Asteroid deflection strategies

Asteroid deflection strategies are methods by which near-Earth objects could be diverted, preventing potentially catastrophic impact events. 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.

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

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 mp|2004 MN|4), 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 [ [http://neo.jpl.nasa.gov/apophis/ Predicting Apophis' Earth Encounters in 2029 and 2036 ] ] , (though there is still a slight chance that the 2029 close approach will deflect the asteroid in just the right way to result in a collision in 2036). This is a common pattern, for reasons shown in the figure at the right.

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.Fact|date=October 2007 Depending on speed, objects as small as 100 m in diameter are historically extremely destructive. 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.Fact|date=October 2007

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.

Popular strategies

Strategies fall into two basic sets: destruction and delay.

Destruction 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. As will be shown, this does not always solve the problem, as sufficient amounts of material hitting the Earth at high speed can be devastating even if they are not collected together in a single body.

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. By the same token, the arrival time of the impactor must be known to this accuracy in order to forecast the impact at all, and to determine how to affect its velocity.

Nuclear weapons

One of the often proposed solutions is firing nuclear missiles at the oncoming asteroid to vaporize all or most of it. If not completely vaporized, the resulting reduction of mass from the blast combined with the radiation blast and rocket exhaust effect from ejecta 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 not to 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 1967, 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 [http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=6840 Project Icarus] (MIT Press, 1968), which served as the inspiration [' [http://www.ips.gov.au/IPSHosted/neo/info/refers/Bk_Icarus_MIT.htm 'Project Icarus"] ] for the 1979 film Meteor.

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 [http://www.esa.int/SPECIALS/NEO/SEMZRZNVGJE_0.html Don Quijote] , is 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 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 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.

Mass driver

A mass driver is an (automated) system on the asteroid to eject material into space thus giving the object a slow steady push and decreasing its mass. A mass driver is designed to work as a very low Specific Impulse system, which in general uses a lot of propellant, but very little power.

The idea is that when using local material as propellant, the amount of propellant is not as important as the amount of power, which is likely to be limited.

A mass driver can impart a lot more momentum to an asteroid than a rocket carried there.

Conventional Rocket Motor

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 1E6 m-sec (E.g. adding 1 km/s to a 1 ton vehicle), will have a relatively small effect on a relatively small asteroid that weighs roughly a million times more.

Other proposals

*Wrapping the asteroid in a sheet of reflective plastic such as aluminized PET film, or dusting the object with titanium dioxide to alter its trajectory via radiation pressure
*Dusting the object with soot to alter its trajectory via the Yarkovsky effect
*Attaching a large enough solar sail directly to the object, thus using solar pressure to shift the object's orbit
*Chapman, Durda & Gold's [http://www.boulder.swri.edu/clark/neowp.html white paper] calculates deflections using existing chemical rockets, delivered to the asteroid, then push it sideways, assuming sufficient fuel also delivered
*Planetary scientist Eugene Shoemaker in 1996 proposed [--in a lecture to the Arizona Geological Society in 12-96. ] deflecting a potential impactor by releasing a cloud of steam in the path of the object, hopefully gently slowing it. Nick Szabo in 1990 sketched [http://www.cs.cmu.edu/afs/cs.cmu.edu/usr/mnr/st/std070] 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.

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 changed 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 the 1980s NASA studied evidence of past strikes on planet Earth, and the risk of this happening at our current level of civilization. This led to a program that maps which objects in our 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 the world's 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. [cite web|title=Astronauts push for strategies, spacecraft to prevent calamitous asteroid strike|url=http://www.post-gazette.com/pg/05332/613441.stm|accessdate=2008-01-18]
* It is currently (as of late 2007) believed that there are approximately 20,000 objects capable of crossing Earth's orbit and large enough (140 meters or larger) to warrant concern. [cite web |url=http://www.science.house.gov/press/PRArticle.aspx?NewsID=2036 |title=Subcommittee Questions NASA’s Plan for Detecting Hazardous Asteroids ] On the average, one of these will collide with Earth each 5,000 years, unless preventative measures are undertaken.cite web |url=http://democrats.science.house.gov/media/File/Commdocs/hearings/2007/space/08nov/Yeomans_testimony.pdf |title=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 |author=Donald K. Yeomans |date = 2007-11-08] 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 [http://www.lpl.arizona.edu/css/ Catlalina Sky Survey website] (CSS) is one of four NASA´s 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 310 NEOs in 2005, 400 in 2006 and the record will be broken with 450 NEOs found in 2007. In doing this survey they discovered on November 202007, an asteroid, designated mp|2007 WD|5, which initially was estimated to have a chance of hitting Mars on January 302008, but further observations during the following weeks allowed NASA to rule out an impact. [cite web|title=Catalina Sky Survey Discovers Space Rock That Could Hit Mars|url=http://uanews.org/node/17415|accessdate=2007-12-22] NASA estimated a near miss by 26,000 km. [cite web|title=Recently Discovered Asteroid Could Hit Mars in January|url=http://neo.jpl.nasa.gov/news/news151.html|accessdate=2007-12-22]

Past events

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

A ~10 km (6 mi) wide asteroid struck the Yucatán Peninsula in what is now Mexico, creating the Chicxulub Crater. The impact may have contributed to the eventual extinction of the dinosaurs and much other plant and animal life at the time.

15 million years ago

Several impacts in Bavaria, 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

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. In 2007, NASA announced the results of a revision [cite web |url=http://impact.arc.nasa.gov/news_detail.cfm?ID=179 |title=Tunguska Revision, and a Possible NEA Impact on Mars |accessdate=2008-06-17] that now estimates a smaller size of the piece of comet or asteroid exploding at Tunguska, but it also implies a higher frequency for this type of events. [cite web |url = http://www.sandia.gov/news/resources/releases/2007/asteroid.html| title = Sandia supercomputers offer new explanation of Tunguska disaster| date = 2007-12-17| publisher = Sandia National Laboratories| accessdate = 2007-12-22]

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 seconds, reaching a minimum height of 58 km over Montana. The object appears to have been about 10 m in diameter [cite web |url=http://www.astrosociety.org/pubs/mercury/9806/impact.html |title=Observation of Meteoroid Impacts by Space-Based Sensors |author=Edward Tagliaferri |publisher=Astronomical Society of the Pacific] An impact by an object in this size range would correspond to an impact energy roughly comparable to the Hiroshima bomb, if the object had hit the Earth's surface.

1989

On March 221989 the 300 metre (1,000 ft) 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 six hours before. If the asteroid had impacted it would have created the largest explosion on Earth in recorded history.

2002, 1/3 distance to Moon

NASA reported that an asteroid named 2002 MN missed the Earth by about 120,700 km (75,000 mi) on June 142002. It is estimated to be between 50 and 120 metres in diameter. It was discovered three days after its close to Earth pass. [http://neo.jpl.nasa.gov/news/news128.html]

2004, 400,000 km

NASA also reported asteroid 2004 XP14, that on July 32006 passed by at 1.1 times the distance to the Moon (a little more than 400,000 km). This NEO was discovered in 2004 but was rediscovered only one week before its close approach to Earth. This asteroid was unusually well placed for study by radar, and it was traveling at a relative speed of 17 km/s. [http://impact.arc.nasa.gov/news_detail.cfm?ID=168 Close Pass by XP14 & NASA NEO Workshop]

2008 Sudan impact

On 5 October 2008, scientists calculated that a small asteroid (Near-Earth object 8TA9D69) just sighted that night should impact the Earth on 6 October over Sudan, at 0246 UTC, 5:46 a.m. local time. [cite web |title=Small Asteroid Predicted to Cause Brilliant Fireball over Northern Sudan |author=Don Yeomans |publisher=NASA/JPL Near-Earth Object Program Office |date=October 6, 2008 |accessdate=2008-10-09 |url=http://neo.jpl.nasa.gov/news/news159.html ] [cite web |title=FLASH! Meteor to Explode Tonight |author=Richard A. Kerr |publisher= ScienceNOW Daily News |date=6 October 2008 |accessdate=2008-10-09 |url=http://sciencenow.sciencemag.org/cgi/content/full/2008/1006/2 ] . The asteroid arrived as predicted. [cite web |title=Impact of Asteroid 2008 TC3 Confirmed |author=Don Yeomans |publisher=NASA/JPL Near-Earth Object Program Office |date=October 7, 2008 |accessdate=2008-10-09 |url=http://neo.jpl.nasa.gov/news/news160.html ] [cite web |title=Asteroid Watchers Score a Hit |author=Richard A. Kerr |publisher=ScienceNOW Daily News |date=8 October 2008 |url=http://sciencenow.sciencemag.org/cgi/content/full/2008/1008/1 |accessdate=2008-10-09] . This is the first time that an asteroid impact on Earth has been predicted before it occurred.

Future events

2029 near miss

99942 Apophis (previously known by its provisional designation mp|2004 MN|4) 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.

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 April 162008, the impact probability for 13 April2036 is estimated at 1 in 43,000, so Apophis is a Level 0 Torino impact hazard.

2880 encounter

If (29075) 1950 DA continues on its present orbit, it will approach near to the Earth on March 162880. 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 probability¹⁄₃₀₀.

ocial, 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, [http://neo.jpl.nasa.gov/risk/a99942.html] ), 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. (See also Asteroids in fiction#Collisions with Earth).

Film

*"Meteor" (1979) - A series of orbital platforms armed with nuclear missiles are used to deflect an asteroid.
*"The Last Starfighter" (1984) Ko-Dan leader Xur all but destroys the Rylosian Starfighter base with the mother-ship's "meteor gun" (a keel-mounted mass driver), killing most of the base personnel (excluding Starfighter Navigator Grig) and all of the Starfighter League save the Starfighter who just left: Alex Rogan. Written by Jonathan R. Betuel
*"Starship Troopers" (1997) - Insect-like aliens launch an asteroid at Earth, totally obliterating Buenos Aires. The film was loosely inspired by a novel of the same name (1959) by Robert A. Heinlein. Shortly afterward, orbital defenses are constructed to destroy any future asteroids the aliens may send.
*"Armageddon" (1998) - A pair of newly modified space shuttles are used to drill a hole in an asteroid and plant a nuclear bomb.
*"Deep Impact" (1998) - A manned spacecraft plants a number of nuclear bombs on a comet and is partially successful.
*"Earthstorm" (2006) - Asteroid impact on the lunar surface and a resulting debris storm that strikes the earth, inflicting severe damage. Scientists, along with a bombing expert, bind the moon's core, thereby avoiding a global catastrophe.

Literature

*See also .
*"The Moon Is a Harsh Mistress" (1966) - A lunar penal colony exacts independence by bombarding Earth with large rocks. Written by Robert A. Heinlein.
*"Lucifer's Hammer" (1977) - A comet, which was initially thought unlikely to strike, hits the Earth, resulting in the end of civilization and a decline into tribal warfare over food and resources. Written by Larry Niven and Jerry Pournelle.
*"Shiva Descending" (1980). A swarm of meteors is falling on Earth, but a giant comet, Shiva, is still coming. Written by Gregory Benford and William Rotsler.

*"Footfall" (1985) - An alien race uses controlled meteorite strikes as well as a large asteroid superweapon against Earth. Written by Larry Niven and Jerry Pournelle.
*"The Hammer of God" (1993)- A spacecraft is sent to divert a massive asteroid by using thrusters. Written by Arthur C. Clarke.
*"Moonfall" (1998). A comet is in collision course with the Moon. After the collision, the debris start falling on Earth. Written by Jack McDevitt.
*"Sunstorm" (2005) - Extraterrestrials attempt to cause Earth's destruction by way of a "cosmic bullet" projectile sent into the Sun. Written by Stephen Baxter and Arthur C. Clarke.

Television

*"Power Rangers" (1993) - A meteor is sent towards Earth by evil space aliens in the television series, but is pushed off course by several Megazords.
*"Stargate" (1997) and "" (2001) - Both shows feature an episode with a similar attack to that described in Sunstorm above.
*"Futurama" - The episode "A Big Piece of Garbage" (1999) features a large space object on a collision course with Earth which turns out to be a giant ball of garbage launched into space by New York around 2052.
*"Stratos 4" (2003) - In this anime, a two-staged space and air defense network is established in order to prevent a large group of comets colliding with Earth.
*"" (1968) - episode named The Paradise Syndrome where Captain Kirk (in a state of amnesia) finds a centuries old obelisk which has a deflector beam built in to deflect an asteroid coming to wipe out a primitive race.
*"Babylon 5" (1995) - The Centauri bombard Narn with asteroids fired by mass drivers during the second Centauri invasion in Season 2.
*"Impact Earth" (2007) TV - A comet fragment strike in the Atlantic Ocean destroys Shannon Airport, Ireland with a tsunami. They discover it was from a long period comet that was a Sun Grazer and then discover that it was only a small part and the rest was coming a year later. There is an argument between the main hero scientist as to the efficacy of a nuclear deflection strategy, but they discover in the nick of time that a nuclear bomb would make it worse, so they implement an evacuation strategy and allow it to hit, in Pittsburgh.
*"Danny Phantom" (2007) - The series finale, "Phantom Planet", involved a giant asteroid of the fictional element ecto-ranium from the rings of Saturn almost collide with Earth. This was solved when ghosts had made the planet intangible, hence the title.
*"The Sarah Jane Adventures" (2007) - "Whatever Happened to Sarah Jane?" A meteor on a collision course with the Earth is ultimately deflected back into space by Sarah Jane's alien computer, Mr. Smith.

Gaming

*"Outpost" (1994) and "Outpost 2" (1997) - The player of these two colonization PC games from Sierra Entertainment is given the task of building and managing a space colony in the aftermath of humanity's certain extinction caused by an asteroid collision.
*"The Dig" (1995) - In this adventure PC game from LucasArts, three of five astronauts assigned to blow an asteroid off-course are transported to a distant world.
*"" (2001) - In this combat flight simulator for the Playstation 2 by Namco, a railgun battery is used in an attempt to destroy a massive asteroid with limited success.
*"" (2008) - Almost 90% of mankind has been killed off following devastating meteor strikes which have destroyed much of civilization and caused a massive dust cloud to blot out the sun. The player takes the role of a military leader and tries to protect the survivors in the ruins of civilization.

ee also

*B612 Foundation
*Chicxulub Crater
*Comet Shoemaker-Levy 9 which collided with Jupiter in 1994
*Dinosaur extinction theory first explained by Walter Alvarez
*Doomsday event
*Earth-crosser asteroid
*Impact events
*Lincoln Near-Earth Asteroid Research (LINEAR)
*Near-Earth asteroid
*Palermo scale
*Torino Scale
*Tunguska event
*Vitim event
*Cando event
*Eastern Mediterranean Event
*List of impact craters on Earth

Notes

References

* 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 http://www.esa.int/gsp/ACT/publications/pub-mad.htm - 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 http://www.boulder.swri.edu/clark/neowp.html
* 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 [http://www.csicop.org/si/9705/asteroid.html 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 http://impact.arc.nasa.gov/downloads/NEO_Chapter_1.pdf?ID=113
* Russell L. Schweickart, Edward T. Lu, Piet Hut and Clark R. Chapman; "The Asteroid Tugboat"; Scientific American (November 2003).
* [http://arstechnica.com/journals/science.ars/2008/04/04/how-to-deflect-an-asteroid How to Deflect an Asteroid] (Ars Technica)

External links

* [http://www.esa.int/gsp/ACT/mad/op/AsteroidsAndNEOs/maywedeflect.htm European Space Agency's ACT - May we deflect Asteroids? (what the Dinosaurs did not know!!)]
* [http://www.spaceref.com/news/viewpr.html?pid=18788 Asteroid belt collision generates dust cloud around Earth] causing climate change a few million years ago.
* [http://www.asteroidoccultations.com/ Asteroid Occultation Updates]
* [http://www.bbc.co.uk/science/horizon/2003/armageddon.shtml BBC Horizon - Averting Armageddon (summary)]
* [http://www.griffithobs.org/StarAward.html Best Astronomy web sites]
* [http://www.nearearthobjects.co.uk/ British Government FAQ on Near Earth Orbit risks]
* [http://pw2.netcom.com/~speaker6/doomsday.html Doomsday Asteroid dot Com]
* [http://www.fair-society.org Future Asteroid Interception Research]
* [http://www.npl.washington.edu/AV/altvw50.html Killer Asteroids and You]
* [http://lifeboat.com/ex/asteroid.shield Lifeboat Foundation AsteroidShield]
* [http://www.ll.mit.edu/LINEAR/ LINEAR a USAF NASA joint effort operated by M.I.T. Lincoln Laboratory] .
* [http://meteorite.org/ Meteorite FAQ]
* [http://impact.arc.nasa.gov/ NASA Asteroid and Comet Hazards]
* [http://www.earthsky.org/shows/astrophysics_interviews.php?id=49240 Near Earth Asteroid principal investigator Raymond Bambery interviewed by Earth and Sky]
* [http://newton.dm.unipi.it/cgi-bin/neodys/neoibo Near Earth Objects Directory] also [http://www.cfa.harvard.edu/iau/NEO/ToConfirm.html here] .
* [http://neo.jpl.nasa.gov/risk/ Near Earth Object Risks] .
* [http://near.jhuapl.edu/ NEAR-Shoemaker, NASA space mission to study asteroid Eros for one year] .
* [http://neat.jpl.nasa.gov/ NEAT (Near Earth Asteroid Tracking)] by [http://www.fas.org/spp/military/program/track/geodss.htm GEODSS {USAF/Ground-based Electro-Optical Deep Space Surveillance}] under the 21st Space Wing, headquartered at Peterson Air Force Base, Colorado.
** [http://neat.jpl.nasa.gov/msss.htm NEAT on MSS (Maui Space Surveillance System 1.2-m telescope)]
* [http://www.permanent.com/ PERMANENT (Projects to Employ Resources of the Moon and Asteroids Near Earth in the Near Term)]
* [http://janus.astro.umd.edu/astro/impact.html Simulate the collision of an asteroid or a comet with any planet in Solar System] , or [http://www.lpl.arizona.edu/impacteffects/ calculate effects] based on size and speed of the bombardment, and how far witness is from impact site.
* [http://spacewatch.lpl.arizona.edu/ Space Watch Observatory at University of Arizona]

Spaceguard around Earth

* [http://www1.tpgi.com.au/users/tps-seti/spacegd.html Australian SpaceGuard]
* [http://www.spaceguarduk.com/ British SpaceGuard]
* [http://earn.dlr.de/ EARN (European Asteroid Research Node)]
* [http://web.mit.edu/rpb/wgneo/ IAU (International Astronomical Union) Working Group on Near Earth Objects] .
* [http://spaceguard.esa.int/ Spaceguard Foundation]
** SpaceGuard Foundation's [http://spaceguard.esa.int/tumblingstone/ Tumbling Stone on-Line Magazine]