Late Heavy Bombardment

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Artist's impression of the Moon during the Late Heavy Bombardment (above) and today (below).

The Late Heavy Bombardment (abbreviated LHB and also known as the lunar cataclysm) is an event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. During this interval, a disproportionately large number of asteroids are theorized to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.

The Late Heavy Bombardment happened after the Earth and other rocky planets had formed and accreted most of their mass, but still quite early in Earth's history.

Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses attempt to explain the apparent spike in the flux of impactors (i.e. asteroids and comets) in the inner Solar System, but no consensus yet exists. The Nice model, popular among planetary scientists, postulates that the giant planets underwent orbital migration and in doing so, scattered objects in the asteroid and/or Kuiper belts into eccentric orbits, and into the path of the terrestrial planets. Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, and that the apparent clustering of impact-melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin. They also note that the rate of impact cratering could differ significantly between the outer and inner zones of the Solar System.

Evidence for a cataclysm

The main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are believed to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The Apollo 15, 16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, and Serenitatis basins, respectively. 

The apparent clustering of ages of these impact melts, between about 3.8 and 4.1 Ga, led to postulation that the ages record an intense bombardment of the Moon. They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds. At the time, the conclusion was considered controversial. 

As more data has become available, particularly from lunar meteorites, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga. Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga.

Dating of howardite, eucrite and diogenite (HED) meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.

Criticisms of the cataclysm hypothesis

While the cataclysm hypothesis has recently gained in popularity, particularly among dynamicists who have identified possible causes for such a phenomenon, the cataclysm hypothesis is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1 Ga is related to all such samples having been pulverized, or their ages being reset. 

The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin. The Imbrium impact basin is the youngest and largest of the multi-ring basins found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9 Ga simply reflects material being collected from a single impact event, Imbrium, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in 40Ar/39Ar dating could also be produced by an episodic early crust formation followed by partial 40Ar losses as the impact rate declined.

A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1 Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods. Latest reinterpretation of crater statistics suggests that the flux on the Moon and on Mars may have been lower in general. Thus, the recorded crater population can be explained without any peak in the earliest bombardment of the inner Solar System.

Geological consequences on Earth

If a cataclysmic cratering event truly occurred on the Moon, the Earth would have been affected as well. Extrapolating lunar cratering rates to Earth at this time suggests that the following number of craters would have formed:

• 22,000 or more impact craters with diameters >20 km (12 mi),
• about 40 impact basins with diameters about 1,000 km (620 mi),
• several impact basins with diameters about 5,000 km (3,100 mi),

Before the formulation of the LHB theory, geologists generally assumed that the Earth remained molten until about 3.8 Ga. This date could be found in many of the oldest-known rocks from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, uranium–lead dating of zircons. As no older rocks could be found, it was generally assumed that the Earth had remained molten until this date, which defined the boundary between the earlier Hadean and later Archean eons. Nonetheless, more recently, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the Acasta Gneiss of the Slave Craton in northwestern Canada.

Older rocks could be found, however, in the form of asteroid fragments that fall to Earth as meteorites. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6 Ga, which is assumed to be the time when the first solids formed in the protoplanetary disk around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of the Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of the Earth into a solid body as the gravitational potential energy of accretion was released.

Later calculations showed that the rate of collapse and cooling depends on the size of the rocky body. Scaling this rate to an object of Earth mass suggested very rapid cooling, requiring only 100 million years. The difference between measurement and theory presented a conundrum at the time.

The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8 Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the Acasta Gneiss in the North American cratonic shield and the gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (~3.8 Ga old) rock.

The Jack Hills zircon led to something of a revolution in our understanding of the Hadean eon. Older references generally show that Hadean Earth had a molten surface with prominent volcanos. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek Hades. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see Cool early Earth).

Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in Greenland were a relic of organic matter. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8 Ga old, and others suggesting a more "modest" 3.6 Ga. In either case it was a very short time for abiogenesis to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible; life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. Recent studies suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85 Ga, suggesting the latter possibility is the most likely answer. More recent studies have found no evidence for the isotopically light carbon ratios that were the basis for the original claims.

More recently, a similar study of Jack Hills rocks shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25 Ga. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life.

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of the Earth would have been sterilized, hydrothermal vents below the Earth's surface could have incubated life by providing a sanctuary for heat-loving microbes.

In April 2014, scientists reported finding evidence of the largest terrestrial meteor impact event to date near the Barberton Greenstone Belt. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37 to 58 kilometres (23 to 36 miles) wide. The crater from this event, if it still exists, has not yet been found.

Possible causes

Giant-planet migration

Simulation showing outer planets and planetesimal belt: a) Early configuration, before Jupiter (green) and Saturn (orange) reach 2:1 resonance b) Scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue) c) After ejection of planetesimals by planets.

 

In the Nice model the Late Heavy Bombardment is the result of a dynamical instability in the outer Solar System. The original Nice model simulations by Gomes et al. began with the Solar System's giant planets in a tight orbital configuration surrounded by a rich trans-Neptunian belt. Objects from this belt stray into planet crossing orbits causing the orbits of the planets to migrate over several hundred million years. Jupiter and Saturn's orbits drift apart slowly until they cross a 2:1 orbital resonance causing the eccentricities of their orbits to increase. The orbits of the planets become unstable and Uranus and Neptune are scattered onto wider orbits that disrupt the outer belt, causing a bombardment of comets as they enter planet crossing orbits. Interactions between the objects and the planets also drive a faster migration of Jupiter and Saturn's orbits. This migration causes resonances to sweep through the asteroid belt, increasing the eccentricities of many asteroids until they enter the inner Solar System and impact the terrestrial planets.

The Nice model has undergone some modification since its initial publication. The giant planets now begin in a multi-resonant configuration due an early gas-driven migration through the protoplanetary disk. Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years. The encounters between planets that follow include one between an ice giant and Saturn the that propels the ice giant onto a Jupiter-crossing orbit followed by an encounter with Jupiter which drives the ice giant outward. This jumping-Jupiter scenario quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets. While this is required to preserve the low eccentricities of the terrestrial planets and avoid leaving the asteroid belt with too many high eccentricity asteroids, it also reduces the fraction of asteroids removed from the main asteroid belt, leaving a now nearly depleted inner band of asteroids as the primary source of the impactors of the LHB. The ice giant is often ejected following its encounter with Jupiter leading some to propose that the Solar System began with five giant planets. Recent works, however, have found that impacts from this inner asteroid belt would be insufficient to explain the formation of ancient impact spherule beds and the lunar basins, and that the asteroid belt was probably not the source of the Late Heavy Bombardment.

Late Uranus/Neptune formation

According to one planetesimal simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years. Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion. This "late appearance" of these planets has therefore been suggested as a different reason for the LHB. However, recent calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that Jovian planets formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.

Planet V hypothesis

The Planet V hypothesis posits that a fifth terrestrial planet created the Late Heavy Bombardment when its meta-stable orbit entered the inner asteroid belt. The hypothetical fifth terrestrial planet, Planet V, had a mass less than half of Mars and originally orbited between Mars and the asteroid belt. Planet V's orbit became unstable due to perturbations from the other inner planets causing it to intersect the inner asteroid belt. After close encounters with Planet V, many asteroids entered Earth-crossing orbits producing the Late Heavy Bombardment. Planet V was ultimately lost, likely plunging into the Sun. In numerical simulations, an uneven distribution of asteroids, with the asteroids heavily concentrated toward the inner asteroid belt, has been shown to be necessary to produce the LHB via this mechanism. An alternate version of this hypothesis in which the lunar impactors are debris resulting from Planet V impacting Mars, forming the Borealis Basin, has been proposed to explain a low number of giant lunar basins relative to craters and a lack of evidence of cometary impactors.

Disruption of Mars-crossing asteroid

A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid. This Vesta-sized asteroid was a remnant of a population which initially was much larger than the current main asteroid belt. Most of the pre-Imbrium impacts would have been due to these Mars-crossing objects, with the early bombardment extending until 4.1 billion years ago. A lull in basin-forming impacts then followed during which the lunar magnetic field decayed. Then roughly 3.9 billion years ago a catastrophic impact disrupted the Vesta-sized asteroid radically increasing the population of Mars-crossing objects. Many of these objects then evolved onto Earth-crossing orbits producing a spike in the lunar impact rate during which the last few lunar impact basins are formed. Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size-frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis. The timing and the cause of the change in the size-frequency distribution of craters is controversial.

Other potential sources

A number of other possible sources of the Late Heavy Bombardment have been investigated. Among these are additional Earth satellites orbiting independently or as lunar trojans, planetesimals left over from the formations of the terrestrial planets, Earth or Venus co-orbitals, and the breakup of a large main belt asteroid. Additional Earth satellites on independent orbits were shown to be quickly captured into resonances during the Moon's early tidally-driven orbital expansion and were lost or destroyed within in a few million years Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii. Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins. The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB. Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500 km parent body with the most favorable initial conditions. Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.

Exosystem with possible Late Heavy Bombardment

Evidence has been found for Late Heavy Bombardment-like conditions around the star Eta Corvi.

Asteroid impact prediction

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2008 TC3 was the first successfully predicted asteroid impact. This picture shows the estimated path and altitude of the meteor in red, with the possible location for the METEOSAT IR fireball (bolide) as orange crosshairs and the infrasound detection of the explosion in green

Asteroid impact prediction is the prediction of the dates and times of asteroids impacting Earth, along with the locations and severities of the impacts.

The process of impact prediction follows three major steps:

1. Discovery of an asteroid and initial assessment of its orbit
2. Follow up observations to improve the accuracy of the orbit data
3. Calculating if, when and where the orbit will intersect with Earth at some point in the future.

In addition, although not strictly part of the prediction process, once an impact has been predicted, an appropriate response needs to be made.

Most asteroids are discovered by a camera on a telescope with a wide field of view. Image differencing software compares a recent photograph with earlier ones of the same part of the sky, detecting objects that have moved, brightened, or appeared. Follow up can be carried out by any telescope powerful enough to see the newly detected object. Orbit intersection calculations are then carried out by two independent systems, one (Sentry) run by NASA and the other (NEODyS) by ESA. 

In order to predict an impact, current systems need several factors to be just right, for example the direction of approach, the weather and the phase of the moon. Because of this they have a low rate of success. Performance is improving as existing systems are upgraded and new systems come on line, but some of the issues the current systems face can only be overcome by a dedicated space based system.

History

In 1992 a report to NASA recommended a coordinated survey (christened Spaceguard) to discover, verify and provide follow-up observations for Earth-crossing asteroids. This survey was scaled to discover 90% of all objects larger than one kilometer within 25 years. Three years later, a further NASA report recommended search surveys that would discover 60–70% of the short-period, near-Earth objects larger than one kilometer within ten years and obtain 90% completeness within five more years.

In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1 km or larger that could represent a collision risk to Earth. The 1 km diameter metric was chosen after considerable study indicated that an impact of an object smaller than 1 km could cause significant local or regional damage but is unlikely to cause a worldwide catastrophe.The impact of an object much larger than 1 km diameter could well result in worldwide damage up to, and potentially including, extinction of the human race. The NASA commitment has resulted in the funding of a number of NEO search efforts, which made considerable progress toward the 90% goal by the target date of 2008 and also produced the first ever successful prediction of an asteroid impact (the 4-meter 2008 TC3 was detected 19 hours before impact). However the 2009 discovery of several NEOs approximately 2 to 3 kilometers in diameter (e.g. 2009 CR₂, 2009 HC82, 2009 KJ, 2009 MS and 2009 OG) demonstrated there were still large objects to be detected. 

One of the 7,000 buildings damaged by the 2013 Chelyabinsk meteor

Three years later, in 2012, the small asteroid 367943 Duende was discovered and successfully predicted to be on close approach to Earth again just 11 months later. This was a landmark prediction as the object was only 20 m × 40 m, and was closely monitored as a result. However, on the day of its closest approach, by coincidence another asteroid was also approaching Earth, unpredicted and undetected, from a direction close to the Sun. Unlike 367943 Duende it was on a collision course and impacted Earth 16 hours before 367943 Duende passed, becoming the Chelyabinsk meteor. It injured 1,500 people and damaged over 7,000 buildings, raising the profile of the dangers of even small asteroid impacts if they occur over populated areas. The asteroid is estimated to have been 17 m across. 

In April 2018, the B612 Foundation reported "It's 100 per cent certain we'll be hit [by a devastating asteroid], but we're not 100 per cent sure when." Also in 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.

Discovery of Near Earth Asteroids

The first step in predicting impacts is detecting asteroids and determining their orbits. Finding faint Near-Earth objects against the background stars is very much a needle in a haystack search. It is achieved by sky surveys that are designed to discover near Earth asteroids. Unlike the majority of telescopes that have a narrow field of view and high magnification, survey telescopes have a wide field of view to scan the entire sky in a reasonable amount of time with enough sensitivity to pick up the faint Near-Earth objects they are searching for. 

NEO focused surveys revisit the same area of sky several times in succession. Movement can then be detected using image differencing techniques. Anything that moves from image to image against the background of stars is compared to a catalogue of all known objects, and if it is not already known is reported as a new discovery along with its astrometry. This then allows other observers to confirm and add to the data about the newly discovered object.

Cataloging vs Warning

Cataloging systems focus on finding larger asteroids years in advance and scan the sky slowly (once per month), but carefully. Warning systems focus on scanning the sky relatively quickly (once per night) and typically cannot detect objects that are as faint as cataloging systems. Some systems compromise and scan the sky say once per week.

Cataloging Systems

For larger asteroids (greater than 100 m to 1 km across), prediction is based on cataloging the asteroid, years to centuries before it could impact. This technique is possible as they can be seen from a long distance due to their large size. Their orbits therefore can be measured and any future impacts predicted long before they are on their final approach to Earth. This long period of warning is necessary as an impact from a 1 km object would cause world-wide damage. As of 2018, the inventory is nearly complete for the kilometer-size objects (around 900) and approximately one third complete for 140 meter objects (around 8500).

Warning Systems

Smaller near-Earth objects are far more numerous (millions) and the vast majority remain undiscovered. They seldom pass close enough to Earth on a previous approach that they become bright enough to observe, and so most can only be observed on final approach. They therefore cannot usually be cataloged well in advance and can only be warned about, a few weeks to days in advance. This is much too late to deflect them away from Earth, but is enough time to mitigate the consequences of the impact by evacuating and otherwise preparing the affected area. 

Current mechanisms for detecting asteroids on final approach rely on ground based telescopes with wide fields of view which currently can monitor the sky at most every second night. They therefore still miss most of the smaller asteroids that more commonly impact Earth, which are bright enough to detect for less than two days. Ground-based telescopes also cannot detect most of the asteroids which impact the day side of the planet. These and other problems mean very few impacts are successfully predicted.

Surveys

The main NEO focussed surveys are listed below, along with future telescopes that are already funded. They have a fairly clear-cut division between 'cataloging surveys' which use larger telescopes to mostly identify larger asteroids well before they come very close to Earth and 'warning surveys' which mostly look for smaller asteroids on their final approach. The existing warning surveys have enough capacity between them to scan the northern sky once per clear night. However, they are concentrated in a relatively small part of the planet. Two (Pan-STARRS and ATLAS) are in Hawaii, which means they see the same parts of the sky at the same time of day, and are affected by the same weather. Two others (Catalina Sky Survey and Zwicky Transient Facility) are located in the southwestern United States and so suffer from similar overlap. These surveys do complement each other to an extent in that some are cataloging surveys and some are warning surveys. However, the resulting coverage across the globe is imperfect. In particular, there are currently no major surveys in the Southern Hemisphere. 

Locations of the major near Earth asteroid surveys, currently clustered in the north west of the globe.

This clustering together of the sky surveys in the Northern hemisphere means that around 15% of the sky at extreme Southern declination is never monitored, and that the rest of the Southern sky is observed over a shorter season than the Northern sky. Moreover as the hours of darkness are fewer in summertime, the lack of a balance of surveys between North and South means that the sky is scanned less often in the Northern summer. Once it is completed, the Large Synoptic Survey Telescope will cover the southern sky, but being at a similar longitude to the other surveys there will still be times every day when it will be in daylight along with all the others. The 3.5 m Space Surveillance Telescope, which was originally also in the southwest United States, was dismantled and moved to Western Australia in 2017. When completed, this would make a significant difference to the global coverage. However, due to the new site being in a cyclone region, construction has not been completed. Unless this issue is resolved, the two new ATLAS telescopes planned for construction in the Southern hemisphere (including one at the South African Astronomical Observatory) by 2020, are currently the only ones which will cover this gap in monitoring the skies (south east of the globe).

Survey	Telescope diameter (m)	Number of telescopes	Time to scan entire visible Sky (when clear)	Limiting magnitude	Hemisphere	Activity	Peak yearly observations	Survey category
ATLAS	0.5	2	2 nights	19	Northern	2016–present	1,908,828	Warning survey
	0.5	2	1 night	19	Southern	2020	NA	Warning survey
Catalina Sky Survey	1.5	1	30 nights	21.5	Northern	1998–present	see Mount Lemmon Survey	Cataloging survey
	0.7	1	7 nights	19.5	Northern	1998–present	1,934,824	Cataloging survey
	0.5	1	?	?	Southern	2004–2013	264,634	Warning survey
Large Synoptic Survey Telescope	8.4	1	3-4 nights	27	Southern	2022	NA	Both
Lincoln Near-Earth Asteroid Research	1.0	2	?	?	Northern	1998-2012	3,346,181	Cataloging survey
Lowell Observatory Near-Earth-Object Search	0.6	1	41 nights	19.5	Northern	1998-2008	836,844	Cataloging survey
Mount Lemmon Survey	1.52	1	?	~21	Northern	2005–present	2,920,211	Cataloging survey
Near-Earth Asteroid Tracking	?	2	?	?	Northern	1995-2007	1,214,008	Cataloging survey
NEO Survey Telescope	1	1	1 night	21	Northern	2020	NA	Warning survey
NEOWISE	0.4	1	~6 months	~22	Earth Orbit	2009–present	2,279,598	Cataloging survey
Pan-STARRS	1.8	2	30 nights	23	Northern	2010–present	5,254,605	Cataloging survey
Space Surveillance Telescope	3.5	1	6 nights	20.5	Northern	2014-2017	6,973,249	Warning survey
					Southern	delayed	NA	Warning survey
Spacewatch	1.8	1	?	?	Northern	1980–1998	1,532,613	Cataloging survey
	0.9	1	?	22
Zwicky Transient Facility	1.2	1	3 nights	20.5	Northern	2018–present	483,822	Warning survey

ATLAS

ATLAS, the "Asteroid Terrestrial-impact Last Alert System" uses two 0.5 metre telescopes located at Haleakala and Mauna Loa on two of the Hawaiian Islands. With a field of view of 30 square degrees each, the telescopes survey the observable sky down to apparent magnitude 19 with 4 exposures every two clear nights. The survey has been fully operational with these two telescopes since 2017, and in 2018 obtained NASA funding for two additional telescopes. Both will be sited in the Southern hemisphere, with one at the South African Astronomical Observatory, and they are expected to take 18 months to build. Their southern locations will provide coverage of the 15% of the sky that cannot be observed from Hawaii, and the doubling of its observing resources will allow ATLAS to survey the observable sky with 4 exposures every clear night rather than every two nights.

Catalina Sky Survey (including Mount Lemmon Survey)

In 1998, the Catalina Sky Survey (CSS) took over from Spacewatch in surveying the sky for the University of Arizona. It uses two telescopes, a 1.5 m Cassegrain reflector telescope on the peak of Mount Lemmon (also known as a survey in its own right, the Mount Lemmon Survey), and a 0.7 m Schmidt telescope near Mount Bigelow (both in the Tucson, Arizona area in the south west of the United States). Both sites use identical cameras which provide a field of view of 5 square degrees on the 1.5-m telescope and 19 square degrees on the Catalina Schmidt. The Cassegrain reflector telescope takes three to four weeks to survey the entire sky, detecting objects fainter than apparent magnitude 21.5. The 0.7 m telescope takes a week to complete a survey of the sky, detecting objects fainter than apparent magnitude 19. This combination of telescopes, one slow and one medium, has so far detected more near Earth Objects than any other single survey. This shows the need for a combination of different types of telescopes.

CSS used to include a telescope in the Southern Hemisphere, the Siding Spring Survey. However operations ended in 2013 after funding was discontinued.

Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope (LSST) is a wide-field survey reflecting telescope with an 8.4-meter primary mirror, currently under construction on Cerro Pachón in Chile. It will survey the entire available sky around every three nights. Science operations are due to begin in 2022. Scanning the sky relatively fast but also being able to detect objects down to apparent magnitude 27, it should be good at detecting nearby fast moving objects as well as excellent for larger slower objects that are currently further away.

NEO Survery Telescope

The Near Earth Object Survery TELescope (NEOSTEL) is an ESA funded project, starting with an initial prototype currently under construction. The telescope is of a new "fly-eye" design that combines a single reflector with multiple sets of optics and CCDs, giving a very wide field of view (around 45 square degrees). When complete it will have the widest field of view of any telescope and will be able to survey the majority of the visible sky in a single night. If the initial prototype is successful, three more telescopes are planned for installation around the globe. Because of the novel design, the size of the primary mirror is not directly comparable to more conventional telescopes, but is equivalent to a conventional 1 metre telescope.

The telescope itself should be complete by end of 2019, and installation on Mount Mufara, Sicily should be complete in 2020.

NEOWISE

Viewed from space by WISE using a thermal camera, asteroid 2010 AB78 appears redder than the background stars as it emits most of its light at longer infrared wavelengths. In visible light it is very faint and difficult to see.

The Wide-field Infrared Survey Explorer is a 0.4 m infrared-wavelength space telescope launched in December 2009, and placed in hibernation in February 2011. It was re-activated in 2013 specifically to search for near-Earth objects under the NEOWISE mission. By this stage, the spacecraft's cryogenic coolant had been depleted and so only two of the spacecraft's four sensors could be used. Whilst this has still led to new discoveries of asteroids not previously seen from ground-based telescopes, the productivity has dropped significantly. In its peak year when all four sensors were operational, WISE made 2.28 million observations. In recent years, with no cryogen, NEOWISE typically makes approximately 0.15 million observations annually. The next generation of infrared space telescopes has been designed so that they do not need cryogenic cooling.

Pan-STARRS

Pan-STARRS, the "Panoramic Survey Telescope And Rapid Response System", currently (2018) consists of two 1.8 m Ritchey–Chrétien telescopes located at Haleakala in Hawaii. It has discovered a large number of new asteroids, comets, variable stars, supernovae and other celestial objects. Its primary mission is now to detect mear-Earth objects that threaten impact events, and it is expected to create a database of all objects visible from Hawaii (three-quarters of the entire sky) down to apparent magnitude 24. The Pan-STARRS NEO survey searches all the sky north of declination −47.5. It takes three to four weeks to survey the entire sky.

Space Surveillance Telescope

The Space Surveillance Telescope (SST) is a 3.5 m telescope that detects, tracks, and can discern small, obscure objects, in deep space with a wide field of view system. The SST mount uses an advanced servo-control technology, that makes it one of the quickest and most agile telescopes of its size. It has a field of view of 6 square degrees and can scan the visible sky in 6 clear nights down to apparent magnitude 20.5. Its primary mission is tracking orbital debris. This task is similar to that of spotting near-Earth asteroids and so it is capable of both.

The SST was initially deployed for testing and evaluation at the White Sands Missile Range in New Mexico. On December 6, 2013, it was announced that the telescope system would be moved to the Naval Communication Station Harold E. Holt in Exmouth, Western Australia. The SST was moved to Australia in 2017, but due to the new site being in a cyclone region, construction has been delayed, pending a redesign that can withstand cyclone force winds.

Spacewatch

Spacewatch was an early sky survey focussed on finding near Earth asteroids, originally founded in 1980. It was the first to use CCD image sensors to search for them, and the first to develop software to detect moving objects automatically in real-time. This led to a huge increase in productivity. Before 1990 a few hundred observations were made each year. After automation, annual productivity jumped by a factor of 100 leading to tens of thousands of observations per year. This paved the way for the surveys we have today.

Although the survey is still in operation, in 1998 is was superseded by Catalina Sky Survey. Since then it has focused on following up on discoveries by other surveys, rather than making new discoveries itself. In particular it aims to prevent high priority PHOs from being lost after their discovery. The survey telescopes are 1.8 m and 0.9 m. The two follow up telescopes are 2.3 m and 4 m.

Zwicky Transient Facility

The Zwicky Transient Facility (ZTF) was commissioned in 2018, superseding the Intermediate Palomar Transient Factory (2009–2017). It is designed to detect transient objects that rapidly change in brightness as well as moving objects, for example supernovae, gamma ray bursts, collisions between two neutron stars, comets and asteroids. The ZTF is a 1.2 m telescope that has a field of view of 47 square degrees, designed to image the entire northern sky in three nights and scan the plane of the Milky Way twice each night to a limiting magnitude of 20.5. The amount of data produced by ZTF is expected to be 10 times larger than its predecessor.

Follow up observations

Once a new asteroid has been discovered and reported, other observers can confirm the finding and help define the orbit of the newly discovered object. The International Astronomical Union Minor Planet Center (MPC) acts as the global clearing house for information on asteroid orbits. It publishes lists of new discoveries that need verifying and accepts the resulting follow up observations from around the world. Unlike the initial discovery, which typically requires unusual and expensive wide-field telescopes, ordinary telescopes can be used to confirm the object as its position is now approximately known. There are far more of these around the globe, and even a well equipped amateur astronomer can contribute valuable follow-up observations of moderately bright asteroids. For example, the Great Shefford Observatory in the back garden of amateur Peter Birtwhistle typically submits thousands of observations to the Minor Planet Center every year. Nonetheless, some surveys (for example CSS and Spacewatch) have their own dedicated follow up telescopes.

Follow up observations from professionals and amateurs alike are important because once a sky survey has reported a discovery it may not return to observe the object again for days or weeks, by which time it may be too faint for it to detect. The more observations that are made of an object and the longer the elapsed time it is observed over, the greater the accuracy of the orbit model. This is important for two reasons:

1. for imminent impacts it helps to make a better prediction of where the impact will occur and whether there is any danger of hitting a populated area.
2. for asteroids that will miss Earth this time round, the more accurate the orbit model is, the further into the future its position can be predicted. This allows impacts to be predicted years in advance.

Estimating impact severity using infrared (IR)

Assessing the size of the asteroid is important for predicting the severity of the impact, and therefore the actions that need to be taken (if any). Because of this, one key follow up observation is to view the asteroid in the thermal infrared spectrum (long-wavelength infrared), using an infrared telescope. The amount of thermal radiation given off by an asteroid allows a much more accurate assessment of its size than how bright it appears (apparent magnitude) to an ordinary telescope that operates in the visible spectrum. Using thermal infrared, it is possible to estimate the size to within about 10% of the true size. With reflected visible light observed by a conventional telescope, the object could be anything from 50% to 200% of the estimated size.

One example of a such a follow up observation was for 3671 Dionysus by UKIRT, the worlds largest infrared telescope at the time (1997). However such follow ups are rare. The size estimates of most near-Earth asteroids are based on visible light only.

It should be noted that if the object was discovered by an infrared survey telescope initially, then an accurate size estimate will already be available, and infrared follow up will not be needed. However none of the ground-based survey telescopes listed above operate at thermal infrared wavelengths, and although NEOWISE used to have two thermal infrared sensors, they have stopped working since the cryogen ran out. There are therefore currently no active or planned thermal infrared sky surveys which are focused on discovering near-Earth objects.

Impact calculation

Minimum orbit intersection distance

The minimum orbit intersection distance (MOID) between an asteroid and the Earth is the distance between the closest points of their orbits. This first check is a coarse measure that does not allow an impact prediction to be made, but is based solely on the orbit parameters and gives an initial measure of how close to Earth the asteroid could come. If the MOID is large then the two objects never come near each other. In this case, unless the orbit of the asteroid is perturbed so that the MOID is reduced at some point in the future, it will never impact Earth and can be ignored. However if the MOID is small then it is necessary to carry out more detailed calculations to determine if an impact will happen in the future. NASA considers asteroids with a MOID of less than 0.05 AU and an absolute magnitude brighter than 22 to be a potentially hazardous asteroid.

Projecting into the future

Orbit and positions of 2018 LA and Earth, 30 days before impact. The diagram shows how orbit data can be used to predict impacts well in advance. This particular asteroid's orbit was only known a few hours before impact. The diagram was made later.

Once the orbit model of an asteroid has been refined, its position can be forecast sufficiently far into the future to check its projected position against the future projected position of Earth. If the distance between the centre of the asteroid and the centre of the Earth is less than Earth radius then an impact is predicted. To take account of the uncertainties (however small) in the orbit of the asteroid, several future projections are made (simulations). Each simulation has slightly different parameters within the range of the uncertainty. This allows a percentage chance of impact to be estimated. For example if 1,000 simulations are carried out and 739 result in an impact, then the prediction would be a 73.9% chance of impact.

NEODyS

NEODyS (Near Earth Objects Dynamic Site) is a European Space Agency service that provides information on near Earth objects. It is based on a continually and (almost) automatically maintained database of near earth asteroid orbits. The site provides a number of services to the NEO community. The main service is an impact monitoring system (CLOMON2) of all near-Earth asteroids covering a period until the year 2100.

The NEODyS website includes a Risk Page where all NEOs with probabilities of hitting the Earth greater than 10⁻¹¹ from now until 2100 are shown in a risk list. In the table of the risk list the NEOs are divided into:

• "special", as it is the case of (99942) Apophis
• "observable", objects which are presently observable and which critically need a follow up in order to improve their orbit
• "possible recovery", objects which are not visible at present, but which are possible to recover in the near future
• "lost", objects which have an absolute magnitude (H) brighter than 25 but which are virtually lost, their orbit being too uncertain; and
• "small", objects with an absolute magnitude fainter than 25 and, even if they are "lost", they are considered too small to result in heavy damage on the ground (though it should be noted that the Chelyabinsk meteor would have been fainter than this).

Each object has its own impactor table (IT) which shows many parameters useful to determine the risk assessment.

Sentry prediction system

NASA's Sentry System continually scans the MPC catalog of known asteroids, analyzing their orbits for any possible future impacts. Like ESA's NEODyS, it gives a MOID for each near-Earth object, and a list of possible future impacts, along with the probability of each. It uses a slightly different algorithm to NEODyS, and so provides a useful cross-check and corroboration. 

Currently, no impacts are predicted (the single highest probability impact currently listed is ~7 m asteroid 2010 RF12, which is due to pass Earth in September 2095 with only a 5% predicted chance of impacting).

Impact probability calculation pattern

Why predicted asteroid impact probability often goes up, then down.

The ellipses in the diagram on the right show the predicted position of an example asteroid at closest Earth approach. At first, with only a few asteroid observations, the error ellipse is very large and includes the Earth. Further observations shrink the error ellipse, but it still includes the Earth. This raises the predicted impact probability, since the Earth now covers a larger fraction of the error region. Finally, yet more observations (often radar observations, or discovery of a previous sighting of the same asteroid on archival images) shrink the ellipse revealing that the Earth is outside the error region, and the impact probability is near zero.

For asteroids that are actually on track to hit Earth the predicted probability of impact continues to increase as more observations are made. This very similar pattern makes it difficult to differentiate between asteroids which will only come close to Earth and those which will actually hit it. This in turn makes it difficult to decide when to raise an alarm as gaining more certainty takes time, which reduces the time available to react to a predicted impact. However raising the alarm too soon has the danger of causing a false alarm and creating a Boy Who Cried Wolf effect if the asteroid in fact misses Earth.

Response to predicted impact

Once an impact has been predicted the potential severity needs to be assessed, and a response plan formed. Depending on the time to impact and the predicted severity this may be as simple as giving a warning to citizens. For example, although unpredicted, the 2013 impact at Chelyabinsk was spotted through the window by teacher Yulia Karbysheva. She thought it prudent to take precautionary measures by ordering her students to stay away from the room's windows and to perform a duck and cover maneuver. The teacher, who remained standing, was seriously lacerated when the blast arrived and window glass severed a tendon in one of her arms and left thigh, but none of her students, whom she ordered to hide under their desks, suffered cuts. If the impact had been predicted and a warning had been given to the entire population, similar simple precautionary actions could have vastly reduced the number of injuries. Children who were not in her class were injured.

If a more severe impact is predicted, the response may require evacuation of the area, or an avoidance mission to repel the asteroid. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.

Improving impact prediction

In addition to the already funded telescopes mentioned above, two separate approaches have been suggested by NASA to improve impact prediction. Both approaches focus on the first step in impact prediction (discovering near-Earth asteroids) as this is the largest weakness in the current system. The first approach uses more powerful ground-based telescopes similar to the LSST. This will improve detection, but being ground-based will only observe part of the sky around Earth. In particular, all ground-based telescopes have a large blind spot for any asteroids coming from the direction of the Sun. In addition, they are affected by weather conditions, airglow and the phase of the Moon. 

Ground based telescopes can only detect objects approaching on the night-side of the  planet, away from the Sun. Roughly half of impacts occur on the day-side of the planet.

To get around all of these issues, the second approach suggested is the use of space-based telescopes which can observe a much larger region of the sky around Earth. Although they still cannot point directly towards the Sun, they do not have the problem of blue sky to overcome and so can detect asteroids much closer in the sky to the Sun than ground-based telescopes. Unaffected by weather or airglow they can also operate 24 hours per day all year round. Finally, telescopes in space have the advantage of being able to use infrared sensors without the interference of the Earth's atmosphere. These sensors are better for detecting asteroids than optical sensors, and although there are some ground based infrared telescopes such as UKIRT, they are not designed for detecting asteroids. Space-based telescopes are more expensive, however, and tend to have a shorter lifespan. Therefore, Earth-based and space-based technologies complement each other to an extent. Although the majority of the IR spectrum is blocked by Earth's atmosphere, the very useful thermal (long-wavelength infrared) frequency band is not blocked (see gap at 10 μm in the diagram below). This allows for the possibility of ground based thermal imaging surveys designed for detecting near earth asteroids, though none are currently planned. 

A diagram of the electromagnetic spectrum and the types of telescope used to view different parts of it

Opposition effect

There is a further issue that even telescopes in Earth orbit do not overcome (unless they operate in the thermal infrared spectrum). This is the issue of illumination. Asteroids go through phases similar to the lunar phases. Even though a telescope in orbit may have an unobstructed view of an object that is close in the sky to the Sun, it will still be looking at the dark side of the object. This is because the Sun is shining primarily on the side facing away from the Earth, as is the case with the Moon when it is in a crescent phase. Because of the opposition effect, objects are far less bright in these phases than when fully illuminated, which makes them difficult to detect (see diagram below). 

Due to the opposition effect over half (53%) of the discoveries of near Earth objects were made in 3.8% of the sky, in a 22.5° cone facing directly away from the Sun, and the vast majority (87%) were made in 15% of the sky, in a 45° cone facing away from the Sun.

This problem can be solved by the use of thermal infrared surveys (either ground based or space based). Ordinary telescopes depend on observing light reflected from the Sun, which is why the opposition effect occurs. Telescopes which detect thermal infrared light depend only on the temperature of the object. Its thermal glow can be detected from any angle, and is particularly useful for differentiating asteroids from the background stars which have a different thermal signature.

This problem can also be solved without using thermal infrared, by positioning a space telescope away from Earth, closer to the Sun. The telescope can then look back towards Earth from the same direction as the Sun, and any asteroids closer to Earth than the telescope will then be in opposition, and much better illuminated. There is a point between the Earth and Sun where the gravities of the two bodies are perfectly in balance, called the Sun-Earth L1 Lagrange point (SEL1). It is approximately 1 million miles from Earth, about four times as far away as the Moon, and is ideally suited for placing such a space telescope. One problem with this position is Earth glare. Looking outward from SEL1, Earth itself is at full brightness, which prevents a telescope situated there from seeing that area of sky. Fortunately, this is the same area of sky that ground-based telescopes are best at spotting asteroids in, so the two complement each other. 

Another possible position for a space telescope would be even closer to the Sun, for example in a Venus-like orbit. This would give a wider view of Earth orbit, but at a greater distance. Unlike a telescope at the SEL1 Lagrange point, it would not stay in sync with Earth but would orbit the Sun at a similar rate to Venus. Because of this, it would not often be in a position to provide any warning of asteroids shortly before impact, but it would be in a good position to catalog objects before they are on final approach, especially those which primarily orbit closer to the Sun. One issue with being as close to the Sun as Venus is that the craft may be too warm to use infrared wavelengths. A second issue would be communications. As the telescope will be a long way from Earth for most of the year (and even behind the Sun at some points) communication would often be slow and at times impossible, without expensive improvements to the Deep Space Network.

Solutions to problems: summary table

This table summarises which of the various problems encountered by current telescopes are solved by the various different solutions. 

Proposed solution	Global coverage	Clouds	Blue sky	Full moon	Opposition Effect	Thermal Infrared	Airglow
Geographically separated ground based survey telescopes	✓
More powerful ground based survey telescopes					✓
Infrared ground based NEO survey telescopes					✓	✓
Telescope in Earth orbit	✓	✓	✓	✓			✓
Infrared Telescope in Earth orbit	✓	✓	✓	✓	✓	✓	✓
Telescope at SEL1	✓	✓	✓	✓	✓		✓
Infrared Telescope at SEL1	✓	✓	✓	✓	✓	✓	✓
Telescope in Venus-like orbit		✓	✓	✓	✓		✓

NEOCAM

In 2017 NASA proposed a number of alternative solutions to detect 90% of near-Earth objects of size 140 m or larger over the next few decades, which will also improve detection rates for the smaller objects which impact Earth more often. Several of the proposals use a combination of an improved ground based telescope and a space based telescope positioned at the SEL1 Lagrange point such as NEOCAM. However none of these proposals have yet been funded. As this is a global issue, and noting that to date NASA-sponsored surveys have contributed over 95% of all near earth object discoveries, in 2018 the Trump administration asked NASA to find international partners to help fund the improvements.

List of successfully predicted asteroid impacts

Below is the list of all near-Earth objects which may have impacted the Earth and which were successfully predicted beforehand. This list would also include any objects identified as having greater than 50% chance of impacting in the future, but none of the future impacts have been predicted at this time. As asteroid detection ability increases it is expected that prediction will become more successful in the future. 

Date of impact	Date discovered	Object	Observation arc (minutes)	Warning period (days)	Cataloged	Size (m)	(H) (abs. mag)	Velocity (km/s)	Explosion Altitude (km)	Impact Energy (kt)
2008-10-07	2008-10-06	2008 TC3	1,145	0.7	No	4.1	30.4	12.8	37	0.98
2014-01-02	2014-01-01	2014 AA	69	0.8	No	2–4	30.9	35.0	unknown	unknown
2018-01-22	2018-01-22	A106fgF	39	0.4	No	1–4	31.1	unknown	N/A (impact unconfirmed)	N/A (impact unconfirmed)
2018-06-02	2018-06-02	2018 LA	227	0.3	No	2.6–3.8	30.6	17	28.7	1