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Planetary Defense: Preventing a World of Trouble
(Released November 2005)

 
  by Salvatore A. Vittorio  

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Review Article
Our mission, should we choose to accept it: (1) Make up to six billion people understand the danger that faces this world. (2) Make them care enough. (3) Keep them caring long enough for it to matter. (4) Give them what they need to stop what's coming.
- Larry Niven, February 20, 2004 (from a presentation at the 2004 Planetary Defense Conference, sponsored by the American Institute of Aeronautics and Astronautics)

I. Introduction - The Threat

Planetary defense encompasses protecting the Earth from potential destruction due to impact by a large piece of space debris. Astronomical telescopes and deep space radar systems have verified the existence of a large number of near Earth objects (NEOs), such as asteroids, meteoroids, and comets that potentially could destroy most life on Earth. Where NEOs intersect Earth's orbit, there exists a risk of a collision. [1] An asteroid with a diameter of 1-10 km would strike the Earth with a power rivaling the strength of a multiple warhead attack with the most powerful thermonuclear explosives known to man. Computational fluid dynamics studies have indicated that an ocean strike by such an asteroid may create a gigantic tsunami that would flood and obliterate coastal regions. Perhaps even more significantly, a land strike may eject a massive dust cloud, rivaling that from the most powerful volcanic explosion, which could seriously affect climate on the scale of two to three years. It could alter our biosphere to the point that life as we know it would cease to exist. As recently as 1998, the astronomical and astrophysics community thought that most of the known NEOs do not pose a near-term threat, and therefore do not present any danger to the Earth and its biosphere. However, the relatively recent collision of the comet Shoemaker-Levy 9 with the planet Jupiter on July 16, 1994, and continuing discoveries of non-cataloged asteroids passing near Earth without any advanced warning, have increased concerns. It is worthwhile to note that one striking feature of practically every celestial body in our solar system is the abundance of impact craters. [2]

Geological evidence and observations of planetary bodies confirm the existence of a threat, albeit small, that Earth could one day be struck by a comet, meteor, or asteroid on a collision course from space that would be large enough to cause widespread destruction of modern society. In the last 100 years, a massive impact took place in Siberia (Tunguska, 1908) that devastated 2000 square kilometers and carried the destructive force of 12.5 megatons of TNT. This impactor was only 60 meters in diameter. The Earth's surface still shows scars of previous larger-scale impacts. A 100-meter diameter meteor over 20,000 years ago is believed to have caused the Meteor Crater in Arizona. In the more massive Cretaceous-Tertiary impact (K-T impact), a 10-km diameter object struck off the Yucatan Peninsula some 65.5 million years ago. Scientists found the crater around 1995 using seismic monitoring equipment designed to hunt for oil. Probably the size of a small city, the Chicxulub impact is widely believed to have triggered a mass dinosaur die-off, either through a global firestorm caused by earthquakes releasing pockets of methane set afire by lightning, or through massive long-term environmental changes. The impact likely also triggered giant tsunamis across the ocean and earthquakes that reverberated around the world. While such enormous impacts are certainly infrequent, objects with diameters of approximately 1 km can be expected to intercept the Earth every 100,000 years. [3]

Some thought and planning, therefore, is needed to provide a reasonable level of protection against such disastrous events. Identification and cataloging of NEOs and celestial bodies is an important first step. Observation and tracking of small (1 km or less) objects is a difficult task given the low albedos of the target bodies and their small size. However, accurate long-term orbital prediction models must be developed to allow for adequate response time. NASA and the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, California, maintain an active list of NEOs sorted by a weighted scale indicating their approach distance and destructive potential (the Palermo Technical Impact Hazard Scale). In 2003, over 10 objects were identified that passed within 1.5 times the distance from the Earth to the Moon (1 lunar distance = 384,000 km). The smallest objects in the JPL database are 20 meters in diameter with typical NEOs in the 500-meter to 1-km diameter range. [14, 3] Currently, the highest active object on the Palermo Technical Impact Hazard Scale (Palermo scale) is asteroid 2004 VD17, with a cumulative Palermo scale value of -1.06. [4]
[Note 1]

NASA has proposed new observatories that will be able to detect even smaller objects. For example, NASA's Revolutionary Aerospace Systems Concepts (RASC) program conducted a study called CAPS (Comet Asteroid Protection System) that promoted a lunar telescope installation for conducting NEO detection research. The question remains what should be done if a planetary impactor on a collision course with Earth is actually confirmed. [3]

II. Categorizing the Threat

The large number of circumstances in which a NEO might threaten the Earth can be characterized by several parameters, such as warning time; NEO size, mass, and orbital parameters; impact location; certainty of impact; NEO orbital class (e.g., those of the Atens, Apollo, and Amor families of asteroids, the trans-Neptunian objects, and the comets); NEO composition; and NEO spin.

The concept of a response decision chart (see Figure 1), constructed by researchers Peter Nicolas, Andrew Barton, Douglas Robinson, and Jean Marc Salotti, could be applied to break down the problem into manageable sub-categories, where each branch of the tree corresponds to a particular class of NEO threat scenarios for which response options would be similar. [7] Thus, for any threat scenario, it can be determined what responses are appropriate. The aim of such a classification tree is to provide a global tool providing more useful information to the policy maker by laying down in a simple but comprehensive way all possible NEO impact scenarios, consequences, and responses. In addition to the above parameters, other parameters might require expert scientific advice, such as detectability and destructibility. [6]

Warning
Time
Damage
Potential
Response
Limitations
Hit
Location
Responses
short → destructible → fragment-
ation
small → ocean → pulverize
medium → partially →
destructible
fragment-
ation
large → deflectible → small
evacuation
long → land → large
evacuation
neither → relocation
Figure 1: Response decision chart (simplified version) [7]
Click here for complete chart

The specific classification breakdown of the parameters of warning time, damage potential, hit location, and responses are discussed below. In addition, another parameter, certainty of impact, is also addressed.

A. Warning time

The classification of 'Short' is intended for threats with less than one year of warning time. Such scenarios exclude the development of new space missions, allowing only ground-based responses. If the warning time is in the scale of decades, there are many opportunities for both space-based and Earth-based mitigation efforts. The category named 'Medium' encompasses the warning time frame from 2 to 30 years, presenting the possibility of new space missions with existing technology. The 'Long' warning time category encompasses all NEO impact scenarios with the impact occurring more than 30 years in the future, so one can only speculate about far future mitigation technologies and approaches. [6]

B. Damage potential

This refers to the effects that an NEO would have if it impacts the Earth. The effects are largely dependent on the size and mass of the NEO. The 'Large' category includes all NEOs larger than 2 km since these objects would cause very large disasters and therefore require global response efforts. The 'Small' category is intended to include any NEO threats that would be of local or regional significance. It is generally thought that such NEOs can be successfully deflected or destroyed with modern technology if there is sufficient warning time. Excluded are the very small NEOs that would burn up in the Earth's atmosphere (less than 30 m), since these require little or no response. [6]

C. Response limitations

The branches at this point of the tree in Figure 1 distinguish between the different types of space-based responses. The category named 'Deflectable' is for NEOs that are small enough or with long enough warning time to enable deflection of the body with sufficient delta-v to prevent its impact with Earth. The object's accessibility should also be taken into account. When possible, this category is the most desirable response since it doesn't affect the Earth. 'Destructible' is a possibility for objects that are not deflectable. In some cases the NEO is held together too weakly to be able to absorb the energy transfer required for deflection. There are also cases where 'Partially Destructible' is a possibility. Reducing the NEO into many smaller fragments by nuclear blasts may reduce the scale of disaster on Earth, although it also risks spreading the effects (including nuclear fallout) over a wider area. Finally, 'Neither' is included for the cases where the NEO is neither destructible nor deflectable. [6]

D. Hit location

The two most important distinctions for a NEO striking the Earth's surface are between 'Ocean' impacts and 'Land' impacts. The most devastating possible effect of an ocean impact would be a tsunami, which would damage coasts in the region or around the world. On the other hand, a similar size land impact would lead to more debris and dust being ejected into the atmosphere, likely causing more severe climatic changes. Accurate prediction of the impact location is not always possible since it depends on accurate orbital parameters for the NEO. In some cases, data will be insufficient to determine whether the NEO will hit the land or the ocean (due to long warning times, incomplete orbital observations, etc.). A dispersed impact zone including both ocean and land regions is also possible. Furthermore, the presence of the Earth's atmosphere may also lead to dispersion of the body over a wide area. [6]

E. Certainty of impact

Another important tool for categorizing the Earth impact hazard associated with newly discovered NEOs is the Torino Scale, which is equivalent to the "Richter Scale" but for NEOs. This scale was created by Professor Richard P. Binzel at the Massachusetts Institute of Technology and revised at an international conference on NEOs held in Torino, Italy, in June 1999. The Torino scale utilizes numbers that range from 0 to 10, where 0 indicates an object that has a zero or negligibly small chance of collision with the Earth, or that is too small to penetrate the Earth's atmosphere intact in the event that a collision does occur. A 10 indicates that a collision is certain, and the impacting object is so large that it is capable of precipitating a global disaster. An object is assigned a value based on its collision probability and its kinetic energy (proportional to its mass times the square of its encounter velocity). [6]

III. Some Possible Response Options

For all we know, a large asteroid may be heading this way right now, and you'll never get this [conversation] on the air. The danger of asteroid or comet impact is one of the best reasons for getting into space. I'm very fond of quoting my friend Larry Niven: 'The dinosaurs became extinct because they didn't have a space program.'And if we become extinct because we don't have a space program, it'll serve us right!
-Arthur C. Clarke

Deflecting or destroying a NEO in space is still in the realm of science fiction. However, if an impact does occur, many steps can be taken on Earth to mitigate its effects. It would be best, of course, to avoid the initial catastrophe. The idea of the deflection strategy is to change the orbit of the NEO in order to prevent an impact with Earth. According to C. Gritzner deflection is the only solution in case of a NEO larger than about 100m, because the destruction strategy might worsen the situation. [15] However, the deflection of a NEO years ahead of its impact requires that detection be achieved soon enough and its orbital elements precisely computed.

Destruction—the second best strategy—refers to breaking the object into many pieces. The destruction of the NEO body is generally proposed when there is very little time left before impact, weeks or months, so that any deflection would be insufficient. The timing of the interception is crucial in the response decision process and depends on the threat's detection time. If there is time for an interception, then a space-based response is possible; if not, then the only solution is to evacuate the impact location. [6]

A. Space-based responses Space-based response strategies to threatening NEOs can be summarized as follows:

  1. Non-nuclear or Kinetic deflection: A non-nuclear novel concept is employed to deflect an NEO. Alternatively, for kinetic deflection, a large spacecraft or several spacecraft, or a missile, is/are sent to impact and deflect the NEO using only kinetic energy.
  2. Nuclear deflection: Nuclear explosions are triggered at a distance, on the surface or after penetration, provoking the ejection of rocks from the NEO, which in turn reacts by a small deflection.
  3. Nuclear destruction: In some cases, the explosion might cause the partial fragmentation or even the pulverization of the NEO.
  4. Mass driver: If the action time is significant, it is possible to land a device that would regularly eject some matter from the asteroid and therefore slowly deflect it from its original trajectory.
  5. Billiards shot: This option consists of deflecting a small asteroid, putting it on a collision course with the Earth-threatening NEO.

The selection of the best option depends on the time available for action and the diameter (size) of the NEO. The effects of a large blast near a NEO depend not only on the NEO's mass but also on its composition and structure. For rubble-pile asteroids—loose aggregations of rock, presumably the result of a collision—deflection is impractical. In cases where deflection is desired, nuclear blasts should not be triggered too close to the NEO to avoid its fragmentation. When NEOs are too large for nuclear destruction or deflection, the billiards shot option is theoretically possible but remains uncertain in terms of accuracy and technical feasibility. In this option, a small NEO's orbit is changed in order to achieve a collision with the (larger) NEO on a collision course with Earth. This method could deflect even 10-km-class NEOs. [6]

Particular scenarios or specifics concerning the above space-based response strategies are discussed and/or considered below. Focus is placed mainly on threatening near-Earth asteroids (NEAs). Near-Earth asteroids, a subset of the NEOs, are asteroids whose orbit intersects Earth's orbit and which may therefore pose a collision danger, as well as being most easily accessible for spacecraft from Earth. [NEA Information]

1. Non-nuclear and kinetic deflection concepts

A. Mirror Ablation approach One method of deflecting Earth-threatening asteroids uses a solar collector that focuses sunlight on the surface of the asteroid. This strongly heats a small spot, and vaporizes enough material so that the thrust from the expanding jet of gas and dust can, over a period of years, divert the asteroid. The primary difficulty with this scheme is the danger of fouling of the last optical element in the system by evaporated material. Overall, this scheme does not involve a large extension of present technology. It is effective on comets, the orbits of which are perturbed by forces generated in reaction to the jets of gas and dust that emanate from their surfaces during passage through the inner solar system [16, 17, 18].

Asteroids, by definition, do not emit such jets, and their orbits are therefore much more predictable than those of comets.However, if some means were found to create such comet-like jets even on asteroids, the thrust of these jets could be used to steer a threatening asteroid out of a collision course with the Earth. This is the basic idea behind the Mirror Ablation approach, shown in Figure 2. Sunlight, freely available in space, could be concentrated on the surface of the asteroid, just as a burning glass can raise the temperature of wood and leaves on the Earth's surface to the ignition point. If the sunlight is sufficiently concentrated, even the surfaces of rocky asteroids may become hot enough to evaporate and develop a steady thrust that will, over time, impart a velocity increment sufficient to avert a collision with the Earth. [Note 2] [8]

mirror ablation mission
Figure 2. Schematic illustration of the basic components of the Mirror Ablation Mission to deflect an Earth-threatening asteroid [8]

B. Kinetic energy (KE) projectile

This asteroid deflection technique is based on a fictitious threat scenario. The mission objective is to prevent the collision of the virtual 200-meter binary asteroid Athos with Earth on February 29, 2016. The detection date of the asteroid is assumed to be February 22, 2005, which gives a short span from the time of detection to the launch date of the deflection spacecraft. The technique is based on the momentum transfer from an impacting spacecraft on the hazardous object. The impacting process coincides with the ejection of crater material, where the total momentum change of the target object is the momentum of the escaping ejecta plus the momentum carried with the projectile. For nonporous targets the ratio between ejecta momentum and projectile momentum can be as large as 13, whereas for porous targets this could be decreased to 0.2, yielding a momentum enhancement factor of 14 and 1.2, respectively. [23] In this particular scenario a momentum enhancement factor of 3 is assumed, which was deduced from the physical properties of Athos and experimental factors for analogous materials. [Note 3] A state-of-the-art launch system arsenal is thus envisioned that comprises two rockets, which could both be used for KE interaction. These are the American Delta IV Heavy and the European Ariane 5 capable of launching 12.4 and 12 tons (planned for 2006) into geostationary transfer orbit (GTO), respectively. A KE mission opportunity is identified, which maximizes the product of spacecraft mass and relative velocity (in Athos' direction of flight) when impacting on Athos in April 2012. The launch from GTO is scheduled for December 27, 2011.

A velocity change of 787 m/s, applied during perigee passage, is required to get onto an interplanetary orbit. After a low-demanding deep space maneuver (4 m/s) the spacecraft swings by Earth on February 15, 2012. This gravity assist from Earth enables the high energy impact with Athos on April 5, 2012. [Note 4] Since launch systems cannot be started within short intervals, the spacecraft are to be launched into geostationary transfer parking orbits, beginning one year in advance of interplanetary transfer to Athos. Last, but not least, it has to be ensured that the projectile shelling will not cause fragmentation of Athos into large boulders that possibly remain on collision course with Earth. The technology readiness level of KE projectiles has been evaluated as high, and no technological problems are expected. Possible development efforts could deal with the shape and composition of a KE spacecraft. [9]

2. Nuclear Deflection Concept - An asteroid interceptor

In one particular study a conceptual design was developed for an asteroid interceptor vehicle using a nuclear explosive. [10] The proposed target for this mission is the 200-m asteroid threat object Athos (see above). The proposed interceptor's basic functions include locating the NEO, delivering a nuclear device to it, exploding the nuclear device at the appropriate time/location, and verifying the system's performance. In order to reduce interceptor propellant requirements, a direct intercept approach, rather than a rendezvous approach, was chosen in this scenario. Direct intercept permits the use of a relatively large nuclear device, but places severe constraints on the intercept conditions and affects the potential accuracy of weapon delivery. This study limited the interceptor launch to existing vehicles in order to enable a near-term deployment. An overall mission objective is to provide opportunities for multiple shots, if necessary, which implies that it would be useful to verify the performance of each shot to determine if another shot is required. For this reason, it was decided to provide a separate cruise stage on the interceptor for the purpose of sensing the NEO and the kill vehicle that carries the nuclear weapon, and relaying observations back to Earth. Other reasons for including the cruise stage are a desire to lighten the kill vehicle and the possible use of cooperative target tracking between the cruise stage and kill vehicle.

The proposed interceptor consists of a cruise stage and a kill-vehicle stage. The function of the cruise stage is to perform spacecraft housekeeping for the interceptor during transit, relay data to and from the kill vehicle during endgame, and verify system performance during and shortly after intercept. The function of the kill-vehicle stage is to deliver the weapon to the desired target point and detonate. A mass goal for the entire interceptor stack of 6000 kg is based on the launch vehicle capacity for a Delta IV Heavy and the intercept trajectory requirements. Conceptual mass estimates for both interceptor stages were developed using the spreadsheet-based Aerospace Concurrent Engineering Model (CEM). [19] This model utilizes historical spacecraft design relations and basic physical principles to parametrically estimate space vehicle mass.

Payload capacity for the nuclear device was estimated by systematically increasing the weapon mass until a launch mass limit of 6000 kg was reached for the entire launch stack. Based on the analysis, the estimated weapon capacity for the proposed conceptual design is 1500 kg or an approximately 1.9 megaton yield with a warhead optimized for neutron emission. [20] High neutron emission enables maximum momentum transfer to the NEO target. [10]

3. Nuclear Destruction Concept - Nuclear fragmentation

The equations used to model the catastrophic fragmentation of a near-Earth solid body asteroid derive from the work of Thomas J. Ahrens, California Institute of Technology, and Alan W. Harris, Jet Propulsion Laboratory, [21] based on the assumption that an explosive device is placed deep enough below the asteroid's surface to produce near-optimum fragmentation. The location for optimum fragmentation is generally considered to be the target object's geometric center.

This assumes ideal destruction conditions, namely, (1) the asteroid is a perfectly spherical homogeneous structure, (2) the explosive charge is placed at the exact geometric center, and (3) the explosion fractures the target body into pieces no larger than 10 meters in diameter. Although open to debate, it is generally assumed that fragments of this size would be much less likely than the original body to survive entry through Earth's atmosphere. Even if any fragments did reach the ground, the impact of these relatively small objects, spread over a large area, would be less damaging from a global point of view than a single massive asteroid strike.

Asteroids greater than 2 km in diameter would be considered catastrophic to the Earth. Unfortunately, there are currently no existing nuclear devices that could catastrophically fragment an asteroid greater than 2 km in diameter.

One idea considered to place an explosive device at the geometric center of an asteroid that can be destroyed (those about less than 2 km in diameter) of the target body is to use the same technology that is found in the " long-rod bunker buster" ordinance that the U.S. military employs against underground facilities. This idea has the advantage of not requiring a delta-v breaking maneuver to rendezvous with and "soft" land on the target. Instead the outbound kinetic energy is used to bury the device to the optimum depth. It is assumed that the explosive device can be successfully delivered kinetically to the "center" of a 200 m diameter asteroid; anything larger may require the use of some sort of drilling or auger device.

4. Mass Driver Concept - The Modular/Swarm Architecture

A modular architecture of smaller devices can provide the means to build up defensive capability immediately while allowing for system improvements and modifications over time. An approach conceptualized by SpaceWorks Engineering, Inc. (SEI, Atlanta, Georgia) is to subtly change the orbit of a potential impactor far from the point of impact. Mass drivers landed on the impactor will be used to eject small pieces of the asteroid's own mass to gradually affect its velocity. SEI advocates the use of multiple, "small" lander spacecraft to provide a modular, scalable, and rapid response to planetary defense.

Their solution consists of hundreds or thousands of identical spacecraft that will intercept the target body and conduct mass driver/ejector operations to perturb the target body's trajectory to the point where an impact with Earth can be avoided. In the nominal configuration, each spacecraft will be independently controlled and powered, but will work in loose coordination with other members of the network. The spacecraft will be nuclear powered, possibly pre-deployed outside of low Earth orbit (LEO) (likely an Earth-Moon or Earth-Sun libration point), and be capable of using chemical propulsive boost to rapidly intercept an incoming target. Upon arrival, each spacecraft will begin to eject small amounts of mass from the asteroid that will, over time, slightly change target's heliocentric (Sun-centered) orbit so that impact is avoided. SEI's modular approach offers the unique advantages of overall mission reliability through massive redundancy, economies-of-scale during spacecraft production, flexible and practical launch and transfer to an on-orbit pre-deployment location, a tailorable response depending on the size and nature of the incoming threat, and the production of only small bits of ejecta that will not independently survive Earth atmospheric entry. Trade studies conducted by SEI have assumed that the launch rail that will eject the mined mass from the impactor (asteroid) should be as long as reasonable, but launch packaging and stiffness considerations will limit it to no more than 10 meters. As the launch velocity increases for a given shot mass, the mass driver power increases in a cubic fashion, thus driving up the size and mass of the mass driver capacitor units. The energy (or work) used to accelerate the ejecta increases proportional to the square of the launch velocity, thus requiring a larger spacecraft power supply (or alternately a longer cycle time between shots) to recharge the capacitor units. The compressive force on the lander and rail increases relative to the square of the launch velocity. The downward force will benefit the mining process to some degree, but an excessive compressive load would require a massive lander structure and thus exacerbate the launch and deployment problem for the spacecraft.

Based on these results, SEI has established a working baseline of a 10-meter launch rail, 0.5-kg ejecta mass per shot, and a launch velocity of 1000 m/s (well within the capability of today's rail launchers). For this configuration, the ejecta will undergo an acceleration of almost 5100 Earth g's (gravitational acceleration) for a period of 0.02 seconds, which means a mass driver power of 12.5 megawatts per shot. [11]

5. Billiards Shot Concept - Artemis target

The billiards shot strategy consists in deflecting a smaller asteroid (the striker) so that it impacts and destroys the threatening asteroid before the predicted collision with Earth. The Artemis asteroid is considered here, which is 119.1 km in diameter, and has the potential of striking Earth in 2033. The kinetic energy of a small striker is more powerful than nuclear weapons for the destruction of big NEOs. Ten tons of nuclear energy is roughly equivalent to 10,000 kilotons of TNT. A 100 meters large asteroid weighs approximately 1 megaton. If the relative velocity at impact is 20 km/s, the kinetic energy is about 100,000 kilotons of TNT, which is 10 times more than the energy of a strong nuclear blast. Asteroids as small as 100 meters in diameter, therefore, could be used for a billiards shot. In the case of Artemis, a 100-meter diameter asteroid is sufficient. Considering a plane change maneuver to match Artemis's orbital plane, followed by a modification of the semi-major axis, it has been shown that the 1999 VK12 is the best striker of the billiards shot against Artemis. The closest encounter occurs in April 2027. [Note 5] Theoretically, the billiards shot option is cheap in terms of energy; however the feasibility of the maneuver has yet to be checked. In terms of energy, it could sometimes be easier to deflect a small striker and to put it on a collision course with a big target asteroid than to deflect the latter in order to avoid the collision with our planet. In the case of Artemis, however, the data are not ideal.

In general, in order to deflect the striker, numerous nuclear blasts have to be triggered close to it. The striker asteroid should be able to resist the numerous blasts. It should not be a rubble pile or a friable asteroid. Since the composition of a potential striker is usually unknown, a specific mission should be designed to assess its ability to play the striker role. Complementary studies remain to be performed to assess astronautic capabilities and to examine the billiards shot without exact matching of the planes. However, in the case of a real threat, the billiards shot could be the only option that allows deflection or destruction of a large NEO (greater than 10 km) at relatively short notice. [12]

orbital paths of the Earth
Figure 3: Orbital paths of the Earth (small circular orbit), Artemis (top), and 1999VK12 (bottom) and positions around the sun in April 2027 [12]

The selection of the best of the above five options depends on the time available for action and the diameter of the NEO. For space-based responses, the key parameter is the action time, which is as a function of the warning time and orbital parameters. Figure 4 is a graph showing space-based responses. [6]

 space-based responses
Figure 4: Selection of the best option according to the action time (in years) and the diameter of the NEO (in km) [6]

B. Earth-Based Responses

Of course, if a successful interception, destruction, or mass reduction to a safe level of the threatening near Earth object is impossible due to lack of time or incapability, then Earth based responses are the only recourse. Earth-based responses can be divided into two groups depending on the NEO disaster: (1) relocation of the endangered populations, and (2) sanctuary in appropriate shelters.
If the likelihood of a NEO impact with a particular region of the Earth is established, the most obvious step is to plan the evacuation of the population from that zone. Currently there are very few procedures for coping with this problem, and none at an international level. The International Strategy for Disaster Reduction (ISDR) is yet to recognize the NEO threat, although it has procedures to deal with large-scale disasters on an international level. [22] Furthermore there is no internationally agreed link in the NEO detection community between who would give the warning of an impact and these disaster response authorities. Thus, the time delay between confirmation of a detection and the official decision makers in government learning of the threat is probably on the order of days or weeks. The construction of shelters to protect the population is a last resort that would have to be attempted in desperate cases. Even in the other cases, in which a space mission is the main response option, a shelter plan would still need to be developed as a contingency. [6]

1. Evacuation and relocation

The term evacuation encompasses all emergency efforts to move people from the area prior to an impact. Depending on the impact's size, the time scale for evacuations would be hours to weeks. The concept of relocation refers to a broader effort to save not only humans but wildlife, cultural heritage, etc., and to establish a temporary or permanent habitat for them. The time scale for relocation is assumed to be enough for planning (weeks to decades). [6]

2. Shelters

The 'Shelter' response category refers to any attempts to provide protection from the effects of a NEO impact on Earth. Depending on the size of the NEO, there may be 'Long-Term' or 'Short-Term' sheltering. Long-term shelters are envisioned to sustain human life on Earth after a large impact has substantially changed the atmosphere and climate. This is in some ways analogous to attempts at supporting human life on other planets. Short-term shelters encompass all attempts to mitigate against the direct effects of the NEO's impact, such as a blast wave, earthquakes, etc. [6]

IV. Conclusion

It is extremely unlikely that Earth will be hit by a very large asteroid (size above 1 km) that would cause global destruction, but chances are much larger for impact with lesser asteroids (100 m or so in size) that would cause a local catastrophe. A sudden unpredicted impact may be the most probable scenario due to a currently incomplete NEOs survey. In terms of risk analysis, the main uncertainty comes from the incomplete knowledge of the number, size distribution and orbital parameters of the NEO population. The natural hazard of NEO impact could be mitigated by adequate advance action. Responding to the varying magnitudes of NEO threats involves a very broad range of disciplines and makes the planning for NEO responses a complex task. However, Earth-based responses to NEO threats can take advantage of existing natural disaster mitigation strategies applied to earthquakes, floods, hurricanes, etc. The magnitude of a NEO catastrophe could be much higher, ranging from a regional to a continental to a global scale, whereas traditional natural disaster consequences range from local to regional scales. A major step in assessing any NEO hazard is to identify the threatening object and to characterize it, either by Earth or space-based observing facilities, long enough before the possible impact to design and operate mitigation missions. Most attention to the NEO threat is currently focused on the detection issue, and the NEO response work has been almost exclusively on technology studies of possible space-based strategies. However, the problem of mitigating the effects of a large Earth-impact having large regional or even global consequences, and adapting international laws related to the choice and management of NEO response missions, needs to be tackled. [6] Given the fact that Earth encounters with near-Earth asteroids as early as 2027 or 2033 (see above) are possible, the window of opportunity for planning and organizing for such a possibility grows narrower with each passing year. In reality, we really don't know with absolute certainty when the next large asteroid (on the order of that of the K-T impact) will strike Earth. Therefore, the time to start preparing is always now.

In a paper delivered at the 2004 Planetary Defense Conference, R. Dale Brownfield of the Gaiashield Group stated:

Any Global Strategic Planetary Defense Policy on this issue must be firmly anchored in a clear and unmitigated assessment of the threat, an acute awareness of our current collective inability to reliably deal with it and a full appreciation of the magnitude of the loss when we do not. Any rational policy must appreciate that until we have deployed an effective planetary defense and mitigated this threat, TNLA [The Next Large Asteroid on its way to strike Earth] is a dire Clear and Present Danger for the whole world. That this is not just another partisan campaign issue but a new status quo for mankind requiring administration by an agency as autonomous and immune to the vagaries of economics and politics as is possible - a global human martial authority. In this arena the Mission must dictate the Policy not the other way around. Statecraft, Economics and to some degree even Science must take a backseat to the demands of Planetary Defense. [13]

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