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 not a reference to its shape.

The Apophis Deflection

This section may require cleanup to meet Wikipedia's quality standards. Please improve this section if you can. (March 2009)

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 estimate 1 in 250,000 chance of impact in 2036.^[1][2] Apophis is estimated to be as large as 1,300 feet (400 m) across, and if it hit the earth could cause millions of casualties.^[3] 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.^[3] Scientists 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 – about 31 in. per day – then in three years its orbit would be deflected by more than a mile, enough to miss the keyhole.^[4] 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).^[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]

References