Gravitational Keyhole
The "big guys" of our solar system -- the planets -- frequently push around the "little guys" -- like the mountain-sized rocks known as asteroids. But sometimes, when the push is just right, the little guys can react in nasty ways.
For a while, astronomers were concerned that Earth might push one asteroid in just the right way, setting up a collision several years later.
Earth and the other planets move the little guys around because they're much bigger and more massive, so their gravity is much stronger. When an asteroid passes close by, the planet's gravity changes the smaller body's orbit. The planet's orbit changes, too, but by an amount that's insignificant compared to the change in the asteroid's orbit.
If the asteroid passes the planet at just the right spot -- known as a gravitational keyhole -- it's pushed into an orbit that brings the planet and asteroid back together.
The size of the keyhole depends on several factors, including the masses of the planet and asteroid, and the asteroid's original orbital path. When an asteroid enters the keyhole, it sets up an alignment between its orbit and that of the planet. If the alignment is just right, the two bodies can ram together.
Astronomers thought that an asteroid might pass through a keyhole when it flies past Earth in 2029. There's still a chance that it could happen, but the odds are getting slimmer all the time. More about the asteroid Apophis tomorrow.
Script by Damond Benningfield, Copyright 2006
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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