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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.

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

File:2004mn4d5 s.gif

A diagram in which the white bar represents the more likely positions of asteroid 99942 Apophis 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 2004 MN₄), 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 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.

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

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 weapons 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.

Dan Durda (homepage) has argued that if an asteroid was a rubble pile, had a low enough density and was porous enough, it could absorb enough energy from a stand-off explosion to not be deflected.

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 Dario Izzo (homepage), 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

Setting up an automated mass driver machine on the asteroid to eject material into space thus giving the object a slow steady push and decreasing its mass
- Any spacecraft propulsion device would have a similar effect of giving a steady push, hopefully forcing the asteroid onto a trajectory that takes it away from Earth.
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 white paper calculates deflections using existing chemical rockets, delivered to the asteroid, then push it sideways, assuming sufficient fuel also delivered
The late Eugene Shoemaker once proposed^[1] deflecting a potential impactor by releasing a cloud of steam in the path of the object, hopefully gently slowing it.
If the asteroid gets too close to Earth, a last-minute strategy could involve directing it towards the moon in the hope that its gravity would pull the asteroid into it, removing 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 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

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 to map which objects in our solar system both cross Earth orbit and are large enough to do us 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.
It is currently believed that there are approximately 1500 objects capable of crossing Earth's orbit and large enough to warrant concern. The odds are that within a 1,000 year period, some of them will collide with Earth, unless preventative measures are undertaken. It is now anticipated that by year 2008, all such objects that are 1 km or more in diameter will have been identified and will be monitored.

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

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 dinosaurs, 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

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.[1]

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. [2]

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, [3]), 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

Meteor (1979) - A series of orbital platforms armed with nuclear missiles are used to deflect an asteroid.
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.
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.

Literature

See also Asteroids in fiction: Collisions with Earth.
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.
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.
Sunstorm (2005) - Extraterrestrials attempt to cause Earth's destruction by way of a "cosmic bullet" projectile sent into the Sun. Written by Charles Sheffield 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 Star Trek: Enterprise (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.

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, astronauts assigned to blow an asteroid off-course are transported to a distant world.
Ace Combat 04 (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.
Final Fantasy VII (1997) - In the 7th game of the Final Fantasy series, the main protagonist, Cloud Strife, is trying to stop the main antagonist, Sephiroth, from destroying the planet using a giant meteor impact.

Notes

^ --in a lecture to the Arizona Geological Society in 12-96.

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 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).

External links

European Space Agency's ACT - May we deflect Asteroids? (what the Dinosaurs did not know!!)
Asteroid belt collision generates dust cloud around Earth causing climate change a few million years ago.
Asteroid Occultation Updates
BBC Horizon - Averting Armageddon (summary)
Best Astronomy web sites
British Government FAQ on Near Earth Orbit risks
Doomsday Asteroid dot Com
Killer Asteroids and You
Lifeboat Foundation AsteroidShield
LINEAR a USAF NASA joint effort operated by M.I.T. Lincoln Laboratory.
Meteorite FAQ
NASA Asteroid and Comet Hazards
Near Earth Asteroid principal investigator Raymond Bambery interviewed by Earth and Sky
Near Earth Objects Directory also here.
Near Earth Object Risks.
NEAR-Shoemaker, NASA space mission to study asteroid Eros for one year.
NEAT (Near Earth Asteroid Tracking) by GEODSS {USAF/Ground-based Electro-Optical Deep Space Surveillance} under the 21st Space Wing, headquartered at Peterson Air Force Base, Colorado.
- NEAT on MSS (Maui Space Surveillance System 1.2-m telescope)
PERMANENT (Projects to Employ Resources of the Moon and Asteroids Near Earth in the Near Term)
Simulate the collision of an asteroid or a comet with any planet in Solar System, or calculate effects based on size and speed of the bombardment, and how far witness is from impact site.
Space Watch Observatory at University of Arizona

Spaceguard around Earth

[1] --in a lecture to the Arizona Geological Society in 12-96.

[1]

@@ Line 141: / Line 141: @@
 *''[[The Dig]]'' (1995) - In this adventure PC game from [[LucasArts]], astronauts assigned to blow an asteroid off-course are transported to a distant world.
 *''[[Ace Combat 04]]'' (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.
-*''[[Final Fantasy VII]]'' (1996) - In the 7th game of the [[Final Fantasy|Final Fantasy series]], the main protagonist, [[Cloud Strife]], is trying to stop the main antagonist, [[Sephiroth]], from destroying the planet using a giant meteor impact.
+*''[[Final Fantasy VII]]'' (1997) - In the 7th game of the [[Final Fantasy|Final Fantasy series]], the main protagonist, [[Cloud Strife]], is trying to stop the main antagonist, [[Sephiroth]], from destroying the planet using a giant meteor impact.
 ==See also==