Asteroid deflection strategies

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Artist's impression of a major impact event. The collision between Earth and an asteroid a few km in diameter releases as much energy as several million nuclear bombs simulatenously.

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 tidal waves or, by placing large quantities of dust into the stratosphere blocking sunlight, cause 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 political situation, think they are being attacked by another nation and retaliate. If this had happened to a superpower during the Cold War, it may have thought it was under nuclear attack and "returned" fire.

The probability that any one of Near-Earth object will hit Earth is extremely low, but the damage from any one strike can be catastrophic; consequently the odds of being killed by an asteroid are comparable to the odds of being killed by an airplane crash.

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.

Contents

Early detection

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. 10 years or more of advance warning would be needed for an asteroid larger than 200 meters across.

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.

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 the Icarus Project.

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. The energy of a nuclear explosion on Earth is initially released in gamma rays and neutrons with X-rays and debris within the surroundings determining how much energy is produced as blast. In the vacuum of space the gamma rays and neutrons might be free to impact unimpeded; however, the dynamics of a nuclear blast in space are unknown. This plan would require testing nuclear devices in space, which is illegal for any country that signed the Outer Space Treaty.

Some skeptics suggest that nuclear scientists are simply trying to find new excuses to keep nuclear missiles around and to keep improving them, as their careers have been endangered since the end of the Cold War. In particular, accusations focused on the efforts of Edward Teller.

Detonating internally

One strategy is to plant powerful explosives inside the asteroid, detonate them and break the asteroid into pieces. This technique, as with launching nuclear weapons from Earth, might cause subsequent impact events of large fragments of the asteroid. A large asteroid could be blown apart by a nuclear device detonated in its core only to have gravity draw the asteroid back together, essentially nullifying the effect of the explosion.

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.

In the case of 99942 Apophis it has been demonstrated by ESA 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 (carried out by the Advanced Concepts Team of the European Space Agency) Dario Izzo ([homepage]) argued that a strategy called 'kinetic impactor deflection' was more efficient than others.

Asteroid 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

NASA study of a solar sail. The sail would be 0.5 km wide.

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
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

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 defense timeline

In the 1980s NASA studied evidence of past strikes on planet Earth, and 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 us. The working conclusion is that there are approximately 1500 such objects, and that the odds are that within a 1,000 year period, some of them are going to hit us, unless we do something about it. It is now anticipated that by year 2008, we will have identified and be tracking all such objects that are 1 km or more in diameter.
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.

Formation of the 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 theories of planetary formation and the chemistry of the Earth and Moon.

65 million years ago

The Chicxulub Crater 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 became 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 Noerdlinger Ries impact crater destroyed large parts of Europe.

50,000 years ago

Arizona crater, 1 km across, energy equivalent to 500 Hiroshima bombs. Winslow, Arizona, iron body 50 m across.

1908
A ~50 m chunk of cometary material exploded over the Stony Tunguska River of Siberia, Russia, with damage the equivalent of 600 Hiroshima-size nuclear bombs, without creating any crater, leveling trees for miles around in the Siberian forest, with a blast felt hundreds of miles away.
* More info on the [Tunguska event]

1972, Earth atmosphere

An asteroid 100 m across actually dipped into Earth's atmosphere, creating a spectacular fireball, but 'skipped' back into space.

1989

An asteroid 800 m in diameter crossed Earth orbit just 6 hours before Earth would be in that same place.

2002, 1/3 distance to Moon

NASA reported that a soccer field-sized asteroid missed earth by about 75,000 miles in June 2002. It was discovered slightly after closest approach.

2029 near miss

99942 Apophis will pass within 4 Earth radii of the Earth's center. Scientists tell people not to worry.

2036 possible impact

After analyzing new data, scientists have now predicted that there is a slim chance 99942 Apophis will pass through a "gravitational keyhole" approximately 400m across, which could cause the asteroid to hit earth in April 2036.

Social, economic, and necessity concerns

Since the last destructive impact is estimated to have occurred millions of years ago, and meaningfully large bodies within the Solar System hypothetically capable of harming Earth are currently discounted while in the process of being identified, serious economic and social concerns arise in relation to any claim of necessity to spend potentially trillions of monetary units of any currency on the technologies described here, once the Near Earth Object Program is completed (Near-Earth Object, [link]), and assuming no meaningful objects are found. Such expenditure and technology, assuming no objects are found upon completion of the NEO program, would effectively weaponize Earth, along with the social and economic maintenance costs of that weaponization, for tens or hundreds of years, spending and waiting, to deal with a highly unlikely and therefore highly speculative threat in terms of its immediacy, and in relation to drawing finite resources from many other well-known, immediate, obvious, existing problems. The misleading phrase "near miss" of 100 miles in the context of impact is objectively the same as a miss of 100 billion miles in the context of impact. Applicable phrases are, "A miss is as good as a mile," and, "Close only counts in horseshoes." Proximity has no objective meaning in context with impact. It is the same as saying a random die roll result makes it more likely for a particular number to show on a later discrete roll of a random die. Because asteroids are discrete, one from the other, claiming a history of previous proximity of one asteroid as being in objective context with future impact of another asteroid is fantasy, and abusive to accurate perception of reality. To quote 99942 Apophis as of 2 July 2006, the heavily annotated Wikipedia page describing the asteroid said to be currently the biggest threat, "It must be stressed that the odds of impact are now known to be very low. Hence, the possible effects of an impact are largely irrelevant." The social, economic, and necessity concern is that assuming no meaningful objects are found by NEO, development of highly expensive, currently nonexistent weapons technology for the purpose of deflecting asteroids is irrelevant in its entirety, and harmful to society, now and in the meaningful future.

Fiction

The movie poster for Deep Impact

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.

Science Fiction authors Charles Sheffield and Arthur C. Clarke (Sunstorm) have also written of a "cosmic bullet" extraterrestrial intelligence threat consisting of a projectile aimed at our sun at near light speed. In the opening move of the war in the movie Starship Troopers, inspired by a novel of the same name by Robert A. Heinlein, insectoid aliens launch an asteroid at Earth, totally obliterating Buenos Aires. A similar attack is made in one of the [[Star Trek: Enterprise]] episodes, and one is averted in a Stargate episode using advanced technology. A meteor is sent towards Earth by evil space aliens in the Power Rangers television series, but is pushed off course by several Megazords.

Notable works include:

Deep Impact - An attempt by the Messiah spacecraft to plant a number of nuclear bombs on a comet is partially successful.
Armageddon - A pair of modified new space shuttles are used to drill a hole in an asteroid and plant a nuclear bomb
Meteor - A series of orbital platforms armed with nuclear missiles are used to deflect an asteroid.
The Dig - An adventure Personal Computer game from LucasArts in which astronauts assigned to blow an asteroid off-course are transported to a distant world.
Final Fantasy VII - A fantasy adventure Sony PlayStation game from Square-Enix (previously known as Square) in which a summon magic spell has summoned the ultimate black magic in the form a red Meteor.

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 controverial 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) available in www.esa.int/gsp/ACT/studies_publications.htm
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 1573920665. (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

[ACT - May we deflect Asteroids?]
[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]
[Meteorite FAQ]
[LINEAR a USAF NASA joint effort operated by M.I.T. Lincoln Laboratory].
[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 ] 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]

[Australian SpaceGuard]
[British SpaceGuard]
[EARN (European Asteroid Research Node)]
[IAU (International Astronomical Union) Working Group on Near Earth Objects].
[Spaceguard Foundation]
* SpaceGuard Foundation's [Tumbling Stone on-Line Magazine]

The minor planets
Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: , , asteroid moons and the Solar system
For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.

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