Original:  A 2003 NASA study of a follow-on program suggests spending US $250–450 million to detect 90\% of all near-Earth asteroids 140 meters and larger by 2028. NEODyS is an online database of known NEOs. Sentinel mission The B612 Foundation is a private nonprofit foundation with headquarters in the United States, dedicated to protecting the Earth from asteroid strikes. It is led mainly by scientists, former astronauts and engineers from the Institute for Advanced Study, Southwest Research Institute, Stanford University, NASA and the space industry. As a non-governmental organization it has conducted two lines of related research to help detect NEOs that could one day strike the Earth, and find the technological means to divert their path to avoid such collisions. The foundation's current goal is to design and build a privately financed asteroid-finding space telescope, Sentinel, to be launched in 2017–2018. The Sentinel's infrared telescope, once parked in an orbit similar to that of Venus, will help identify threatening NEOs by cataloging 90\% of those with diameters larger than , as well as surveying smaller Solar System objects.Powell, Corey S. "Developing Early Warning Systems for Killer Asteroids" , Discover, August 14, 2013, pp. 60–61 (subscription required). Data gathered by Sentinel will help identify asteroids and other NEOs that pose a risk of collision with Earth, by being forwarded to scientific data-sharing networks, including NASA and academic institutions such as the Minor Planet Center.Broad, William J. Vindication for Entrepreneurs Watching Sky: Yes, It Can Fall , The New York Times website, February 16, 2013 and in print on February 17, 2013, p. A1 of the New York edition. Retrieved June 27, 2014. The foundation also proposes asteroid deflection of potentially dangerous NEOs by the use of gravity tractors to divert their trajectories away from Earth,Powell, Corey S. How to Deflect a Killer Asteroid: Researchers Come Up With Contingency Plans That Could Help Our Planet Dodge A Cosmic Bullet , Discover website, September 18, 2013 (subscription required), and in print as "How to Dodge a Cosmic Bullet", October 2013. Retrieved July 15, 2014. 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 Large Synoptic Survey Telescope, currently under construction, is expected to perform a comprehensive, high-resolution survey starting in the early 2020s. 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.Hearing Charter: Near-Earth Objects: Status of the Survey Program and Review of NASA's 2007 Report to Congress | SpaceRef Canada – Your Daily Source of Canadian Space News 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. Impact probability calculation pattern Why 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 similar pattern makes it 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 Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet.C. D. Hall and I. M. Ross, "Dynamics and Control Problems in the Deflection of Near-Earth Objects," Advances in the Astronautical Sciences, Astrodynamics 1997, Vol.97, Part I, 1997, pp.613–631. These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception,Solem, J. C.; Snell, C. (1994). "Terminal intercept for less than one orbital period warning ", a chapter in Hazards Due to Comets and Asteroids, Geherels, T., ed. (University of Arizona Press, Tucson), pp. 1013–1034. 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 12,750 km in diameter and moves at approx. 30 km per second 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.Planetary Defense Conference 2007, Washington D.C. Head-On Impact Deflection of NEAs: A Case Study for 99942 Apophis. Bernd Dachwald, Ralph Kahle, Bong Wie, Published in 2007.pg 3 If an asteroid breaks into fragments, any fragment larger than 35 meters 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.Further RAGE modeling of Asteroid mitigation, surface and subsurface explosions in porous objects. Weaver et al. 2011 Nuclear explosive device 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.Operation CASTLE Commander's Report 4:00Declassified U.S. Nuclear Test Film #34 0800034 – Project Gnome – 1961 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.Los Alamos Science No. 28, 2003 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.Solem, J. C. (1995). "Interception and disruption", in Proceedings of Planetary Defense Workshop, Livermore, CA, May 22–26, 1995, CONF-9505266 (LLNL, Livermore, CA), pp. 219–228 (236–246). 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 thermal heat upon encountering the object's surface matter, ablatively vaporizing all line of sight exposed surface areas of the object to a shallow depth, turning the surface material it heats up into ejecta, and, analogous to the ejecta from a chemical rocket engine exhaust, changing the velocity, or "nudging", the object off course by the reaction, following Newton's third law, with ejecta going one way and the object being propelled in the other. 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. A Hypervelocity Asteroid Mitigation Mission for Emergency Response (HAMMER) has been proposed. 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."Systems Engineering: Avoiding an Asteroid" , Time, June 16, 1967.Day, Dwayne A., "Giant bombs on giant rockets: Project Icarus" , The Space Review, Monday, July 5, 2004 The design study was later published as Project IcarusKleiman Louis A., Project Icarus: an MIT Student Project in Systems Engineering , Cambridge, Massachusetts : MIT Press, 1968 which served as the inspiration for the 1979 film Meteor.Project Icarus "MIT Course precept for movie" , The Tech, MIT, October 30, 1979 A NASA analysis of deflection alternatives, conducted in 2007, stated: Nuclear standoff explosions are assessed to be 10–100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks. Near-Earth Object Survey and Deflection Analysis of Alternatives Report to Congress March 2007 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, bundled together and lofted by an Ares V vehicle sometime in the 2020s, with each B83 being fuzed to detonate over the asteroid's surface at a height of ("1/3 of the objects diameter" as its stand-off), one after the other, with hour-long intervals between each detonation. The results of this study indicated that a single employment of this option "can deflect NEOs of [ diameter] two years before impact, and larger NEOs with at least five years warning".Near Earth Object (NEO) Mitigation Options Using Exploration Technologies 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.Towards Designing an Integrated Architecture for NEO Characterization, Mitigation, Scientific Evaluation, and Resource Utilization Surface and subsurface use 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 previously), began to study strategies that could deal with objects when the time to Earth impact was less than one year. He concluded that to provide the required energy, a nuclear explosion or other event that could deliver the same power, are the only methods that can work against a very large asteroid within these time constraints. This work resulted in the creation of a conceptual Hypervelocity Asteroid Intercept Vehicle (HAIV), which combines a kinetic impactor to create an initial crater for a follow-up subsurface nuclear detonation within that initial crater, which would generate a high degree of efficiency in the conversion of the nuclear energy that is released in the detonation into propulsion energy to the asteroid. Nuking Dangerous Asteroids Might Be the Best Protection, Expert Says. Includes a supercomputer simulation video provided by Los Alamos National Laboratory. 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. At the 2014 NASA Innovative Advanced Concepts (NIAC) conference, Wie and his colleagues stated that "we have the solution, using our baseline concept, to be able to mitigate the asteroid-impact threat, with any range of warning." For example, according to their computer models, with a warning time of 30 days, a asteroid would be neutralized by using a single HAIV, with less than 0.1\% of the destroyed object's mass potentially striking Earth, which by comparison would be more than acceptable.An Innovative Solution to NASA's NEO Impact Threat Mitigation Grand Challenge and Flight Validation Mission Architecture Development, 2014 As of 2015, Wie has collaborated with the Danish Emergency Asteroid Defence Project (EADP),EADP partners which ultimately intends to crowdsource sufficient funds to design, build, and store a non-nuclear HAIV spacecraft as planetary insurance. For threatening asteroids too large and/or too 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 swapped in, under international oversight, when conditions arise that necessitate it.Asteroid Defence, Emergency Asteroid Defence Project Comet deflection possibility "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 ByronAs quoted in Conversations of Lord Byron with Thomas Medwin (1832). 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.Planetary defense workshop LLNL 1995A new use for nuclear weapons: hunting rogue asteroids A persistent campaign by weapons designers to develop a nuclear defense against extraterrestrial rocks slowly wins government support 2013 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 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. In 2013, the related National Laboratories of the US and Russia signed a deal that includes an intent to cooperate on defense from asteroids.United States, Russia Sign Agreement to Further Research and Development Collaboration in Nuclear Energy and Security Present capability An April 2014 GAO report notes that the NNSA is retaining canned sub assemblies (CSAs—nuclear secondary stages) in an indeterminate state pending a senior-level government evaluation of their use in planetary defense against earthbound asteroids." In its FY2015 budget request, the NNSA noted that the nine-megaton B53 component disassembly was "delayed", leading some observers to conclude they might be the warhead CSAs being retained for potential planetary defense purposes. Law The use of nuclear explosive devices is an international issue and will need to be addressed by the United Nations Committee on the Peaceful Uses of Outer Space. The 1996 Comprehensive Nuclear-Test-Ban Treaty technically bans nuclear weapons in space. However, it is unlikely that a nuclear explosive device, fuzed to be detonated only upon interception with a threatening celestial object, Nuking Dangerous Asteroids Might Be the Best Protection, Expert Says. Includes a supercomputer simulation video provided by Los Alamos National Laboratory. "Wie admitted that sending nuclear weapons into space would be politically controversial. However, he said there are a number of safety features that could be built into the spacecraft to prevent the nuclear warhead from detonating in the event of a launch failure." with the sole intent of preventing that celestial body from impacting Earth would be regarded as an un-peaceful use of space, or that the explosive device sent to mitigate an Earth impact, explicitly designed to prevent harm to come to life, would fall under the classification of a "weapon". Kinetic impact The 2005 Deep Impact collision with the comet Tempel 1.Chapter 10 – Comets Astronomy 9601 The impact flash and resulting ejecta are clearly visible. The impactor delivered 19 gigajoules (the equivalent of 4.8 tons of TNT) upon impact.NASA deep impact impactor Deep Impact: Excavating Comet Tempel 1 It generated a predicted velocity change in the comet's orbital motion and decreased its perihelion distance by .The Orbital History of Comet 9P/Tempel 1 After the impact, a newspaper reported that the comet's orbit was changed by ." 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. A NASA analysis of deflection alternatives, conducted in 2007, stated: Non-nuclear kinetic impactors are the most mature approach and could be used in some deflection/mitigation scenarios, especially for NEOs that consist of a single small, solid body. The European Space Agency (ESA) is studying the preliminary design of two space missions for ~2020, named AIDA (formerly Don Quijote), and if flown, they would be the first intentional asteroid deflection mission. ESA's Advanced Concepts Team has also demonstrated theoretically that a deflection of 99942 Apophis could be achieved by sending a simple spacecraft weighing less than one ton to impact against the asteroid. During a trade-off study one of the leading researchers argued that a strategy called 'kinetic impactor deflection' was more efficient than others. The European Union's NEOShield-2 Mission is also primarily studying the Kinetic Impactor mitigation method. The principle of the kinetic impactor mitigation method is that the NEO or Asteroid is deflected following an impact from an impactor spacecraft. The principle of momentum transfer is used, as the impactor crashes into the NEO at a very high velocity of or more. The momentum of the impactor is transferred to the NEO, causing a change in velocity and therefore making it deviate from its course slightly. As of mid-2018, the AIDA mission has been partly approved. The NASA Double Asteroid Redirection Test (DART) kinetic impactor spacecraft has entered phase C (detailed definition). The goal is to impact the asteroidal moon of near-Earth Asteroid 65803 Didymos, nicknamed Didymoon. The impact will occur in October 2022 when Didymos is relatively close to Earth, allowing Earth-based telescopes and planetary radar to observe the event. The result of the impact will be to change the orbital velocity and hence orbital period of Didymoon, by a large enough amount that it can be measured from Earth. This will show for the first time that it is possible to change the orbit of a small asteroid, around the size most likely to require active mitigation in the future. The second part of the AIDA missionthe ESA HERA spacecrafthas entered phase B (Preliminary Definition) and requires approval by ESA member states in October 2019. If approved, it would reach the Didymos system in 2024 and measure both the mass of Didymoon and the precise effect of the impact on that body, allowing much better extrapolation of the AIDA mission to other targets. Asteroid gravity tractor Another alternative to explosive deflection is to move the asteroid slowly over time. A small but constant amount of thrust accumulates to deviate an object sufficiently from its course. Edward T. Lu and Stanley G. Love have proposed using a massive unmanned spacecraft hovering over an asteroid to gravitationally pull the asteroid into a non-threatening orbit. Though both objects are gravitationally pulled towards each other, the spacecraft can counter the force towards the asteroid by, for example, an ion thruster, so the net effect would be that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. While slow, this method has the advantage of working irrespective of the asteroid's composition or spin rate; rubble pile asteroids would be difficult to deflect by means of nuclear detonations, while a pushing device would be hard or inefficient to mount on a fast-rotating asteroid. A gravity tractor would likely have to spend several years beside the asteroid to be effective. A NASA analysis of deflection alternatives, conducted in 2007, stated: "Slow push" mitigation techniques are the most expensive, have the lowest level of technical readiness, and their ability to both travel to and divert a threatening NEO would be limited unless mission durations of many years to decades are possible. Ion beam shepherd Another "contactless" asteroid deflection technique has been proposed by C.Bombardelli and J.Peláez from the Technical University of Madrid. The method involves the use of a low-divergence ion thruster pointed at the asteroid from a nearby hovering spacecraft. The momentum transmitted by the ions reaching the asteroid surface produces a slow-but-continuous force that can deflect the asteroid in a similar way as the gravity tractor, but with a lighter spacecraft. Focused solar energy H. J. Melosh with I.V. Nemchinov proposed deflecting an asteroid or comet by focusing solar energy onto its surface to create thrust from the resulting vaporization of material. This method would first require the construction of a space station with a system of large collecting, concave mirrors similar to those used in solar furnaces. Orbit mitigation with highly concentrated sunlight is scalable to achieving the predetermined deflection within a year even for a global-threatening body without prolonged warning time. Such a hastened strategy may become topical in the case of late detection of a potential hazard, and also, if required, in providing the possibility for some additional action. Conventional concave reflectors are practically inapplicable to the high-concentrating geometry in the case of a giant shadowing space target, which is located in front of the mirrored surface. This is primarily because of the dramatic spread of the mirrors' focal points on the target due to the optical aberration when the optical axis is not aligned with the Sun. On the other hand, the positioning of any collector at a distance to the target much larger than its size does not yield the required concentration level (and therefore temperature) due to the natural divergence of the sunrays. Such principal restrictions are inevitably at any location regarding the asteroid of one or many unshaded forward-reflecting collectors. Also, in the case of secondary mirrors use, similar to the ones found in Cassegrain telescopes, would be prone to heat damage by partially concentrated sunlight from primary mirror. In order to remove the above restrictions, V.P. Vasylyev proposed to apply an alternative design of a mirrored collector – the ring-array concentrator. This type of collector has an underside lens-like position of its focal area that avoids shadowing of the collector by the target and minimizes the risk of its coating by ejected debris. Provided the sunlight concentration ~ 5 × 103 times, a surface irradiance of around 4-5 MW/m2 leads to a thrusting effect ~ 103 N. Intensive ablation of the rotating asteroid surface under the focal spot will lead to the appearance of a deep "canyon", which can contribute to the formation of the escaping gas flow into a jet-like one. This may be sufficient to deflect a 0.5-km asteroid within several months and no addition warning period, only using ring-array collector size ~ 0.5 of asteroid diameter. For such a prompt deflection of the larger NEOs, 1.3-2.2 km, the required collector sizes are comparable to the target diameter. In the case of a longer warning time, the required size of the collector may be significantly decreased. center|Artist's impression of asteroid deflection using an innovative ring-array solar collector. Mass driver A mass driver is an (automated) system on the asteroid to eject material into space thus giving the object a slow steady push and decreasing its mass. A mass driver is designed to work as a very low specific impulse system, which in general uses a lot of propellant, but very little power. The idea is that when using local material as propellant, the amount of propellant is not as important as the amount of power, which is likely to be limited. Conventional rocket engine Attaching any spacecraft propulsion device would have a similar effect of giving a push, possibly forcing the asteroid onto a trajectory that takes it away from Earth. An in-space rocket engine that is capable of imparting an impulse of 106 N·s (E.g. adding 1 km/s to a 1000 kg vehicle), will have a relatively small effect on a relatively small asteroid that has a mass of roughly a million times more. Chapman, Durda, and Gold's white paperChapman, Clark R. and Daniel D. Durda. The Comet/Asteroid Impact Hazard: A Systems Approach , Boulder, CO: Office of Space Studies, Southwest Research Institute, Space Engineering and Technology Branch, Johns Hopkins University Applied Physics Laboratory. calculates deflections using existing chemical rockets delivered to the asteroid. Such direct force rocket engines are typically proposed to use highly-efficient electrically powered spacecraft propulsion, such as ion thrusters or VASIMR. Asteroid laser ablation Similar to the effects of a nuclear device, it is thought possible to focus sufficient laser energy on the surface of an asteroid to cause flash vaporization / ablation to create either in impulse or to ablate away the asteroid mass. This concept, called asteroid laser ablation was articulated in the 1995 SpaceCast 2020 white paper "Preparing for Planetary Defense", and the 1996 Air Force 2025 white paper "Planetary Defense: Catastrophic Health Insurance for Planet URL Early publications include C. R. Phipps "ORION" concept from 1996, Colonel Jonathan W. Campbell's 2000 monograph "Using Lasers in Space: Laser Orbital Debris Removal and Asteroid Deflection", and NASA's 2005 concept Comet Asteroid Protection System (CAPS). Typically such systems require a significant amount of power, such as would be available from a Space-Based Solar Power Satellite. Another proposal is the Phillip Lubin's DE-STAR proposal. The DE-STAR project,Philip Lubin: A space-based array for planetary defense (video), SPIE Newsroom, 22 November 2013 proposed by researchers at the University of California, Santa Barbara, is a concept modular solar powered 1 µm, near infrared wavelength, laser array. The design calls for the array to eventually be approximately 1 km squared in size, with the modular design meaning that it could be launched in increments and assembled in space. In its early stages as a small array it could deal with smaller targets, assist solar sail probes and would also be useful in cleaning up space debris. Other proposals NASA study of a solar sail. The sail would be wide. Wrapping the asteroid in a sheet of reflective plastic such as aluminized PET film as a solar sail "Painting" or dusting the object with titanium dioxide (white) to alter its trajectory via increased reflected radiation pressure or with soot (black) to alter its trajectory via the Yarkovsky effect. Planetary scientist Eugene Shoemaker in 1996 proposed--in a lecture to the Arizona Geological Society in 12–96. deflecting a potential impactor by releasing a cloud of steam in the path of the object, hopefully gently slowing it. Nick Szabo in 1990 sketchedIs an asteroid capture possible/feasible?; Asteroid movement/retrieval; Asteroid relocation/mining; etceras... , Space-tech Digest #70 [bulletin board], Carnegie Mellon University, July 19–25, 1990. a similar idea, "cometary aerobraking", the targeting of a comet or ice construct at an asteroid, then vaporizing the ice with nuclear explosives to form a temporary atmosphere in the path of the asteroid. Coherent digger array multiple 1-ton flat tractors able to dig and expel asteroid soil mass as a coherent fountain array, coordinated fountain activity may propel and deflect over years. Attaching a tether and ballast mass to the asteroid to alter its trajectory by changing its center of mass. Magnetic flux compression to magnetically brake and or capture objects that contain a high percentage of meteoric iron by deploying a wide coil of wire in its orbital path and when it passes through, Inductance creates an electromagnet solenoid to be generated. Deflection technology concerns Carl Sagan, in his book Pale Blue Dot, expressed concern about deflection technology, noting that any method capable of deflecting impactors away from Earth could also be abused to divert non-threatening bodies toward the planet. Considering the history of genocidal political leaders and the possibility of the bureaucratic obscuring of any such project's true goals to most of its scientific participants, he judged the Earth at greater risk from a man-made impact than a natural one. Sagan instead suggested that deflection technology be developed only in an actual emergency situation. All low-energy delivery deflection technologies have inherent fine control and steering capability, making it possible to add just the right amount of energy to steer an asteroid originally destined for a mere close approach toward a specific Earth target. According to former NASA astronaut Rusty Schweickart, the gravitational tractor method is controversial because, during the process of changing an asteroid's trajectory, the point on the Earth where it could most likely hit would be slowly shifted across different countries. Thus, the threat for the entire planet would be minimized at the cost of some specific states' security. In Schweickart's opinion, choosing the way the asteroid should be "dragged" would be a tough diplomatic decision. Analysis of the uncertainty involved in nuclear deflection shows that the ability to protect the planet does not imply the ability to target the planet. A nuclear explosion that changes an asteroid's velocity by 10 meters/second (plus or minus 20\%) would be adequate to push it out of an Earth-impacting orbit. However, if the uncertainty of the velocity change was more than a few percent, there would be no chance of directing the asteroid to a particular target. Planetary defense timeline upright=1.25|The 1984 Strategic Defense Initiative concept of a generic space based Nuclear reactor pumped laser or a hydrogen fluoride laser satellite, firing on a target, causing a momentum change in the target object by laser ablation. With the proposed Space Station Freedom (ISS) in the background. In their 1964 book, Islands in Space, Dandridge M. Cole and Donald W. Cox noted the dangers of planetoid impacts, both those occurring naturally and those that might be brought about with hostile intent. They argued for cataloging the minor planets and developing the technologies to land on, deflect, or even capture planetoids. In 1967, students in the Aeronautics and Astronautics department at MIT did a design study, "Project Icarus," of a mission to prevent a hypothetical impact on Earth by asteroid 1566 Icarus. The design project was later published in a book by the MIT Press and received considerable publicity, for the first time bringing asteroid impact into the public eye. In the 1980s NASA studied evidence of past strikes on planet Earth, and the risk of this happening at the current level of civilization. This led to a program that maps objects in the Solar System that both cross Earth's orbit and are large enough to cause serious damage if they hit. In the 1990s, US Congress held hearings to consider the risks and what needed to be done about them. This led to a US$ 3 million annual budget for programs like Spaceguard and the near-Earth object program, as managed by NASA and USAF.
Revised:  A 2003 NASA study of a follow-on program suggests spending US $250–450 million to detect 90\% of all near-Earth asteroids 140 meters and larger by 2028. NEODyS is an online database of known NEOs. Sentinel mission The B612 Foundation is a private nonprofit foundation with headquarters in the United States, dedicated to protecting the Earth from asteroid strikes. It is led mainly by scientists, former astronauts and engineers from the Institute for Advanced Study, Southwest Research Institute, Stanford University, NASA and the space industry. As a non-governmental organization it has conducted two lines of related research to help detect NEOs that could one day strike the Earth, and find the technological means to divert their path to avoid such collisions. The foundation's current goal is to design and build a privately financed asteroid-finding space telescope, Sentinel, to be launched in 2017–2018. The Sentinel's infrared telescope, once parked in an orbit similar to that of Venus, will help identify threatening NEOs by cataloging 90\% of those with diameters larger than , as well as surveying smaller Solar System objects.Powell, Corey S. "Developing Early Warning Systems for Killer Asteroids" , Discover, August 14, 2013, pp. 60–61 (subscription required). Data gathered by Sentinel will help identify asteroids and other NEOs that pose a risk of collision with Earth, by being forwarded to scientific data-sharing networks, including NASA and academic institutions such as the Minor Planet Center.Broad, William J. Vindication for Entrepreneurs Watching Sky: Yes, It Can Fall , The New York Times website, February 16, 2013 and in print on February 17, 2013, p. A1 of the New York edition. Retrieved June 27, 2014. The foundation also proposes asteroid deflection of potentially dangerous NEOs by the use of gravity tractors to divert their trajectories away from Earth,Powell, Corey S. How to Deflect a Killer Asteroid: Researchers Come Up With Contingency Plans That Could Help Our Planet Dodge A Cosmic Bullet , Discover website, September 18, 2013 (subscription required), and in print as "How to Dodge a Cosmic Bullet", October 2013. Retrieved July 15, 2014. 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 Large Synoptic Survey Telescope, currently under construction, is expected to perform a comprehensive, high-resolution survey starting in the early 2020s. 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.Hearing Charter: Near-Earth Objects: Status of the Survey Program and Review of NASA's 2007 Report to Congress | SpaceRef Canada – Your Daily Source of Canadian Space News 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. Impact probability calculation pattern Why 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 similar pattern makes it 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 Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet.C. D. Hall and I. M. Ross, "Dynamics and Control Problems in the Deflection of Near-Earth Objects", Advances in the Astronautical Sciences, Astrodynamics 1997, Vol.97, Part I, 1997, pp.613–631. These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception,Solem, J. C.; Snell, C. (1994). "Terminal intercept for less than one orbital period warning ", a chapter in Hazards Due to Comets and Asteroids, Geherels, T., ed. (University of Arizona Press, Tucson), pp. 1013–1034. 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 12,750 km in diameter and moves at approx. 30 km per second 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.Planetary Defense Conference 2007, Washington D.C. Head-On Impact Deflection of NEAs: A Case Study for 99942 Apophis. Bernd Dachwald, Ralph Kahle, Bong Wie, Published in 2007.pg 3 If an asteroid breaks into fragments, any fragment larger than 35 meters 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.Further RAGE modeling of Asteroid mitigation, surface and subsurface explosions in porous objects. Weaver et al. 2011 Nuclear explosive device 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.Operation CASTLE Commander's Report 4:00Declassified U.S. Nuclear Test Film #34 0800034 – Project Gnome – 1961 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.Los Alamos Science No. 28, 2003 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.Solem, J. C. (1995). "Interception and disruption", in Proceedings of Planetary Defense Workshop, Livermore, CA, May 22–26, 1995, CONF-9505266 (LLNL, Livermore, CA), pp. 219–228 (236–246). 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 thermal heat upon encountering the object's surface matter, ablatively vaporizing all line of sight exposed surface areas of the object to a shallow depth, turning the surface material it heats up into ejecta, and, analogous to the ejecta from a chemical rocket engine exhaust, changing the velocity, or "nudging", the object off course by the reaction, following Newton's third law, with ejecta going one way and the object being propelled in the other. 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. A Hypervelocity Asteroid Mitigation Mission for Emergency Response (HAMMER) has been proposed. 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."Systems Engineering: Avoiding an Asteroid" , Time, June 16, 1967.Day, Dwayne A., "Giant bombs on giant rockets: Project Icarus" , The Space Review, Monday, July 5, 2004 The design study was later published as Project IcarusKleiman Louis A., Project Icarus: an MIT Student Project in Systems Engineering , Cambridge, Massachusetts : MIT Press, 1968 which served as the inspiration for the 1979 film Meteor.Project Icarus "MIT Course precept for movie" , The Tech, MIT, October 30, 1979 A NASA analysis of deflection alternatives, conducted in 2007, stated: Nuclear standoff explosions are assessed to be 10–100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks. Near-Earth Object Survey and Deflection Analysis of Alternatives Report to Congress March 2007 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, bundled together and lofted by an Ares V vehicle sometime in the 2020s, with each B83 being fuzed to detonate over the asteroid's surface at a height of ("1/3 of the objects diameter" as its stand-off), one after the other, with hour-long intervals between each detonation. The results of this study indicated that a single employment of this option "can deflect NEOs of [ diameter] two years before impact, and larger NEOs with at least five years warning".Near Earth Object (NEO) Mitigation Options Using Exploration Technologies 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.Towards Designing an Integrated Architecture for NEO Characterization, Mitigation, Scientific Evaluation, and Resource Utilization Surface and subsurface use 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 previously), began to study strategies that could deal with objects when the time to Earth impact was less than one year. He concluded that to provide the required energy, a nuclear explosion or other event that could deliver the same power, are the only methods that can work against a very large asteroid within these time constraints. This work resulted in the creation of a conceptual Hypervelocity Asteroid Intercept Vehicle (HAIV), which combines a kinetic impactor to create an initial crater for a follow-up subsurface nuclear detonation within that initial crater, which would generate a high degree of efficiency in the conversion of the nuclear energy that is released in the detonation into propulsion energy to the asteroid. Nuking Dangerous Asteroids Might Be the Best Protection, Expert Says. Includes a supercomputer simulation video provided by Los Alamos National Laboratory. 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. At the 2014 NASA Innovative Advanced Concepts (NIAC) conference, Wie and his colleagues stated that "we have the solution, using our baseline concept, to be able to mitigate the asteroid-impact threat, with any range of warning." For example, according to their computer models, with a warning time of 30 days, a asteroid would be neutralized by using a single HAIV, with less than 0.1\% of the destroyed object's mass potentially striking Earth, which by comparison would be more than acceptable.An Innovative Solution to NASA's NEO Impact Threat Mitigation Grand Challenge and Flight Validation Mission Architecture Development, 2014 As of 2015, Wie has collaborated with the Danish Emergency Asteroid Defence Project (EADP),EADP partners which ultimately intends to crowdsource sufficient funds to design, build, and store a non-nuclear HAIV spacecraft as planetary insurance. For threatening asteroids too large and/or too 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 swapped in, under international oversight, when conditions arise that necessitate it.Asteroid Defence, Emergency Asteroid Defence Project Comet deflection possibility "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 ByronAs quoted in Conversations of Lord Byron with Thomas Medwin (1832). 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.Planetary defense workshop LLNL 1995A new use for nuclear weapons: hunting rogue asteroids A persistent campaign by weapons designers to develop a nuclear defense against extraterrestrial rocks slowly wins government support 2013 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 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. In 2013, the related National Laboratories of the US and Russia signed a deal that includes an intent to cooperate on defense from asteroids.United States, Russia Sign Agreement to Further Research and Development Collaboration in Nuclear Energy and Security Present capability An April 2014 GAO report notes that the NNSA is retaining canned sub assemblies (CSAs—nuclear secondary stages) in an indeterminate state pending a senior-level government evaluation of their use in planetary defense against earthbound asteroids." In its FY2015 budget request, the NNSA noted that the nine-megaton B53 component disassembly was "delayed", leading some observers to conclude they might be the warhead CSAs being retained for potential planetary defense purposes. Law The use of nuclear explosive devices is an international issue and will need to be addressed by the United Nations Committee on the Peaceful Uses of Outer Space. The 1996 Comprehensive Nuclear-Test-Ban Treaty technically bans nuclear weapons in space. However, it is unlikely that a nuclear explosive device, fuzed to be detonated only upon interception with a threatening celestial object, Nuking Dangerous Asteroids Might Be the Best Protection, Expert Says. Includes a supercomputer simulation video provided by Los Alamos National Laboratory. "Wie admitted that sending nuclear weapons into space would be politically controversial. However, he said there are a number of safety features that could be built into the spacecraft to prevent the nuclear warhead from detonating in the event of a launch failure." with the sole intent of preventing that celestial body from impacting Earth would be regarded as an un-peaceful use of space, or that the explosive device sent to mitigate an Earth impact, explicitly designed to prevent harm to come to life, would fall under the classification of a "weapon". Kinetic impact The 2005 Deep Impact collision with the comet Tempel 1.Chapter 10 – Comets Astronomy 9601 The impact flash and resulting ejecta are clearly visible. The impactor delivered 19 gigajoules (the equivalent of 4.8 tons of TNT) upon impact.NASA deep impact impactor Deep Impact: Excavating Comet Tempel 1 It generated a predicted velocity change in the comet's orbital motion and decreased its perihelion distance by .The Orbital History of Comet 9P/Tempel 1 After the impact, a newspaper reported that the comet's orbit was changed by ." 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. A NASA analysis of deflection alternatives, conducted in 2007, stated: Non-nuclear kinetic impactors are the most mature approach and could be used in some deflection/mitigation scenarios, especially for NEOs that consist of a single small, solid body. The European Space Agency (ESA) is studying the preliminary design of two space missions for ~2020, named AIDA (formerly Don Quijote), and if flown, they would be the first intentional asteroid deflection mission. ESA's Advanced Concepts Team has also demonstrated theoretically that a deflection of 99942 Apophis could be achieved by sending a simple spacecraft weighing less than one ton to impact against the asteroid. During a trade-off study one of the leading researchers argued that a strategy called 'kinetic impactor deflection' was more efficient than others. The European Union's NEOShield-2 Mission is also primarily studying the Kinetic Impactor mitigation method. The principle of the kinetic impactor mitigation method is that the NEO or Asteroid is deflected following an impact from an impactor spacecraft. The principle of momentum transfer is used, as the impactor crashes into the NEO at a very high velocity of or more. The momentum of the impactor is transferred to the NEO, causing a change in velocity and therefore making it deviate from its course slightly. As of mid-2018, the AIDA mission has been partly approved. The NASA Double Asteroid Redirection Test (DART) kinetic impactor spacecraft has entered phase C (detailed definition). The goal is to impact the asteroidal moon of near-Earth Asteroid 65803 Didymos, nicknamed Didymoon. The impact will occur in October 2022 when Didymos is relatively close to Earth, allowing Earth-based telescopes and planetary radar to observe the event. The result of the impact will be to change the orbital velocity and hence orbital period of Didymoon, by a large enough amount that it can be measured from Earth. This will show for the first time that it is possible to change the orbit of a small asteroid, around the size most likely to require active mitigation in the future. The second part of the AIDA missionthe ESA HERA spacecrafthas entered phase B (Preliminary Definition) and requires approval by ESA member states in October 2019. If approved, it would reach the Didymos system in 2024 and measure both the mass of Didymoon and the precise effect of the impact on that body, allowing much better extrapolation of the AIDA mission to other targets. Asteroid gravity tractor Another alternative to explosive deflection is to move the asteroid slowly over time. A small but constant amount of thrust accumulates to deviate an object sufficiently from its course. Edward T. Lu and Stanley G. Love have proposed using a massive unmanned spacecraft hovering over an asteroid to gravitationally pull the asteroid into a non-threatening orbit. Though both objects are gravitationally pulled towards each other, the spacecraft can counter the force towards the asteroid by, for example, an ion thruster, so the net effect would be that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. While slow, this method has the advantage of working irrespective of the asteroid's composition or spin rate; rubble pile asteroids would be difficult to deflect by means of nuclear detonations, while a pushing device would be difficult or inefficient to mount on a fast-rotating asteroid. A gravity tractor would likely have to spend several years beside the asteroid to be effective. A NASA analysis of deflection alternatives, conducted in 2007, stated: "Slow push" mitigation techniques are the most expensive, have the lowest level of technical readiness, and their ability to both travel to and divert a threatening NEO would be limited unless mission durations of many years to decades are possible. Ion beam shepherd Another "contactless" asteroid deflection technique has been proposed by C. Bombardelli and J. Peláez from the Technical University of Madrid. The method involves the use of a low-divergence ion thruster pointed at the asteroid from a nearby hovering spacecraft. The momentum transmitted by the ions reaching the asteroid surface produces a slow-but-continuous force that can deflect the asteroid in a similar way as the gravity tractor, but with a lighter spacecraft. Focused solar energy H. J. Melosh with I. V. Nemchinov proposed deflecting an asteroid or comet by focusing solar energy onto its surface to create thrust from the resulting vaporization of material. This method would first require the construction of a space station with a system of large collecting, concave mirrors similar to those used in solar furnaces. Orbit mitigation with highly concentrated sunlight is scalable to achieving the predetermined deflection within a year even for a global-threatening body without prolonged warning time. Such a hastened strategy may become topical in the case of late detection of a potential hazard, and also, if required, in providing the possibility for some additional action. Conventional concave reflectors are practically inapplicable to the high-concentrating geometry in the case of a giant shadowing space target, which is located in front of the mirrored surface. This is primarily because of the dramatic spread of the mirrors' focal points on the target due to the optical aberration when the optical axis is not aligned with the Sun. On the other hand, the positioning of any collector at a distance to the target much larger than its size does not yield the required concentration level (and therefore temperature) due to the natural divergence of the sunrays. Such principal restrictions are inevitably at any location regarding the asteroid of one or many unshaded forward-reflecting collectors. Also, in the case of secondary mirrors use, similar to the ones found in Cassegrain telescopes, would be prone to heat damage by partially concentrated sunlight from primary mirror. In order to remove the above restrictions, V.P. Vasylyev proposed to apply an alternative design of a mirrored collector – the ring-array concentrator. This type of collector has an underside lens-like position of its focal area that avoids shadowing of the collector by the target and minimizes the risk of its coating by ejected debris. Provided the sunlight concentration ~ 5 × 103 times, a surface irradiance of around 4-5 MW/m2 leads to a thrusting effect ~ 103 N. Intensive ablation of the rotating asteroid surface under the focal spot will lead to the appearance of a deep "canyon", which can contribute to the formation of the escaping gas flow into a jet-like one. This may be sufficient to deflect a 0.5-km asteroid within several months and no addition warning period, only using ring-array collector size ~ 0.5 of asteroid diameter. For such a prompt deflection of the larger NEOs, 1.3-2.2 km, the required collector sizes are comparable to the target diameter. In the case of a longer warning time, the required size of the collector may be significantly decreased. center|Artist's impression of asteroid deflection using an innovative ring-array solar collector. Mass driver A mass driver is an (automated) system on the asteroid to eject material into space thus giving the object a slow steady push and decreasing its mass. A mass driver is designed to work as a very low specific impulse system, which in general uses a lot of propellant, but very little power. The idea is that when using local material as propellant, the amount of propellant is not as important as the amount of power, which is likely to be limited. Conventional rocket engine Attaching any spacecraft propulsion device would have a similar effect of giving a push, possibly forcing the asteroid onto a trajectory that takes it away from Earth. An in-space rocket engine that is capable of imparting an impulse of 106 N·s (E.g. adding 1 km/s to a 1000 kg vehicle), will have a relatively small effect on a relatively small asteroid that has a mass of roughly a million times more. Chapman, Durda, and Gold's white paperChapman, Clark R. and Daniel D. Durda. The Comet/Asteroid Impact Hazard: A Systems Approach , Boulder, CO: Office of Space Studies, Southwest Research Institute, Space Engineering and Technology Branch, Johns Hopkins University Applied Physics Laboratory. calculates deflections using existing chemical rockets delivered to the asteroid. Such direct force rocket engines are typically proposed to use highly-efficient electrically powered spacecraft propulsion, such as ion thrusters or VASIMR. Asteroid laser ablation Similar to the effects of a nuclear device, it is thought possible to focus sufficient laser energy on the surface of an asteroid to cause flash vaporization / ablation to create either in impulse or to ablate away the asteroid mass. This concept, called asteroid laser ablation was articulated in the 1995 SpaceCast 2020 white paper "Preparing for Planetary Defense", and the 1996 Air Force 2025 white paper "Planetary Defense: Catastrophic Health Insurance for Planet URL Early publications include C. R. Phipps "ORION" concept from 1996, Colonel Jonathan W. Campbell's 2000 monograph "Using Lasers in Space: Laser Orbital Debris Removal and Asteroid Deflection", and NASA's 2005 concept Comet Asteroid Protection System (CAPS). Typically such systems require a significant amount of power, such as would be available from a Space-Based Solar Power Satellite. Another proposal is the Phillip Lubin's DE-STAR proposal: The DE-STAR project,Philip Lubin: A space-based array for planetary defense (video), SPIE Newsroom, 22 November 2013 proposed by researchers at the University of California, Santa Barbara, is a concept modular solar powered 1 µm, near infrared wavelength, laser array. The design calls for the array to eventually be approximately 1 km squared in size, with the modular design meaning that it could be launched in increments and assembled in space. In its early stages as a small array it could deal with smaller targets, assist solar sail probes and would also be useful in cleaning up space debris. Other proposals NASA study of a solar sail. The sail would be wide. Wrapping the asteroid in a sheet of reflective plastic such as aluminized PET film as a solar sail "Painting" or dusting the object with titanium dioxide (white) to alter its trajectory via increased reflected radiation pressure or with soot (black) to alter its trajectory via the Yarkovsky effect. Planetary scientist Eugene Shoemaker in 1996 proposed--in a lecture to the Arizona Geological Society in 12–96. deflecting a potential impactor by releasing a cloud of steam in the path of the object, hopefully gently slowing it. Nick Szabo in 1990 sketchedIs an asteroid capture possible/feasible?; Asteroid movement/retrieval; Asteroid relocation/mining; etceras... , Space-tech Digest #70 [bulletin board], Carnegie Mellon University, July 19–25, 1990. a similar idea, "cometary aerobraking", the targeting of a comet or ice construct at an asteroid, then vaporizing the ice with nuclear explosives to form a temporary atmosphere in the path of the asteroid. Coherent digger array multiple 1-ton flat tractors able to dig and expel asteroid soil mass as a coherent fountain array, coordinated fountain activity may propel and deflect over years. Attaching a tether and ballast mass to the asteroid to alter its trajectory by changing its center of mass. Magnetic flux compression to magnetically brake and or capture objects that contain a high percentage of meteoric iron by deploying a wide coil of wire in its orbital path and when it passes through, Inductance creates an electromagnet solenoid to be generated. Deflection technology concerns Carl Sagan, in his book Pale Blue Dot, expressed concern about deflection technology, noting that any method capable of deflecting impactors away from Earth could also be abused to divert non-threatening bodies toward the planet. Considering the history of genocidal political leaders and the possibility of the bureaucratic obscuring of any such project's true goals to most of its scientific participants, he judged the Earth at greater risk from a man-made impact than a natural one. Sagan instead suggested that deflection technology be developed only in an actual emergency situation. All low-energy delivery deflection technologies have inherent fine control and steering capability, making it possible to add just the right amount of energy to steer an asteroid originally destined for a mere close approach toward a specific Earth target. According to former NASA astronaut Rusty Schweickart, the gravitational tractor method is controversial because, during the process of changing an asteroid's trajectory, the point on the Earth where it could most likely hit would be slowly shifted across different countries. Thus, the threat for the entire planet would be minimized at the cost of some specific states' security. In Schweickart's opinion, choosing the way the asteroid should be "dragged" would be a tough diplomatic decision. Analysis of the uncertainty involved in nuclear deflection shows that the ability to protect the planet does not imply the ability to target the planet. A nuclear explosion that changes an asteroid's velocity by 10 meters/second (plus or minus 20\%) would be adequate to push it out of an Earth-impacting orbit. However, if the uncertainty of the velocity change was more than a few percent, there would be no chance of directing the asteroid to a particular target. Planetary defense timeline upright=1.25|The 1984 Strategic Defense Initiative concept of a generic space based Nuclear reactor pumped laser or a hydrogen fluoride laser satellite, firing on a target, causing a momentum change in the target object by laser ablation. With the proposed Space Station Freedom (ISS) in the background. In their 1964 book, Islands in Space, Dandridge M. Cole and Donald W. Cox noted the dangers of planetoid impacts, both those occurring naturally and those that might be brought about with hostile intent. They argued for cataloging the minor planets and developing the technologies to land on, deflect, or even capture planetoids. In 1967, students in the Aeronautics and Astronautics department at MIT did a design study, "Project Icarus", of a mission to prevent a hypothetical impact on Earth by asteroid 1566 Icarus. The design project was later published in a book by the MIT Press and received considerable publicity, for the first time bringing asteroid impact into the public eye. In the 1980s NASA studied evidence of past strikes on planet Earth, and the risk of this happening at the current level of civilization. This led to a program that maps objects in the Solar System that both cross Earth's orbit and are large enough to cause serious damage if they hit. In the 1990s, US Congress held hearings to consider the risks and what needed to be done about them. This led to a US$ 3 million annual budget for programs like Spaceguard and the near-Earth object program, as managed by NASA and USAF.
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