Gravitational keyhole
A gravitational keyhole, or resonance keyhole, is a small region of space that could alter the course of a passing asteroid 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 no reference to its shape.
The Apophis Deflection
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Astronomers have identified an asteroid named Apophis that was once estimated to have a 2.7% (1 in 37) chance of striking the earth in 2029. Further observations and revisions of the estimated path of the asteroid have resulted in an estimated 1 in 250,000 chance (0.0004%) of impact in 2036.[1][2] Apophis is estimated to be as large as 1,300 feet (400 m) across, and could cause millions of casualties if it were to hit Earth.[3] Astronomers think that Apophis will most likely miss a 2,000-foot (610 m) wide keyhole in 2029 which, if passed through, would cause it to hit Earth in 2036.[3]
Scientists from the B612 Foundation, a private foundation dedicated to protecting the Earth from asteroid strikes, have proposed that Apophis be nudged out of its present orbit into an orbit that takes it further from the keyhole.[3] 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."[3]
Additional observations of the trajectory of Apophis revealed the keyhole would likely be missed. 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.
Size of Keyhole
The gravitational keyhole for Apophis is only 2,000 feet (610 m) in diameter.[4] Calculations showed that if Apophis' velocity could be changed by only 0.0001 mph, then in three years its orbit would be deflected by more than a mile, enough to miss the keyhole. 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.[4] Orbit projections made in 2006 for Apophis in 2029 have a margin of error of about 2,000 miles (3,200 km), which as of early 2010 is well within the estimated distance between Apophis (18,600 miles) and the location of the keyhole (18,893 miles).[4] As time passes the error ellipse will be reduced but NASA may have to wait until enough data accumulates to reduce the error ellipse to one mile (1.6 km) before it knows if Apophis will hit the keyhole or not.[4]
See also
References
- ^ Dwayne Brown (2008-04-16). "NASA Refines Asteroid Apophis' Path Toward Earth". NASA. Retrieved 2009-10-08.
- ^ http://en.wikipedia.org/wiki/99942_Apophis
- ^ a b c d Time Magazine Online. "How to Save the Planet: An Asteroid Named Apophis may put us to the Test" by Leon Jaroff. August 13, 2005.
- ^ a b c d Popular Mechanics. "The Threat is Out There" by David Noland. December, 2006.