Asteroid impact avoidance comprises a number of methods by which near-Earth objects (NEO) could be diverted, preventing destructive impact events. A sufficiently large impact by an asteroid or other NEOs would cause, depending on its impact location, massive tsunamis, multiple firestorms and an impact winter caused by the sunlight-blocking effect of placing large quantities of pulverized rock dust, and other debris, into the stratosphere.
A collision between the Earth and an approximately 10-kilometre-wide object 66 million years ago is thought to have produced the Chicxulub Crater and the Cretaceous–Paleogene extinction event, widely held responsible for the extinction of the dinosaurs.
While the chances of a major collision are not great in the near term, there is a high probability that one will happen eventually unless defensive actions are taken. Recent astronomical events—such as the Shoemaker-Levy 9 impacts on Jupiter and the 2013 Chelyabinsk meteor along with the growing number of objects on the Sentry Risk Table—have drawn renewed attention to such threats. NASA warns that the earth is unprepared.
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 3.5/t × 10−2 m·s−1 (where t is the number of years until potential impact) is needed to successfully deflect a body on a direct collision trajectory. In addition, under certain circumstances, 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 passing through a ‘keyhole’ and 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 ms−1.
An impact by a 10 kilometres (6.2 mi) 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 coming into 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 50 metres (160 ft) in diameter, which are far more common, are historically extremely destructive regionally.
History of government mandates
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. The impact of an object much larger than 1 km diameter could well result in worldwide damage up to, and potentially including, extinction of the human species. The NASA commitment has resulted in the funding of a number of NEO search efforts that are making considerable progress toward the 90% goal by 2008.[needs update] The 2009 discovery of an NEO approximately 2 to 3 kilometers in diameter demonstrated there were still large objects to be detected.
United States Representative George E. Brown, Jr. (D-CA) was quoted as voicing his support for planetary defense projects in Air & Space Power Chronicles, saying “If some day in the future we discover well in advance that an asteroid that is big enough to cause a mass extinction is going to hit the Earth, and then we alter the course of that asteroid so that it does not hit us, it will be one of the most important accomplishments in all of human history.”
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 U.S. Congress has declared that the general welfare and security of the United States require that the unique competence of NASA be directed to detecting, tracking, cataloguing, and characterizing near-Earth asteroids and comets in order to provide warning and mitigation of the potential hazard of such near-Earth objects to the Earth. The NASA Administrator shall plan, develop, and implement a Near-Earth Object Survey program to detect, track, catalogue, and characterize the physical characteristics of near- Earth objects equal to or greater than 140 meters in diameter in order to assess the threat of such near-Earth objects to the Earth. It shall be the goal of the Survey program to achieve 90% completion of its near-Earth object catalogue (based on statistically predicted populations of near-Earth objects) within 15 years after the date of enactment of this Act. The NASA Administrator shall transmit to Congress not later than 1 year after the date of enactment of this Act an initial report that provides the following: (A) An analysis of possible alternatives that NASA may employ to carry out the Survey program, including ground-based and space-based alternatives with technical descriptions. (B) A recommended option and proposed budget to carry out the Survey program pursuant to the recommended option. (C) Analysis of possible alternatives that NASA could employ to divert an object on a likely collision course with Earth.
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).
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 which 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.
Spacewatch, which uses a 90 centimeter 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 Dr. Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by Dr. McMillan. The Spacewatch project has acquired a 1.8 meter 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), Catalina Sky Survey, 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, now in operation, conducts frequent scans of the 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 140 meters and larger by 2028.
NEODyS is an online database of known NEOs.
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 140 metres (460 ft), as well as surveying smaller Solar System objects.
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. 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.
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 is to resume operations sometime in early 2014.
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.
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.
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 2004 MN4), which in 2009 temporarily had an impact probability of about 3% for the year 2029. Additional observations revised this probability down to zero.
Impact probability calculation pattern
The ellipses in the diagram at right show the likely asteroid position 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 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 until the Earth is outside the error region, and the impact probability returns to near zero.
Collision avoidance strategies
Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet. 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, rendezvous, or remote station).
Strategies fall into two basic sets: destruction and delay. Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or 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. The indirect methods, such as gravity tractors, attaching rockets or mass drivers, are much slower and require traveling to the object, time to change course up to 180 degrees to fly alongside it, and then take 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 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.
Nuclear explosive device
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 thermal heat upon encountering the objects 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 in order to avoid hitting the Earth.
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 20-meter or greater stand-off height above its surface, so as not to fracture the potentially loosely held together object. Providing this stand-off strategy was done far enough in advance, the force from a sufficient number of nuclear blasts would be enough to alter the object’s trajectory to avoid an impact, according to computer simulations and experimental evidence from meteorites exposed to the thermal X-ray pulses of the Z-machine.
The 1964 book Islands in Space calculates that the nuclear megatonnage necessary for several deflection scenarios exists. 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 1.4 kilometer wide asteroid 1566 Icarus, an object which 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 maximum yield of the Soviets’ 1961 Tsar Bomba 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:
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.
In the same year NASA released a study where the asteroid Apophis (with a diameter ~300 m) was assumed to have a much lower rubble pile density (“1500 kg/m^3”) and therefore mass than is now known, 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 that are 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 100 m (“1/3 of the objects diameter” as its stand-off), one after the other, with hour long intervals between each successive detonation. The results of this study indicated that a single employment of this “option can deflect NEOs of [100-500m diameter] two years before impact, and larger NEOs with at least five years warning”. 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.
Surface and subsurface use
The director of the Asteroid Deflection Research Center at Iowa State University, Dr. Wie, who had published kinetic impactor deflection studies in the past, began in 2011 to study strategies that could deal with 50 to 500 meter diameter objects when the time to Earth impact was under a year or so. He concluded that to provide the required energy, a nuclear explosion or other events 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.
Another proposed approach along similar lines is the use of a surface detonating nuclear device, in place of the prior mentioned kinetic impactor, in order to create the initial crater, with the resulting crater that forms then again being used 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 1,000-foot-wide (300 m) asteroid would be neutralized by using a single HAIV, with less than 0.1 percent of the destroyed object’s mass potentially striking Earth, which by comparison would be more than acceptable.
As of 2015 Dr. Wie has collaborated with the Danish Emergency Asteroid Defence Project (EADP), 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 in this configuration than when compared to the stand-off strategy are intended to be swapped-in, under international oversight, when conditions arise that necessitate it.
Comet deflection possibility
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 1 gigaton nuclear explosive device, which would be equivalent to the kinetic energy of a 1 km diameter asteroid. The theoretical 1 Gt device would weigh about 25-30 tons, light enough to be lifted on the Energia rocket and it could be used to instantaneously vaporize a 1 km asteroid, divert the paths of extinction event class asteroids (greater than 10 km in diameter) within a few months of short notice, while with 1-year notice, at an interception location no closer than Jupiter, it would also be capable of dealing with the even rarer short period comets which can come out of the Kuiper belt and transit past Earth orbit within 2 years. For comets of this class, with a maximum estimated 100 km diameter, Charon 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.
An April 2014 GAO report notes that the NNSA is retaining canned subassemblies (CSAs) ” 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 9 Mt B53 component disassembly was “delayed”, leading some observers to conclude they might be the warhead CSAs being retained for potential planetary defense purposes.[not in citation given] Following the total disassembly of all 25 Mt high yield B41s in 1976, the B53 is the highest yielding US device presently in the Enduring Stockpile.
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, 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”.
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 (spacecraft) & the earlier Don Quijote, and if flown, they would be the first intentional asteroid deflection mission ever designed. ESA’s Advanced Concepts Team has also demonstrated theoretically that a deflection of 99942 Apophis could be achieved by sending a simple spacecraft[when?] weighing less than one ton to impact against the asteroid. During a trade-off study one of the leading researchers[who?] argued that a strategy called ‘kinetic impactor deflection’ was more efficient than others.[dubious ]
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 10 km/s or more. The mass and velocity of the impactor (the momentum) are transferred to the NEO, causing a change in velocity and therefore making it deviate from its course slightly.
Asteroid gravity tractor
One more alternative to explosive deflection is to move the asteroid slowly over a time. Tiny constant thrust accumulates to deviate an object sufficiently from its predicted course. Edward T. Lu and Stanley G. Love have proposed using a large heavy unmanned spacecraft hovering over an asteroid to gravitationally pull the latter into a non-threatening orbit. The spacecraft and the asteroid mutually attract one another. If the spacecraft counters the force towards the asteroid by, e.g., an ion thruster, the net effect is 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 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 recently 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 done by the gravity tractor but with a lighter spacecraft.
Use of focused solar energy
H. Jay Melosh proposed to deflect an asteroid or comet by focusing solar energy onto its surface to create thrust from the resulting vaporization of material, or to amplify the Yarkovsky effect. Over a span of months or years enough solar radiation can be directed onto the object to deflect it.
This method would first require the construction of a space station with a system of gigantic lenses. Then the station would be transported toward the Sun.
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.
Another possibility is to use a mass driver on the Moon aimed at the NEO to take advantage of the Moon’s orbital velocity and inexhaustible supply of “rock bullets”.
Conventional rocket engine
Attaching any spacecraft propulsion device would have a similar effect of giving a steady 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 paper 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 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 Earth”. 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.
A recent proposal is the Dr. Phillip Lubin’s DE-STAR proposal.
- The DE-STAR project, 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.
- 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 deflecting a potential impactor by releasing a cloud of steam in the path of the object, hopefully gently slowing it. Nick Szabo in 1990 sketched 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.
- 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 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 only be developed 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 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
- 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 Pressand 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 which objects in the Solar System both cross Earth’s orbit and are large enough to cause serious damage if they ever 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.
- In 2005 a number of astronauts published an open letter through the Association of Space Explorers calling for a united push to develop strategies to protect Earth from the risk of a cosmic collision.
- It is currently (as of late 2007) estimated that there are approximately 20,000 objects capable of crossing Earth’s orbit and large enough (140 meters or larger) to warrant concern. On the average, one of these will collide with Earth every 5,000 years, unless preventative measures are undertaken. It is now anticipated that by year 2008, 90% of such objects that are 1 km or more in diameter will have been identified and will be monitored. The further task of identifying and monitoring all such objects of 140m or greater is expected to be complete around 2020.
- The Catalina Sky Survey (CSS) is one of NASA´s four funded surveys to carry out a 1998 U.S. Congress mandate to find and catalog by the end of 2008, at least 90 percent of all near-Earth objects (NEOs) larger than 1 kilometer across. CSS discovered over 1150 NEOs in years 2005 to 2007. In doing this survey they discovered on November 20, 2007, an asteroid, designated 2007 WD5, which initially was estimated to have a chance of hitting Mars on January 30, 2008, but further observations during the following weeks allowed NASA to rule out an impact. NASA estimated a near miss by 26,000 kilometres (16,000 mi).
- In January 2012, after a near pass-by of object 2012 BX34, a paper entitled “A Global Approach to Near-Earth Object Impact Threat Mitigation,” is released by researchers from Russia, Germany, the United States, France, Britain and Spain which discusses the “NEOShield” project.
- Earth woefully unprepared for surprise comet or asteroid, Nasa scientist warns The Guardian
- S.-Y. Park and I. M. Ross, “Two-Body Optimization for Deflecting Earth-Crossing Asteroids,” Journal of Guidance, Control and Dynamics, Vol. 22, No.3, 1999, pp.415–420.
- Lu, Edward T. and Stanley G. Love. A Gravitational Tractor for Towing Asteroids[dead link], NASA, Johnson Space Center, submitted to arxiv.org September 20, 2005. (PDF documentArchived October 5, 2016, at the Wayback Machine.).
- “Report of the Task Force on potentially hazardous Near Earth Objects” (PDF). British National Space Center. Retrieved 2008-10-21., p. 12.
- U.S.Congress (19 March 2013). “Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II) – Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session” (PDF). United States Congress. p. 147. Retrieved 3 May 2014.
- Canavan, G. H.; Solem, J. C.; Rather, D. G. (1993). “Proceedings of the Near-Earth-Object Interception Workshop, January 14-16, 1992, Los Alamos, NM”. Los Alamos National Laboratory LA–12476-C.
- Morrison, D., 25 January 1992, The Spaceguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop Archived October 13, 2016, at the Wayback Machine., NASA, Washington, D.C.
- Shoemaker, E.M., 1995, Report of the Near-Earth Objects Survey Working Group, NASA Office of Space Science, Solar System Exploration Office
- National Academy of Sciences. 2010.Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies: Final Report. Washington, DC: The National Academies Press. Available at: “Archived copy”. Archived from the original on 2014-08-06. Retrieved 2016-10-02..
- Stokes, GStokes, G.; J. Evans (18–25 July 2004). Detection and discovery of near-Earth asteroids by the linear program. 35th COSPAR Scientific Assembly. Paris, France. p. 4338. Retrieved 2007-10-23.
- “Lincoln Near-Earth Asteroid Research (LINEAR)”. National Aeronautics and Space Administration. 23 October 2007.
- “The Spacewatch Project”. Retrieved 2007-10-23.
- “Near-Earth Objects Search Program”. National Aeronautics and Space Administration. 23 October 2007.
- “NEOShield – Near Earth Object – Asteroid Impact Prevention”.
- “NEOShield – Near Earth Object – Asteroid Impact Prevention”.
- “NEOShield Project”. European Union Consortium. 17 November 2016.
- “NASA Releases Near-Earth Object Search Report”. National Aeronautics and Space Administration. Retrieved 2007-10-23.
- David Morrison. “NASA NEO Workshop”. National Aeronautics and Space Administration.
- Powell, Corey S. “Developing Early Warning Systems for Killer Asteroids” Archived October 28, 2016, at the Wayback Machine., Discover, August 14, 2013, pp. 60–61 (subscription required).
- “The Sentinel Mission”. B612 Foundation. Retrieved September 19, 2012.
- Broad, William J. Vindication for Entrepreneurs Watching Sky: Yes, It Can Fall Archived November 4, 2014, at the Wayback Machine., 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.
- Wall, Mike (July 10, 2012). “Private Space Telescope Project Could Boost Asteroid Mining”. Space.com. Retrieved September 14, 2012.
- Powell, Corey S. How to Deflect a Killer Asteroid: Researchers Come Up With Contingency Plans That Could Help Our Planet Dodge A Cosmic Bullet Archived August 28, 2016, at the Wayback Machine., Discover website, September 18, 2013 (subscription required), and in print as “How to Dodge a Cosmic Bullet”, October 2013. Retrieved July 15, 2014.
- “PROJECT B612: Deflecting an Asteroid using Nuclear-Powered Plasma Drive Propulsion (home page)”. Project B612 (now B612 Foundation). November 26, 2002. Archived from the original on July 12, 2011. Retrieved April 15, 2012.
- Lu, Edward T.; Love, Stanley G. (2005). “Gravitational Tractor For Towing Asteroids”. Nature. 438 (7065): 177–178. arXiv:. Bibcode:2005Natur.438..177L. doi:10.1038/438177a. PMID 16281025.
- “orbit@home is upgrading!”. Orbit.psi.edu. Retrieved 2013-10-29.
- “orbit@home is upgrading!”. Orbit.psi.edu. Retrieved 2013-10-29.
- 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[permanent dead link]
- Hildebrand, A. R.; Tedesco, E. F.; Carroll, K. A.; et al. (2008). The Near Earth Object Surveillance Satellite (NEOSSat) Mission Will Conduct an Efficient Space-Based Asteroid Survey at Low Solar Elongations (PDF). Asteroids, Comets, Meteors. Bibcode:2008LPICo1405.8293H. Paper id 8293.
- Spears, Tom (May 2, 2008). “Canada space mission targets asteroids”. Calgary Herald via Canada.com. Archived from the original on November 6, 2012. Retrieved June 27, 2008.
- We Saw It Coming: Asteroid Monitored from Outer Space to Ground Impact Archived March 3, 2016, at the Wayback Machine. Newswise, Retrieved on March 26, 2009.
- Predicting Apophis’ Earth Encounters in 2029 and 2036Archived October 29, 2016, at the Wayback Machine.
- “Why we have Asteroid “Scares””. Spaceguard UK. Archived from the original on December 22, 2007. (Original Site is no longer available, see Archived Site at )
- Canavan, G. H; Solem, J. C. (1992). “Interception of near-Earth objects”. Mercury. 21 (3): 107–109. ISSN 0047-6773.
- 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.
- Solem, J. C. (1993). “Interception of comets and asteroids on collision course with Earth”. Journal of Spacecraft and Rockets. The American Institute of Aeronautics and Astronautics (AIAA). 30 (2): 222–228. doi:10.2514/3.11531.
- Solem, J. C.; Snell, C. (1994). “Terminal intercept for less than one orbital period warning Archived May 6, 2016, at the Wayback Machine.”, a chapter in Hazards Due to Comets and Asteroids, Geherels, T., ed. (University of Arizona Press, Tucson), pp. 1013-1034.
- Solem, J. C. (2000). “Deflection and disruption of asteroids on collision course with Earth”. Journal of the British Interplanetary Society. 53: 180–196.
- Ross, I. M., Park, S.-Y. and Porter, S. E., “Gravitational Effects of Earth in Optimizing Delta-V for Deflecting Earth-Crossing Asteroids,” Journal of Spacecraft and Rockets, Vol. 38, No. 5, 2001, pp. 759–764.
- 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 Archived March 4, 2016, at the Wayback Machine.
- Dillow, Clay (9 April 2012). “How it Would Work: Destroying an Incoming Killer Asteroid With a Nuclear Blast”. Popular Science. Bonnier. Retrieved 6 January 2013.
- RAGE Hydrocode Modeling of Asteroid Mitigation:Surface and Subsurface Explosions in Porous PHO Objects. Weaver et. al 2011[permanent dead link]
- Further RAGE modeling of Asteroid mitigation, surface and subsurface explosions in porous objects. Weaver et. al 2011[permanent dead link]
- Simonenko, V.; Nogin, V.; Petrov, D.; Shubin, O.; Solem, J. C. (1994). “Defending the Earth against impacts from large comets and asteroids Archived May 16, 2016, at the Wayback Machine.”, a chapter in Hazards Due to Comets and Asteroids, Geherels, T., ed. (University of Arizona Press, Tucson), pp. 929-954.
- 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).
- Solem, J. C. (1999). “Comet and asteroid hazards: Threat and mitigation”. Science of Tsunami Hazards. 17 (3): 141–154.
- “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies ( 2010 ) National Academy of Sciences page 77”.
- “Physics.nist.gov”. Physics.nist.gov. Retrieved 2011-11-08.
- “Archived copy”. Archived from the original on 2016-08-05. Retrieved 2014-08-03. NASA plans ‘Armageddon’ spacecraft to blast asteroid 2007. The warheads would explode at a distance of one-third of the NEO’s diameter and each detonation’s X and gamma rays and neutrons would turn part of the NEO’s surface into an expanding plasma to generate a force to deflect the asteroid.
- How to Stop a Killer Asteroid When a civilization-ending space rock bears down on us, this nuclear option might just save our species. 2015 Archived August 27, 2016, at the Wayback Machine.
- Islands in Space, Dandridge M. Cole and Donald W. Cox, pp. 126–127.
- Goldstein, R. M. (1968). “Radar Observations of Icarus”. Science. 162 (3856): 903–4. Bibcode:1968Sci…162..903G. doi:10.1126/science.162.3856.903. PMID 17769079.
- “Systems Engineering: Avoiding an Asteroid” ArchivedJuly 21, 2013, at the Wayback Machine., Time Magazine, June 16, 1967.
- Day, Dwayne A., “Giant bombs on giant rockets: Project Icarus” Archived April 15, 2016, at the Wayback Machine., The Space Review, Monday, July 5, 2004
- Kleiman Louis A., Project Icarus: an MIT Student Project in Systems Engineering Archived October 17, 2007, at the Wayback Machine., Cambridge, Massachusetts : MIT Press, 1968
- ‘Project Icarus Archived June 2, 2016, at the Wayback Machine.
- “MIT Course precept for movie” Archived November 4, 2016, at the Wayback Machine., The Tech, MIT, October 30, 1979
- “Archived copy”. Archived from the original on 2016-03-05. Retrieved 2015-11-20. Near-Earth Object Survey and Deflection Analysis of Alternatives Report to Congress March 2007
- “NASA plans ‘Armageddon’ spacecraft to blast asteroid”Archived September 5, 2011, at the Wayback Machine. article at Flightglobal.com
- Near Earth Object (NEO) Mitigation Options Using Exploration Technologies Archived July 1, 2015, at the Wayback Machine.
- Towards Designing an Integrated Architecture for NEO Characterization,Mitigation, Scientific Evaluation, and Resource Utilization
- “Archived copy”. Archived from the original on 2016-04-01. Retrieved 2013-07-02. Nuking Dangerous Asteroids Might Be the Best Protection, Expert Says. Includes a supercomputer simulation video provided by Los Alamos National Laboratory.
- Mike Wall (February 14, 2014). “How Nuclear Bombs Could Save Earth from Killer Asteroids”.
- An Innovative Solution to NASA’s NEO Impact Threat Mitigation Grand Challenge and Flight Validation Mission Architecture Development, 2014 Archived March 4, 2016, at the Wayback Machine.
- EADP partners Archived October 25, 2016, at the Wayback Machine.
- Asteroid Defence, Emergency Asteroid Defence Project[permanent dead link]
- Planetary defense workshop LLNL 1995
- Jason Mick (October 17, 2013). “The mother of all bombs would sit in wait in an orbitary platform”.
- A 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 Archived March 20, 2016, at the Wayback Machine.
- United States, Russia Sign Agreement to Further Research and Development Collaboration in Nuclear Energy and SecurityArchived March 4, 2016, at the Wayback Machine.
- “”Actions Needed by NNSA to Clarify Dismantlement Performance Goal”, Report to the Subcommittee on Energy and Water Development, Committee on Appropriations, U.S. Senate, United States Government Accountability Office,” (PDF). April 2014. Retrieved 4 August 2014.
- “Department of Energy FY 2015 Congressional Budget Request for the National Nuclear Security Administration”(PDF). March 2014. Retrieved 4 August 2014.
- “Archived copy”. Archived from the original on 2016-04-01. Retrieved 2013-07-02. 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.
- John Remo, the dilemma if nuclear energy in space, 2015Archived August 20, 2016, at the Wayback Machine.
- Chapter 10 – Comets Astronomy 9601 ArchivedNovember 7, 2016, at the Wayback Machine.
- NASA deep impact impactor Archived June 23, 2016, at the Wayback Machine.
- A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1’s gravity, mass, and density[permanent dead link]
- Photometry and Imaging Results for Comet 9P/Tempel 1 and Deep Impact: Gas Production Rates, Postimpact Light Curves, and Ejecta Plume Morphology[permanent dead link]
- Deep Impact: Excavating Comet Tempel 1 Archived June 28, 2011, at the Wayback Machine.
- The Orbital History of Comet 9P/Tempel 1 Archived March 6, 2016, at the Wayback Machine.
- “Court Rejects Russian Astrologer’s Lawsuit Against NASA”. MosNews.com. August 11, 2005. Archived from the original on May 21, 2007. Retrieved May 11, 2009.
- “Kinetic impactor -“.
- “NEOShield Project”. European Union Consortium. 17 November 2016.
- Chapman, Clark R. and Daniel D. Durda. The Comet/Asteroid Impact Hazard: A Systems Approach Archived March 4, 2016, at the Wayback Machine., Boulder, CO: Office of Space Studies, Southwest Research Institute, Space Engineering and Technology Branch, Johns Hopkins University Applied Physics Laboratory.
- DISRUPTION OF KILOMETER-SIZED ASTEROIDS BY ENERGETIC COLLISIONS Asphaug et. al 1998 ArchivedMarch 6, 2016, at the Wayback Machine.
- “Center for Strategy and Technology”.
- “Space Based Laser. FAS.”.
- Philip Lubin: A space-based array for planetary defense (video), SPIE Newsroom, 22 November 2013 Archived June 9, 2015, at the Wayback Machine.
- –in a lecture to the Arizona Geological Society in 12-96.
- Is an asteroid capture possible/feasible?; Asteroid movement/retrieval; Asteroid relocation/mining; etceras…Archived November 6, 2016, at the Wayback Machine., Space-tech Digest #70 [bulletin board], Carnegie Mellon University, July 19–25, 1990.
- David French (October 2009). “Near-Earth Object Threat Mitigation Using a Tethered Ballast Mass”. J. Aerosp. Engrg.
- “How to Colonize an Asteroid Solenoids”.
- “National Space Society, From Ad Astra, Volume 18 Number 2, Summer 2006”.
- Madrigal, Alexis (16 December 2009). “Saving Earth From an Asteroid Will Take Diplomats, Not Heroes”. WIRED. Retrieved 17 December 2009.
- Islands in Space, Dandridge M. Cole and Donald W. Cox, pp. 7–8.
- “Astronauts push for strategies, spacecraft to prevent calamitous asteroid strike”. Pittsburgh Post-Gazette. November 28, 2005. Retrieved 2008-01-18.
- “Subcommittee Questions NASA’s Plan for Detecting Hazardous Asteroids”.
- Donald K. Yeomans (2007-11-08). “Testimony Before The House Committee On Science And Technology Subcommittee On Space And Aeronautics: Near-Earth Objects (NEOS) – Status Of The Survey Program And Review Of Nasa’s Report To Congress” (PDF).
- Catlalina Sky Survey website Archived October 19, 2016, at the Wayback Machine.
- “Catalina Sky Survey Discovers Space Rock That Could Hit Mars”. Retrieved 2007-12-22.
- “Recently Discovered Asteroid Could Hit Mars in January”. Retrieved 2007-12-22.
- Leonard David. Asteroid Threat to Earth Sparks Global ‘NEOShield’ Project Archived March 9, 2016, at the Wayback Machine., SPACE.com, 26 January 2012.
- Bus-sized asteroid buzzes Earth today passing within 36,000 miles of our atmosphere Archived January 28, 2012, at the Wayback Machine., DailyMail online, 27 January 2012.
- Defenders Of The Planet Archived February 1, 2014, at the Wayback Machine., Off The Fence website. Retrieved April 20, 2013.
- Luis Alvarez et al. 1980 paper in Science magazine on the great mass extinction 65 million years ago that led to the proliferation of mammal species such as the rise of the human race, thanks to asteroid-impact, a controversial theory in its day, now generally accepted.
- Izzo, D., Bourdoux, A., Walker, R. and Ongaro, F.; “Optimal Trajectories for the Impulsive Deflection of NEOs”; Paper IAC-05-C1.5.06, 56th International Astronautical Congress, Fukuoka, Japan, (October 2005). Later published in Acta Astronautica, Vol. 59, No. 1-5, pp. 294–300, April 2006, available in esa.int – The first scientific paper proving that Apophis can be deflected by a small sized kinetic impactor.
- Clark R. Chapman, Daniel D. Durda & Robert E. Gold (February 24, 2001) Impact Hazard, a Systems Approach, white paper on public policy issues associated with the impact hazard, at boulder.swri.edu
- Donald W. Cox, and James H. Chestek. 1996. Doomsday Asteroid: Can We Survive? New York: Prometheus Books. ISBN 1-57392-066-5. (Note that despite its sensationalist title, this is a good treatment of the subject and includes a nice discussion of the collateral space development possibilities.)
- David Morrison Is the Sky Falling?, Skeptical Inquirer 1997.
- David Morrison, Alan W Harris, Geoff Summer, Clark R. Chapman, & Andrea Carusi Dealing with Impact Hazard, 2002 technical summary
- Russell L. Schweickart, Edward T. Lu, Piet Hut and Clark R. Chapman; “The Asteroid Tugboat”; Scientific American (November 2003).
- Kunio M. Sayanagi “How to Deflect an Asteroid” Ars Technica (April 2008).
Source: Wikipedia and trinhmanhdo.com