Near-Earth Objects

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 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 30,503 known near-Earth asteroids (NEAs) and over a hundred known short-period near-Earth comets (NECs). A number of solar-orbiting meteoroids were 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. Asteroids as small as 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 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. , 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

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 , or which are fainter than H = 22.0 (about 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

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 , 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 diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by . 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 showed a potential 2028 close approach 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 , with no chance of collision. By that time, inaccurate reports of a potential impact had caused a media storm.

Risk

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 ''E'' megatonnes is estimated as:

:$f_B = 0.03E^ \;$

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:

:$f_B = 0.00737 E^ \;$

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, 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, will pass the Earth at a safe closest distance (perigee) of on August 31, 2080.

Asteroid 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, , 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, asteroid 99942 Apophis (at the time known only by its provisional designation ) 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, 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.

, is listed with the highest chance of impacting Earth, at 1 in 22 on September 5, 2095. At only 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.

Projects to minimize the threat

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 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). As a result, the ratio of the known and the estimated total number of near-Earth asteroids larger than 1 km in diameter rose from about 20% in 1998 to 65% in 2004, 80% in 2006, and 93% in 2011. The original Spaceguard goal has thus been met, only three years late. , 891 NEAs larger than 1 km have been discovered, or 97% of an estimated total of about 920.

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 or greater, by 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 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.

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

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.

and according to statistics maintained by CNEOS, 27,440 NEOs have been discovered. Only 117 (0.43%) of them are comets, whilst 27,323 (99.57%) are asteroids. 2,224 of those NEOs are classified as potentially hazardous asteroids (PHAs).

, 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.

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 in diameter and 433 Eros is about 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. One way around this opposition bias is to use thermal infrared telescopes that observe their heat emissions instead of the light they reflect.

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.

Near-Earth asteroids (NEAs)

These are asteroids in a near-Earth orbit without the tail or coma of a comet. , 30,503 near-Earth asteroids are known, 2,304 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 astero