What is a gravitational "keyhole"?

Question

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.

Answer 1

Imagine a marble rolling down an inclined valley. The walls of the valley keep the marble in one defined track.

Now imagine a marble rolling down an inclined ridge. In order to stay on the top of the ridge, the marble has to move in *exactly* the right direction. Even a little bit off, and it will roll down the side of the ridge and not stay on top.

That's a little bit like a gravitational keyhole. Apophis is absolutely going to miss the Earth in 2029. But the approach is going to be so darn close that Earth's gravity is going to change its orbit pretty severely. But -- and this is the key -- *exactly* how much the Earth changes the orbit of Apophis is dependent on *exactly* how close Apophis gets. And we've got a pretty good idea, but not that good. So we won't know for sure until the asteroid has already passed us and we can compute its new orbit.

The "keyhole" comes from the fact that there is region of space near Earth that, if Apophis passes through that region, Earth's gravity will change its orbit in such a way that Earth will collide with the asteroid in 2036. (It's called a keyhole because it's a very small region of space.) We now are pretty confident that Apophis will miss the keyhole in 2029, and therefore miss the Earth in 2036.

Answer 2

Gravitational Keyhole

Answer 3

Atomic theory: units of matter, or tiny angels? -- Purple: Um, but gravitation has both law and theory-- law being the description of the process, and theory being the explanation. It's only biology that almost never has both-- in biology, a theory is as consistent as you can get, because of the dynamic nature of living things.

Answer 4

I figure it as a ratio between gravity of the target and the roid.

And the speed of the roid and its angle. or its vector?


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