What is a gravitational "keyhole"?

I been reading an article about an asteroid, Apophis, that may pass, just barely hitting Earth in the year 2029, then possibly entering a gravitational "keyhole," shifting its orbit far enough to make it strike Earth in the year 2036. My question is what exactly is a gravitational "keyhole"?... show more I been reading an article about an asteroid, Apophis, that may pass, just barely hitting Earth in the year 2029, then possibly entering a gravitational "keyhole," shifting its orbit far enough to make it strike Earth in the year 2036.

My question is what exactly is a gravitational "keyhole"? I've tried to google it, wikipedi it, my research was unsucessful. The only answer I've found that it is some sort of imaginary hoop in space.
4 answers 4