Asteroid impact avoidance

thumb|Kinetic impactors such as the one used by the [[Double Asteroid Redirection Test – its impact with the asteroid moon Dimorphos photographed above – are one of many methods, designed to alter the trajectory of an asteroid to prevent its potential collision with Earth.]]

thumb|upright=1.4|Damage caused by the [[Tunguska event. The object was across and exploded above the surface; its explosion flattened 30 million trees and shattered windows hundreds of kilometers away.]]

Asteroid impact avoidance encompasses the methods by which near-Earth objects (NEO) on a potential collision course with Earth could be diverted, preventing destructive impact events. An impact by a sufficiently large asteroid or other NEOs would cause, depending on its impact location, massive tsunamis or multiple firestorms, and an impact winter caused by the sunlight-blocking effect of large quantities of pulverized rock dust and other debris placed into the stratosphere. A collision 66 million years ago between the Earth and an object approximately wide is thought to have produced the Chicxulub crater and triggered the Cretaceous–Paleogene extinction event that the scientific community understands to have caused the extinction of all non-avian dinosaurs.

While the chances of a major collision are low in the near term, it is a near-certainty that one will happen eventually unless defensive measures are taken. Astronomical events—such as the Shoemaker-Levy 9 impacts on Jupiter and the 2013 Chelyabinsk meteor, along with the growing number of near-Earth objects discovered and catalogued on the Sentry Risk Table—have drawn renewed attention to such threats. The popularity of the 2021 movie Don't Look Up helped to raise awareness of the possibility of avoiding NEOs. Awareness of the threat has grown rapidly during the past few decades, but much more needs to be accomplished before the human population can feel adequately protected from a potentially catastrophic asteroid impact.

In 2016, a NASA scientist warned that the Earth is unprepared for such an event. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit by a devastating asteroid, but we're not 100 percent 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.

Several ways of avoiding an asteroid impact have been described. There are two primary ways: to modify the trajectory of the object so that it does not collide with the Earth, or to modify the object by breaking it up so that the resulting fragments do not collide with the Earth or their

smaller size reduces the subsequent hazard posed to the Earth.

Nonetheless, in March 2019, scientists reported that asteroids may be much more difficult to destroy than thought earlier. An asteroid may reassemble itself due to gravity after being disrupted. In May 2021, NASA astronomers reported that 5 to 10 years of preparation may be needed to avoid a virtual impactor based on a simulated exercise conducted by the 2021 Planetary Defense Conference.

In 2022, NASA spacecraft DART impacted Dimorphos, reducing the minor-planet moon's orbital period by 32 minutes. This mission constitutes the first successful attempt at asteroid deflection. In 2027, China plans to launch a deflection mission to the near-Earth object 2015 XF261, with the impact estimated to occur in April 2029.

Deflection efforts

thumb|upright=1.5|Known [[Near-Earth objects as of January 2018 Video (0:55; July 23, 2018) (Earth's orbit in white)]]

thumb|upright=1.5|Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.

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. In June 2018, the US National Science and Technology Council warned that the United States was unprepared for an asteroid impact event, and developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.

Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision-avoidance project, as no known planetary defense hardware has yet been developed. It has been estimated that a velocity change of just (where t is the number of years until potential impact) is needed to successfully deflect a body on a direct collision trajectory. Thus for a large number of years before impact, much smaller velocity changes are needed. For example, it was estimated there was a high chance of 99942 Apophis swinging by Earth in 2029 with a 10−4 probability of returning on an impact trajectory in 2035 or 2036. It was then determined that a deflection from this potential return trajectory, several years before the swing-by, could be achieved with a velocity change on the order of 10−6 m/s.

NASA's Double Asteroid Redirection Test (DART), the world's first full-scale mission to test technology for defending Earth against potential asteroid or comet hazards, launched on a SpaceX Falcon 9 rocket from Space Launch Complex 4 East at Vandenberg Space Force Base in California.

An impact by a asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets entering the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months. Impacts from objects as small as in diameter, which are far more common, are historically extremely destructive regionally (see Barringer crater).

Finding out the material composition of the object is also helpful before deciding which strategy is appropriate. Missions like the 2005 Deep Impact probe and the Rosetta spacecraft, have provided valuable information on what to expect. In October 2022, a method of mapping the insides of a potentially problematic asteroid in order to determine the best area for impact was proposed.

History of US government mandates

Efforts in asteroid impact prediction have concentrated on the survey method. The 1992 NASA-sponsored Near-Earth-Object Interception Workshop hosted by Los Alamos National Laboratory evaluated issues involved in intercepting celestial objects that could hit Earth. In a 1992 report to NASA, a coordinated Spaceguard Survey was recommended to discover, verify and provide follow-up observations for Earth-crossing asteroids. This survey was expected to discover 90% of these objects larger than one kilometer within 25 years. Three years later, another NASA report recommended search surveys that would discover 60–70% of 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.

Because of Congressman Brown's long-standing commitment to planetary defense, a U.S. House of Representatives' bill, H.R. 1022, was named in his honor: The George E. Brown Jr. Near-Earth Object Survey Act. This bill "to provide for a Near-Earth Object Survey program to detect, track, catalogue, and characterize certain near-Earth asteroids and comets" was introduced in March 2005 by Rep. Dana Rohrabacher (R-CA). It was eventually rolled into S.1281, the NASA Authorization Act of 2005, passed by Congress on December 22, 2005, subsequently signed by the President, and stating in part:

The result of this directive was a report presented to Congress in early March 2007. This was an Analysis of Alternatives (AoA) study led by NASA's Program Analysis and Evaluation (PA&E) office with support from outside consultants, the Aerospace Corporation, NASA Langley Research Center (LaRC), and SAIC (amongst others).

Ongoing projects

thumb|Number of NEOs detected by various projects.

thumb|[[NEOWISE first four years of data starting in December 2013 (animated; April 20, 2018)]]

The Minor Planet Center in Cambridge, Massachusetts has been cataloging the orbits of asteroids and comets since 1947. It has recently been joined by surveys that specialize in locating the near-Earth objects (NEO), many (as of early 2007) funded by NASA's Near Earth Object program office as part of their Spaceguard program. One of the best-known is LINEAR that began in 1996. By 2004 LINEAR was discovering tens of thousands of objects each year and accounting for 65% of all new asteroid detections. LINEAR uses two one-meter telescopes and one half-meter telescope based in New Mexico.

The Catalina Sky Survey (CSS) is conducted at the Steward Observatory's Catalina Station, located near Tucson, Arizona, in the United States. It uses two telescopes, a f/2 telescope on the peak of Mount Lemmon, and a f/1.7 Schmidt telescope near Mount Bigelow (both in the Tucson, Arizona area). In 2005, CSS became the most prolific NEO survey surpassing Lincoln Near-Earth Asteroid Research (LINEAR) in total number of NEOs and potentially hazardous asteroids discovered each year since. CSS discovered 310 NEOs in 2005, 396 in 2006, 466 in 2007, and in 2008 564 NEOs were found.

Spacewatch, which uses a telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis equipment to search the skies for intruders, was set up in 1980 by Tom Gehrels and Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by McMillan. The Spacewatch project has acquired a telescope, also at Kitt Peak, to hunt for NEOs, and has provided the old 90-centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability.

Other near-Earth object tracking programs include Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Campo Imperatore Near-Earth Object Survey (CINEOS), Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey. Pan-STARRS completed telescope construction in 2010, and it is now actively observing.

The Asteroid Terrestrial-impact Last Alert System, in operation since 2015, conducts almost nightly scans of the night sky with a view to later-stage detection on the collision stretch of the asteroid orbit. Those would be much too late for deflection, but still in time for evacuation and preparation of the affected Earth region.

Another project, supported by the European Union, is NEOShield, which analyses realistic options for preventing the collision of a NEO with Earth. Their aim is to provide test mission designs for feasible NEO mitigation concepts. The project particularly emphasises on two aspects.

The first one is the focus on technological development on essential techniques and instruments needed for guidance, navigation and control (GNC) in close vicinity of asteroids and comets. This will, for example, allow hitting such bodies with a high-velocity kinetic impactor spacecraft and observing them before, during and after a mitigation attempt, e.g., for orbit determination and monitoring.
The second one focuses on refining Near Earth Object (NEO) characterisation. Moreover, NEOShield-2 will carry out astronomical observations of NEOs, to improve the understanding of their physical properties, concentrating on the smaller sizes of most concern for mitigation purposes, and to identify further objects suitable for missions for physical characterisation and NEO deflection demonstration.

"Spaceguard" is the name for these loosely affiliated programs, some of which receive NASA funding to meet a U.S. Congressional requirement to detect 90% of near-Earth asteroids over 1 km diameter by 2008. A 2003 NASA study of a follow-on program suggests spending US$250–450 million to detect 90% of all near-Earth asteroids and larger by 2028.

NEODyS is an online database of known NEOs.

Notification and response coordination

In October 2013, the United Nations Committee on the Peaceful Uses of Outer Space approved several measures to deal with terrestrial asteroid impacts, including the creation of an International Asteroid Warning Network (IAWN) to act as a clearinghouse for shared information on dangerous asteroids and for any future terrestrial impact events that are identified. Space Missions Planning Advisory Group (SMPAG) should coordinate joint studies of the technologies for deflection missions, and as well provide oversight of actual missions. This is due to deflection missions typically involving a progressive movement of an asteroid's predicted impact point across the surface of the Earth (and also across the territories of uninvolved countries) until the NEO has been deflected either ahead of, or behind the planet at the point their orbits intersect. UN General Assembly endorsed the establishment of IAWN through its resolution 68/75 on 16 December 2023. IAWN's main task is to warn of a possible impact threat, if the following criteria are reached: an impact probability of >1% within the next 20 years, for an object larger than about 10 meters in size. The number of known NEOs was 34,274 as of 30 January 2024, with 2,395 known asteroids whose orbits bring them within 8 million kilometers of Earth's orbit and with diameters larger than about 140 m. Yet, it is estimated only about 44% of the NEOs of that size range have been found so far.

The first time the notification threshold was reached was during the process of refining the orbital parameters of 2024 YR4. The United Nations Office for Outer Space Affairs emailed United States Space Force, among others. Potential impactors are publicly posted on the IAWN home page. There are no firm commitments from spacefaring nations as to what response would be mounted to any dangerous Earth-impacting asteroids. Planning for various scenarios is still underway as of 2025, including impacts affecting the whole planet vs. a spacefaring country vs. an area without the economic means to launch a spacecraft or territory to evacuate effectively. The foundation also proposes asteroid deflection of potentially dangerous NEOs by the use of gravity tractors to divert their trajectories away from Earth, a concept co-invented by the organization's CEO, physicist and former NASA astronaut Ed Lu.

Prospective projects

Orbit@home intends to provide distributed computing resources to optimize search strategy. On February 16, 2013, the project was halted due to lack of grant funding. However, on July 23, 2013, the orbit@home project was selected for funding by NASA's Near Earth Object Observation program and was to resume operations sometime in early 2014. As of July 13, 2018, the project is offline according to its website.

The Vera C. Rubin Observatory began a comprehensive, high-resolution decade-long survey starting in the fall of 2025. It is expected to catalog 80–90% of potentially hazardous asteroids larger than 140 meters.

Detection from space

On November 8, 2007, the House Committee on Science and Technology's Subcommittee on Space and Aeronautics held a hearing to examine the status of NASA's Near-Earth Object survey program. The prospect of using the Wide-field Infrared Survey Explorer was proposed by NASA officials.

WISE surveyed the sky in the infrared band at a very high sensitivity. Asteroids that absorb solar radiation can be observed through the infrared band. It was used to detect NEOs, in addition to performing its science goals. It is projected that WISE could detect 400 NEOs (roughly two percent of the estimated NEO population of interest) within the one-year mission.

NEOSSat, the Near Earth Object Surveillance Satellite, is a microsatellite launched in February 2013 by the Canadian Space Agency (CSA) that will hunt for NEOs in space. Furthermore Near-Earth Object WISE (NEOWISE), an extension of the WISE mission, started in September 2013 (in its second mission extension) to hunt asteroids and comets close to the orbit of Earth.

Deep Impact

Research published in the March 26, 2009 issue of the journal Nature, describes how scientists were able to identify an asteroid in space before it entered Earth's atmosphere, enabling computers to determine its area of origin in the Solar System as well as predict the arrival time and location on Earth of its shattered surviving parts. The four-meter-diameter asteroid, called 2008 TC3, was initially sighted by the automated Catalina Sky Survey telescope, on October 6, 2008. Computations correctly predicted that it would impact 19 hours after discovery and in the Nubian Desert of northern Sudan.

A number of potential threats have been identified, such as 99942 Apophis (previously known by its provisional designation ), which in 2004 temporarily had an impact probability of about 3% for the year 2029. Additional observations revised this probability down to zero.

Double Asteroid Redirection Test

On September 26, 2022 DART impacted Dimorphos, reducing the minor-planet moon's orbital period by 32 minutes. This mission was the first successful attempt at asteroid deflection.

For asteroids that are actually on track to hit Earth the predicted probability of impact continues to increase as more observations are made. This similar pattern makes it initially difficult to differentiate between asteroids that will only come close to Earth and those that 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 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.

Collision avoidance strategies

Cost, risk of failure, complexity, technology readiness, and overall performance are all important trade-offs in weighing collision avoidance strategies. Methods can be differentiated by the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception, rendezvous, or remote station).

Strategies fall into two basic sets: fragmentation and delay. Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or are small enough to burn up in the atmosphere. Delay exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time, or more correctly when some point on Earth's surface intersects the impactor's orbit when the impactor arrives. Since the Earth is approximately in diameter and moves at approximately in its orbit, it travels a distance of one planetary diameter in about 425 seconds, or slightly over seven minutes. Delaying, or advancing the impactor's arrival by times of this magnitude can, depending on the exact geometry of the impact, cause it to miss the Earth.

Collision avoidance strategies can also be seen as either direct, or indirect and in how rapidly they transfer energy to the object. The direct methods, such as nuclear explosives, or kinetic impactors, rapidly intercept the bolide's path. Direct methods are preferred because they are generally less costly in time and money. Their effects may be immediate, thus saving precious time. These methods would work for short-notice and long-notice threats, and are most effective against solid objects that can be directly pushed, but in the case of kinetic impactors, they are not very effective against large loosely aggregated rubble piles. Indirect methods, such as gravity tractors, attaching rockets or mass drivers, are much slower. They require traveling to the object, changing course up to 180 degrees for space rendezvous, and then taking much more time to change the asteroid's path just enough so it will miss Earth.

Many NEOs are thought to be "flying rubble piles" only loosely held together by gravity, and a typical spacecraft sized kinetic-impactor deflection attempt might just break up the object or fragment it without sufficiently adjusting its course. If an asteroid breaks into fragments, any fragment larger than across would not burn up in the atmosphere and itself could impact Earth. Tracking the thousands of buckshot-like fragments that could result from such an explosion would be a very daunting task, although fragmentation would be preferable to doing nothing and allowing the originally larger rubble body, which is analogous to a shot and wax slug, to impact the Earth.

In Cielo simulations conducted in 2011–2012, in which the rate and quantity of energy delivery were sufficiently high and matched to the size of the rubble pile, such as following a tailored nuclear explosion, results indicated that any asteroid fragments, created after the pulse of energy is delivered, would not pose a threat of re-coalescing (including for those with the shape of asteroid Itokawa) but instead would rapidly achieve escape velocity from their parent body (which for Itokawa is about 0.2 m/s) and therefore move out of an Earth-impact trajectory.

Nuclear explosive device

thumb|upright=1.5|In a similar manner to the earlier pipes filled with a [[partial pressure of helium, as used in the Ivy Mike test of 1952, the 1954 Castle Bravo test was likewise heavily instrumented with line-of-sight (LOS) pipes, to better define and quantify the timing and energies of the x-rays and neutrons produced by these early thermonuclear devices. One of the outcomes of this diagnostic work resulted in this graphic depiction of the transport of energetic x-ray and neutrons through a vacuum line, some 2.3 km long, whereupon it heated solid matter at the "station 1200" blockhouse and thus generated a secondary fireball.]]

Initiating a nuclear explosive device above, on, or slightly beneath, the surface of a threatening celestial body is a potential deflection option, with the optimal detonation height dependent upon the composition and size of the object. It does not require the entire NEO to be vaporized to mitigate an impact threat. In the case of an inbound threat from a "rubble pile", the stand off, or detonation height above the surface configuration, has been put forth as a means to prevent the potential fracturing of the rubble pile. The energetic neutrons and soft X-rays released by the detonation, which do not appreciably penetrate matter, are converted into heat upon encountering the object's surface matter, ablatively vaporizing all line of sight exposed surface areas of the object to a shallow depth, Depending on the energy of the explosive device, the resulting rocket exhaust effect, created by the high velocity of the asteroid's vaporized mass ejecta, coupled with the object's small reduction in mass, would produce enough of a change in the object's orbit to make it miss the Earth. While there have been no updates as of 2023 regarding the HAMMER, NASA has published its regular Planetary Defense Strategy and Action Plan for 2023. In it, NASA acknowledges that it is crucial to continue studying the potential of nuclear energy in deflecting or destroying asteroids. This is because it is currently the only option for defense if scientists were not aware of the asteroid within a few months or years, depending on the asteroid's velocity. The report also notes there needs to be research done into the legal implications as well as policy implications on the topic.

Stand-off approach

If the object is very large but is still a loosely-held-together rubble pile, a solution is to detonate one or a series of nuclear explosive devices alongside the asteroid, at a or greater stand-off height above its surface, so as not to fracture the potentially loosely-held-together object. Providing that this stand-off strategy was done far enough in advance, the force from a sufficient number of nuclear blasts would alter the object's trajectory enough to avoid an impact, according to computer simulations and experimental evidence from meteorites exposed to the thermal X-ray pulses of the Z-machine.

In 1967, graduate students under Professor Paul Sandorff at the Massachusetts Institute of Technology were tasked with designing a method to prevent a hypothetical 18-month distant impact on Earth by the asteroid 1566 Icarus, an object that makes regular close approaches to Earth, sometimes as close as 16 lunar distances. To achieve the task within the timeframe and with limited material knowledge of the asteroid's composition, a variable stand-off system was conceived. This would have used a number of modified Saturn V rockets sent on interception courses and the creation of a handful of nuclear explosive devices in the 100-megaton energy range—coincidentally, the same as the maximum yield of the Soviets' Tsar Bomba would have been if a uranium tamper had been used—as each rocket vehicle's payload. The design study was later published as Project Icarus which served as the inspiration for the 1979 film Meteor.

A NASA analysis of deflection alternatives, conducted in 2007, stated:

In the same year, NASA released a study where the asteroid Apophis (with a diameter of around ) was assumed to have a much lower rubble pile density () and therefore lower mass than it is now known to have, and in the study, it is assumed to be on an impact trajectory with Earth for the year 2029. Under these hypothetical conditions, the report determines that a "Cradle spacecraft" would be sufficient to deflect it from Earth impact. This conceptual spacecraft contains six B83 physics packages, each set for their maximum 1.2-megatonne yield, These effectiveness figures are considered to be "conservative" by its authors, and only the thermal X-ray output of the B83 devices was considered, while neutron heating was neglected for ease of calculation purposes.

Research published in 2021 pointed out the fact that for an effective deflection mission, there would need to be a significant amount of warning time, with the ideal being several years or more. The more warning time provided, the less energy will be necessary to divert the asteroid just enough to adjust the trajectory to avoid Earth. The study also emphasized that deflection, as opposed to destruction, can be a safer option, as there is a smaller likelihood of asteroid debris falling to Earth's surface. The researchers proposed the best way to divert an asteroid through deflection is adjusting the output of neutron energy in the nuclear explosion.

Surface and subsurface use

thumb|This early [[Asteroid Redirect Mission artist's impression is suggestive of another method of changing a large threatening celestial body's orbit by capturing relatively smaller celestial objects and using those, and not the usually proposed small bits of spacecraft, as the means of creating a powerful kinetic impact, or alternatively, a stronger faster acting gravitational tractor, as some low-density asteroids such as 253 Mathilde can dissipate impact energy.]]

In 2011, the director of the Asteroid Deflection Research Center at Iowa State University, Dr. Bong Wie (who had published kinetic impactor deflection studies

A similar proposal would use a surface-detonating nuclear device in place of the kinetic impactor to create the initial crater, then using the crater as a rocket nozzle to channel succeeding nuclear detonations.

Wie claimed the computer models he worked on showed the possibility for a asteroid to be destroyed using a single HAIV with a warning time of 30 days. Additionally, the models showed that less than 0.1% of debris from the asteroid would reach Earth's surface. There have been few substantial updates from Wie and his team since 2014 regarding the research.

As of 2015, Wie has collaborated with the Danish Emergency Asteroid Defence Project (EADP), which intends to crowdsource sufficient funds to design, build, and store a non-nuclear HAIV spacecraft as planetary insurance. For threatening asteroids too large or close to Earth impact to effectively be deflected by the non-nuclear HAIV approach, nuclear explosive devices (with 5% of the explosive yield than those used for the stand-off strategy) are intended to be used, under international oversight, when conditions arise that necessitate it.

A study published in 2020 pointed out that a non-nuclear kinetic impact becomes less effective the larger and closer the asteroid. However, researchers ran a model that suggested a nuclear detonation near the surface of an asteroid designed to cover one side of the asteroid with x-rays would be effective. When the x-rays cover one side of an asteroid in the program, the energy would propel the asteroid in a preferred direction. The lead researcher with the study, Dave Dearborn, said a nuclear impact offered more flexibility than a non-nuclear approach, as the energy output can be adjusted specifically to the asteroid's size and location.

Comet deflection possibility

thumb|right|"Who knows whether, when a comet shall approach this globe to destroy it ... men will not tear rocks from their foundations by means of steam, and hurl mountains, as the giants are said to have done, against the flaming mass?" — [[Lord Byron]]

Following the 1994 Shoemaker-Levy 9 comet impacts with Jupiter, Edward Teller proposed, to a collective of U.S. and Russian ex-Cold War weapons designers in a 1995 planetary defense workshop meeting at Lawrence Livermore National Laboratory (LLNL), that they collaborate to design a one-gigaton nuclear explosive device, which would be equivalent to the kinetic energy of a asteroid. The theoretical one-gigaton device would weigh about 25–30 tons, light enough to be lifted on the Energia rocket. It could be used to instantaneously vaporize a one-kilometer asteroid, divert the paths of ELE-class asteroids (greater than in diameter) within short notice of a few months. With one year of notice, and at an interception location no closer than Jupiter, it could also deal with the even rarer short period comets that can come out of the Kuiper belt and transit past Earth orbit within two years. For comets of this class, with a maximum estimated diameter of , Chiron served as the hypothetical threat. The deal was meant to complement New START, but Russia suspended its participation in the treaty in 2023. As of April 2023, there has not been an official update from the White House or Moscow on how Russia's suspended participation will affect adjacent treaties.

Present capability

As of late 2022, the most likely and most effective method for asteroid deflection does not involve nuclear technology. Instead, it involves a kinetic impactor designed to redirect the asteroid, which showed promise in the NASA DART mission. For nuclear technology, simulations have been run analyzing the possibility of using neutron energy put off by a nuclear device to redirect an asteroid. These simulations showed promise, with one study finding that increasing the neutron energy output had a notable effect on the angle of the asteroid's travel. Impact created a crater estimated to be about 150 meters in diameter. The comet "returned to preimpact conditions only 6 days after the event".]]

The impact of a massive object, such as a spacecraft or even another near-Earth object, is another possible solution to a pending NEO impact. An object with a high mass close to the Earth could be sent out into a collision course with the asteroid, knocking it off course.

When the asteroid is still far from the Earth, a means of deflecting the asteroid is to directly alter its momentum by colliding a spacecraft with the asteroid.

thumb|upright=1.2|Compiled timelapse of DART's final 5.5 minutes until impact

A NASA analysis of deflection alternatives, conducted in 2007, stated: