Asteroid deflection strategies Monday, June 22, 2009


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

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

 The following text needs to be harmonized with text in Planetary defense.
A diagram in which the white bar represents the more likely positions of asteroid 99942 Apophis in relation to Earth in 2029
Why asteroid impact probability goes up, then down. The ellipses in this diagram show the likely asteroid position at closest earth approach. At first, with only a few asteroid observations, the error ellipse is very large and includes the Earth. This leads to a small, but non-zero, impact probability. Further observations shrink the error ellipse, but it still includes the Earth. This raises the impact probability, since the Earth now covers a larger fraction of the error region. Finally, yet more observations (often radar observations, or discovery of a previous sighting of the same asteroid on archival images) shrink the ellipse still further. Now the earth is outside the error region, and the impact probability returns to near zero.[1]

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[2], (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.[citation needed] 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.[3]

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

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. Even 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 the thousands of fragments that could result from such an explosion would be a very daunting task.

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. The 1964 book Islands in Space, calculates the nuclear megatonnage necessary for several deflection scenarios.[4] 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 Project Icarus (MIT Press, 1968), which served as the inspiration [5] 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 the preliminary design of a space mission able to demonstrate this futuristic technology. The mission, named 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

Main article: gravitational tractor

One more alternative to explosive deflection is to move the asteroid slowly over a 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

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.

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.

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 has a mass of 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 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[6] 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[7] 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 their 1964 book, Islands in Space, Dandridge M. Cole and Donald W. Cox noted the dangers of planetoid impacts, both those occurring naturally and those that might be brought about with hostile intent. They argued for cataloging the minor planets and developing the technologies to land on, deflect, or even capture planetoids.[8]
  • 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.[9]
  • 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.[10] On the average, one of these will collide with Earth each 5,000 years, unless preventative measures are undertaken.[11] 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[11].
  • The Catalina Sky Survey[12] (CSS) is one of NASA´s four 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 2007 WD5, 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.[13] NASA estimated a near miss by 26,000 km.[14]

Past events

[edit] Formation of the Moon

For more details on this topic, see Giant impact hypothesis.

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 Chicxulub Crater at the tip of the Yucatán Peninsula, the impact of which may have caused the dinosaur extinction

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 1 km wide Arizona meteor crater.

1908 Tunguska event, Siberia

Main article: Tunguska event

A ~50 m chunk of extraterrestrial material is believed to have 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[15] 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.[16]

1972, Earth atmosphere

Main article: The Great Daylight 1972 Fireball

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[17] 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.

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. [1]

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.[18]

2008 Sudan impact

On 5 October 2008, scientists calculated that a small asteroid (2008 TC3, temporary designation 8TA9D69) just sighted that night should impact the Earth on 6 October over Sudan, at 02:46 UTC, 05:46 local time.[19][20]. The asteroid arrived as predicted.[21][22]. This is the first time that an asteroid impact on Earth has been predicted before it occurred.

2009, 66,000 kilometers

On March 2, 2009 NASA's Jet Propulsion Laboratory reported that asteroid 2009 DD45 measuring between 69 feet (21 m) and 154 feet (47 m) in diameter passed just 41,010 miles (66,000 kilometers) above Earth, between our planet and the moon at 13:44 universal time, with a speed of about 12 miles (20 kilometers) a second in the point closest to Earth. It was about the size of one that is thought to have caused Tunguska event.The space rock wasn't noticed by astronomers until February 28, when it showed up as a faint dot in pictures taken at the Siding Spring Observatory in Australia. At the time it was about 1 million miles away.[23]

Future events

2029 near miss

99942 Apophis (previously known by its provisional designation 2004 MN4) 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 16, 2008, the impact probability for April 13, 2036, 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 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 1⁄300.

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

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.
  • The Simpsons (1995): In "Bart's Comet", Bart discovers a comet that is heading directly for Springfield. The town attempts to destroy it with a rocket, but it misses. The comet ends up being destroyed by an extra thick layer of pollution over the city.
  • Stargate (1997) and Star Trek: Enterprise (2001): Both shows feature an episode with a similar attack to that described in Sunstorm above, but for the Earth.
  • 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.
  • Star Trek: The Original Series (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): 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.
  • Homeworld (1999): At the outskirts of the Hiigaran system, the Taiidan attempted to destroy the Kushan Mothership in a last-ditch effort using a large asteroid (somewhere between 15 and 20 km across) with an engine on its back. The asteroid had enough mass and kinetic energy to completely vaporize anything it collided with and was capable of withstanding the combined firepower of the whole Kushan fleet for minutes.
  • Submarine Titans (2000): A real time strategy game by Ellipse Studios in which the Earth is devastated in 2047 by the impact of the Clark Comet and the attached Silicon spacecraft. The impact of the Clark Comet also deposits significant amounts of the fictional element Corium 276, which factors heavily into both the gameplay and the plot of Submarine Titans.
  • Ace Combat 04: Shattered Skies (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.
  • Advance Wars: Days of Ruin (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.

See also

Posted by Prince Hotak at 5:55 AM  

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