Gravitational keyhole

A gravitational keyhole, or resonance keyhole, is a small region of space that if an asteroid were to pass through it, its course would be altered due to Earth's gravity in a way that could cause such an asteroid to collide with the Earth on its next orbital pass. The word "keyhole" contrasts the vastness of space with the relatively tiny region through which a body might pass, and is not a reference to its shape.

Torino Scale

The Torino Scale is a scale from 0 to 10 that indicates the hazards posed by NEOs. Zero on the scale indicates an object has a negligibly small chance of collision with the Earth, compared with the usual "background noise" of collision events, or is too small to penetrate the Earth's atmosphere intact. A 10 indicates that a collision is certain, and the impacting object is large enough to precipitate a global disaster. Only integer values are ever used. An object is assigned a 0 to 10 value based on its collision probability and its kinetic energy (expressed in megatons of TNT).

The Torino Scale was created by Professor Richard P. Binzel in the Department of Earth, Atmospheric, and Planetary Sciences, at the Massachusetts Institute of Technology (MIT). The first version, called "A Near-Earth Object Hazard Index", was presented at a United Nations conference in 1995 and was published by Binzel in the subsequent conference proceedings ("Annals of the New York Academy of Sciences," volume 822, 1997.) A revised version of the "Hazard Index" was presented at a June 1999 international conference on NEOs held in Torino (Turin), Italy. The conference participants voted to adopt the revised version, where the bestowed name "Torino Scale" recognizes the spirit of international cooperation displayed at that conference toward research efforts to understand the hazards posed by NEOs. ("Torino Scale" is the proper usage, not "Turin Scale.") In 2005 a re-worded scale was published to better communicate the risks to the public.

The Apophis Deflection

Astronomers have identified an asteroid named Apophis that has a one in fifty chance of striking the earth, starting in 2029. [http://impact.arc.nasa.gov/news_detail.cfm?ID=162 Time Magazine Online. "How to Save the Planet: An Asteroid Named Apophis may put us to the Test" by Leon Jaroff. August 13, 2005.] ] Apophis is estimated to be as large as 1300 feet across, and if it hit the earth could cause millions of injuries. Astronomers think it is most likely that Apophis will pass close to a keyhole in 2029, but even if it doesn't hit a keyhole in 2029 it will return every seven years and may pose a serious threat the next time in 2036. Scientists have proposed that Apophis be nudged out of its present orbit into an orbit that takes it further from the keyhole. NASA scientist David Morrison says, "After 2029, the deflection would have to be vigorous enough to miss not just a tiny keyhole but the much larger target of the Earth itself. And such a deflection is far beyond present technology for an asteroid this large."

NASA initially estimated the energy that Apophis would have released if it struck Earth as the equivalent of 1480 megatons of TNT. A later, more refined NASA estimate was 880 megatons. [citeweb|title=99942 Apophis (2004 MN4) Earth Impact Risk Summary|url=http://neo.jpl.nasa.gov/risk/a99942.html|publisher="NASA"|accessdate=2008-07-18] The impacts which created the Barringer Crater or caused the Tunguska event are estimated to be in the 3-10 megaton rangecite web
url=http://www.sandia.gov/news/resources/releases/2007/asteroid.html
title=Sandia supercomputers offer new explanation of Tunguska disaster
publisher=Sandia National Laboratories|date=December 17, 2007
accessdate=2008-01-29
quote=The asteroid that caused the extensive damage was much smaller than we had thought,” says Sandia principal investigator Mark Boslough of the impact that occurred June 30, 1908..] The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.The exact effects of any impact would vary based on the asteroid's composition, and the location and angle of impact. Any impact would be extremely detrimental to an area of thousands of square kilometres, but would be unlikely to have long-lasting global effects, such as the initiation of an impact winter.

Additional observations of the trajectory of Apophis revealed the keyhole would likely be missed, and on August 5 2006 Apophis was lowered to a Level 0 on the Torino Scale. As of October 19, 2006, the impact probability for April 13, 2036, was calculated as 1 in 45,000. An additional impact date in 2037 was also identified; the impact probability for that encounter was calculated as 1 in 12.3 million. Most experts agree that Apophis warrants closer scrutiny, and to that end, in February 2008 the Planetary Society awarded $50,000 in prize money to companies and students who submitted designs for space probes that would put a tracking device on or near the asteroid.cite web | author=The Planetary Society| authorlink=Planetary Society | title=Planetary Society Names Winners of $50,000 Asteroid Tagging Competition | url=http://www.planetary.org/about/press/releases/2008/0226_Planetary_Society_Names_Winners_of.html | accessdate=2008-02-26]

ize of Keyhole

The gravitational keyhole for Apophis is only 2000 feet in diameter. Calculations showed that if Apophis' velocity could be changed by only 0.0001 mph—about 31 in. per day then in three years its orbit would be deflected by more than a mile, enough to miss the keyhole. [http://www.popularmechanics.com/science/air_space/4201569.html?page=2 Popular Mechanics. "The Threat is Out There" by David Noland. December, 2006.] ] The problem is that with the keyhole so small it becomes extremely difficult to predict precisely if Apophis will pass through the keyhole or not. Orbit projections made in 2006 for Apophis in 2029 have a margin of error of about 2000 miles. As time passes the error ellipse will be reduced but NASA may have to wait until enough data accumulates to reduce the error elipse to one mile before it knows if Apophis will hit the keyhole or not.

Gravity Tractor

On July 28, 2008 New Scientist reported that Jet Propulsion Labs had completed a study of how a "gravity tractor" could deflect an incoming asteroid away from the keyhole that would determine if the the asteroid would hit earth. [http://space.newscientist.com/article/dn14414-gravity-tractor-could-deflect-asteroids.html New Scientist. "'Gravity tractor' could deflect asteroids" by David L. Chandler. July 28, 2008.] ] If the asteroid misses the keyhole, then it will go on to miss earth. First a spacecraft would be crashed directly into the asteroid, similar to the Deep Impact mission that impacted a comet in 2005. This would provide a big change of direction, but in a less controllable fashion that could push the path of the asteroid into a dangerous keyhole. But then a second spacecraft, the gravity tractor, would come into play, hovering about 150 meters away from the asteroid, to exert a gentle gravitational force, changing the asteroid's velocity by only 0.22 microns per second each day. Over a long enough time, that could steer it away from the keyhole. In the simulation, a simple control system kept the spacecraft in position, and a transponder on the asteroid helped monitor its position and thus determine its trajectory more precisely than would be possible otherwise. 'The gravity tractor is a wimp, but it's a precise wimp,' said astronaut Jack Schweickart. 'It can make very small, precise changes in orbit, and that's what you need to avoid a keyhole.'"

References

"See Also:" 99942 Apophis