. The smallest known near-Earth meteoroid is 2008 TS26 with an absolute magnitude of 33.2 and estimated size of only 1 meter.
Near-Earth asteroids
These are objects in a near-Earth orbit without the tail or coma of a comet. As of 5 February 2017, 15,621 near-Earth asteroids are known, ranging in size from 1 meter up to ~32 kilometers (1036 Ganymed). The estimated number of near-Earth asteroids over one kilometer in diameter was reduced from 1,000–2,000 to 500–1,000 in 2000. In 2011, the estimate of one-kilometre NEAs was narrowed to 981 ± 19 (of which over 90% have been discovered), while the number of NEAs larger than 140 metres across was estimated at 13,200 ± 1,900. There are about 1 million near-Earth asteroids about 40 meters in diameter—of which about 1 percent have been discovered. The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.
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 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. The known asteroid with the greatest known chance of impacting Earth is 2010 RF12 with a 1 in 16 chance of impacting Earth on 5 September 2095. Its 7-meter estimated diameter ensures that an impact would cause little damage.
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.
Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q):
(Note: Some authors define the Atens group 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.
Near-Earth comets
As of 5 February 2017, 106 near-Earth comets have been discovered. 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. Cometary fragmenting may also be responsible for some impacts from near-Earth objects. It is rare for a comet to pass within 0.1 AU (15,000,000 km; 9,300,000 mi) of Earth.
These near-Earth objects were probably derived from the Kuiper belt, beyond the orbit of Neptune.
Impact rate
Stony asteroids with a diameter of 4 meters (13 ft) impact Earth approximately once per year. Asteroids with a diameter of roughly 7 meters enter Earth's atmosphere with as much energy as Little Boy (the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT) about every 5 years. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Every 2,000–3,000 years, objects produce explosions of 10 megatons comparable to the one observed at Tunguska in 1908. Objects with a diameter of one kilometer hit the Earth an average of twice every million years. Large collisions with five kilometer objects happen approximately once every twenty million years.
Assuming that these rates 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.
Close approaches
On August 10, 1972, a meteor that became known as 1972 Great Daylight 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.
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, 12 times as powerful as the Tsar Bomba, the most powerful nuclear bomb ever exploded. 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.
On 13 October 1990 an Earth-grazing meteoroid EN131090 was observed above Czechoslovakia and Poland. It was moving with a speed of 41.74 km/s (25.94 mi/s) along a 409 km (254 mi) trajectory from the south to the north. The closest approach to the Earth was 98.67 km (61.31 mi). 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 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.
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 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 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.
On January 13, 2010, at 12:46 UT, near-Earth asteroid 2010 AL30 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 13-18 kt.
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.
On November 8, 2011, (308635) 2005 YU55 (at about 400m diameter) passed within 324,600 km (201,700 mi) (0.85 lunar distances) of Earth. Ten weeks later, on January 27, 2012, the 10-metre wide asteroid 2012 BX34 passed a mere 60,000 km (37,000 mi) from Earth.
On February 15, 2013, 367943 Duende (2012 DA14) passed approximately 27,700 km (17,200 mi) above the surface of Earth. This was closer than satellites in geosynchronous orbit. The asteroid was not visible to the unaided eye.
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 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 approximately February 1, 2019; it is now known that 2002 NT7 will actually safely pass 0.4078 AU (61,010,000 km; 37,910,000 mi) from the Earth on January 13, 2019.
Asteroid (29075) 1950 DA was lost after its discovery in 1950, since its observations over just 17 days were insufficient to determine its orbit, and then rediscovered on December 31, 2000. It has a diameter of about a kilometer (0.6 miles). The chance that it could impact Earth during its March 16, 2880 close approach was then estimated as 1 in 300. This was roughly 50% greater than the combined chance of impact for all other similarly large objects until 2880. The next radar opportunity for 1950 DA in 2032, will pinpoint our knowledge of the orbit, but additional optical position measurements have already reduced the probability of a 2880 impact to 1 in 20 000.
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
In 1998, the United States Congress mandated the Spaceguard Survey – detection of 90% of near-earth asteroids over 1 km diameter (that threaten global devastation) by 2008. Several surveys have undertaken "Spaceguard" activities (an umbrella term), including Lincoln Near-Earth Asteroid Research (LINEAR), Spacewatch, Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey (CSS), Campo Imperatore Near-Earth Object Survey (CINEOS), Japanese Spaceguard Association, Asiago-DLR Asteroid Survey (ADAS) and Near-Earth Object WISE (NEOWISE). In 2011, it was estimated that 93% of near-Earth asteroids larger than 1 km in diameter have been found, thus the original Spaceguard goal has been met. In 2005, in the USA, the original 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 meters or greater, by 2020.
In May 2016, the precision of asteroid diameter estimates arising from the Wide-field Infrared Survey Explorer and NEOWISE missions have been questioned by Nathan Myhrvold, but the criticism has yet to undergo peer review and faced criticism for its methodology itself.
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.
Exploratory missions
The first near-Earth object to be visited by a spacecraft was asteroid 433 Eros when NASA's Near Earth Asteroid Rendezvous probe orbited it from February 2001 until landing on the asteroid surface in February 2002. A second near-Earth asteroid, 25143 Itokawa, was visited in September 2005 by the JAXA Hayabusa mission, which succeeded in taking material samples back to Earth. A third near-Earth asteroid, 4179 Toutatis, was explored by CNSA's Chang'e 2 spacecraft during a flyby in December 2012.
Notable objects
Notable objects include near-Earth objects and flyby objects as minor planets, comets, asteroids, and meteors:, generally specifically mentioned in news articles.