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Asteroid deflection strategies are methods by which Near-Earth Object s could be diverted, preventing potentially catastrophic Impact Event s. A sufficiently large impact would cause massive Tsunamis and/or, by placing large quantities of dust into the stratosphere blocking sunlight, an Impact Winter . A collision between the earth and a ~10 km object 65 million years ago is believed to have produced the Chicxulub Crater and the extinction of the majority of species preserved in the fossil record. The dangers posed by such collisions go beyond the physical destruction caused by the impacts themselves. A nation hit by less than extinction-destructive force may, depending on their military and/or political situation, think they are being attacked by another nation and retaliate. If this had happened to a superpower during the Cold War , it might have thought it was under nuclear attack and "returned" fire. While in theory the chances of such an event are no greater now than at any other time in history, recent astronomical events (such as Shoemaker-Levy 9 ) have drawn attention to such a threat, and advances in technology have opened up new options. EARLY DETECTION in relation to Earth in 2029]] Almost any deflection effort requires years of warning, allowing time to build a slow-pusher or explosive device to deflect the object. A number of potential threats have been identified, such as 99942 Apophis (previously known by its Provisional Designation ), which had been given an impact probability of ~3% for the year 2029. This probability has been revised to zero on the basis of new observations. An impact by a 10 Km asteroid on the Earth is widely viewed as an Extinction-level Event , likely to cause catastrophic damage to the Biosphere . Depending on speed, objects as small as 100 M in diameter are historically extremely destructive. There is also the threat from Comet s 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. Finding out the material composition of the object is also necessary before deciding which strategy is appropriate. Missions like the 2005 Deep Impact probe have provided valuable information on what to expect. See Also: Spaceguard POPULAR STRATEGIES Nuclear weapons One of the most often proposed solutions is firing Nuclear Missiles at the oncoming asteroid to vaporize all or most of it. While today's nuclear weapons are not powerful enough to destroy a 1 km asteroid, theoretically, Thermonuclear Weapon s can be scaled up to any size so long as enough raw materials are available. If not completely vaporized, the resulting reduction of mass from the blast combined with the radiation blast could produce positive results. The largest problem with this solution is that if the asteroid breaks into fragments, any fragment larger than 35 m across would not burn up in the atmosphere and itself could impact Earth. Tracking of the thousands of fragments that could result would prove daunting. Another proposed solution is to detonate a series of smaller nuclear devices alongside the asteroid, far enough away as to not fracture the object. Providing this was done far enough in advance, the relatively small forces from any number of nuclear blasts could be enough to alter the object's trajectory enough to avoid an impact. This is a form of Nuclear Pulse Propulsion . In 1968, students at the Massachusetts Institute Of Technology designed a system using nuclear explosions to prevent a hypothetical impact on Earth by the asteroid 1566 Icarus . This design study was later published as Project Icarus . Kinetic impact An alternative means of deflecting an asteroid is to attempt to directly alter its Momentum by sending a spacecraft to collide with the asteroid. The European Space Agency is already studying preliminary design of a space mission able to demonstrate this futuristic technology. The mission, named Don Quijote , will be the first real asteroid deflection mission ever designed. In the case of 99942 Apophis it has been demonstrated by ESA 's Advanced Concepts Team that deflection could be achieved by sending a simple spacecraft weighing less than one ton to impact against the asteroid. During a trade-off study one of the leading researchers argued that a strategy called 'kinetic impactor deflection' was more efficient than others. Asteroid Gravitational Tractor The major alternative to explosive deflection is to move the asteroid slowly over a period of time. Tiny constant thrust accumulates to deviate an object sufficiently from its predicted course. Edward T. Lu and Stanley 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., a Nuclear Electric Rocket , 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 or impossible 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. 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. Other proposals
DEFLECTION TECHNOLOGY CONCERNS Carl Sagan , in his book Pale Blue Dot , expressed concerns 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 should only be developed in an actual emergency situation. 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 bomb which gave an asteroid a delta v of 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 DEFENCE TIMELINE
Formation of the Moon It is hypothesized that the Earth collided with a Mars -sized object in its early development. The resulting debris in Earth orbit coalesced to form the Moon . This model is supported by hypotheses of planetary formation and the chemistry of the Earth and Moon. 65 million years ago at the tip of the Yucatán Peninsula , the impact of which may have caused the dinosaur extinction]] The asteroid was ~10 km (6 Mi ) wide, striking the Yucatan peninsula of what ultimately is Mexico , creating the Chicxulub Crater . In addition to the Dinosaur s, this also would have wiped out a great proportion of other animal and plant life on Earth. 15 million years ago Several impacts in Germany associated with the Nördlinger Ries impact crater destroyed large parts of Europe. 50,000 years ago An iron body ~50m in diameter struck near Winslow, Arizona forming the 1km wide Arizona Meteor Crater . 1908 Tunguska event, Siberia See Also: Tunguska event A ~50 m chunk of cometary material exploded over the Stony Tunguska River of Siberia , Russia, with damage the equivalent of 600 Hiroshima nuclear bombs, without creating any crater, leveling trees for miles around in the Siberian forest, with a blast felt hundreds of miles away. 1972, Earth atmosphere A space object actually dipped into Earth's atmosphere, but 'skipped' back into space. A spectacular fireball traveled 1500 km through the atmosphere, from near Salt Lake City, Utah, to near Calgary, Alberta, in about 100 s, reaching a minimum height of 58 km over Montana. Estimates of the object's diameter vary from 3 to 80 m. A size at the low end of that range would correspond to an impact energy comparable to the Hiroshima bomb, if the object had hit the Earth's surface. {Link without Title} 1989 On March 23, 1989 the 300 metre (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometres (400,000 miles) passing through the exact position where the Earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history. 2002, 1/3 distance to Moon NASA reported that an asteroid named 2002 MN missed earth by about 120,700 km (75,000 mi) in June 14 , 2002 . It is estimated to be between 50 and 120 metres in diameter. It was discovered three days after its close to Earth pass. {Link without Title} 2029 near miss 99942 Apophis will pass within 6 Earth radii of the Earth's center. Chances of impact have been revised to zero. There is, however, a very small possibility of a return and impact by Apophis in 2036 due to the Earth's modification of the asteroid's orbit in a 2029 close pass. 2036 possible impact After analyzing new data, scientists have now predicted that there is a slim chance that during the 2029 close encounter with Earth 99942 Apophis will pass through a " Gravitational Keyhole " approximately 400 m across, which could cause the asteroid to hit Earth in April 2036. Additional observations of the trajectory of Apophis revealed the "keyhole" would likely be missed. As of August 5 , 2006 , the impact probability for 13 April 2036 is estimated at 1 in 43,000, so Apophis was lowered to a Level 0 Torino Impact Hazard Scale . 2880 encounter If (29075) 1950 DA continues on its present orbit, it will approach near to the Earth on March 16 , 2880 . Over the intervening time, the rotation of the asteroid will cause its orbit to change (by the Yarkovsky Effect ). A preliminary analysis shows two possible pole directions (Giorgini, et al., 2002 "Asteroid 1950 DA's 2880 Encounter with Earth"). One trajectory misses the Earth by tens of millions of kilometers, while the other has an impact probability of 1⁄300. SOCIAL, ECONOMIC, AND NECESSITY CONCERNS As the likelihood and/or frequency of a destructive impact event is more accurately calculated, serious economic and social concerns may arise in relation to any claim of necessity to spend potentially trillions of monetary units of any currency on the technologies to achieve effective deflection. Assuming no meaningful objects are found by completion of the Near Earth Object Program, {Link without Title} ), it can be argued that any further expenditure of funds and development of technology would simply serve to effectively 'weaponize' Earth (with attendant social and economic maintenance costs) for tens or hundreds of years, to deal with a highly unlikely and speculative threat (in terms of immediacy), while drawing finite resources from many other well-known, immediate, obvious, existing problems. FICTION '']] Asteroid or comet impacts are a common subgenre of Disaster Fiction , and such stories typically feature some attempt - successful or unsuccessful - to prevent the catastrophe. Most involve trying to destroy or explosively redirect an object, perhaps understandably from the direction of dramatic interest. Film
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