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Einstein's gravity protects earth.

Here's a bit of physics trivia. If the universe obeyed only Newton's laws of gravity, the solar system would be a lot less stable than it is. There would be about a 60% chance that Mercury would swing close by the Sun or Venus during the Sun's lifetime and then go on to disrupt orbits in the whole inner solar system, Earth included. But according to a new study, the tiny refinement imposed by Einstein's theory of gravity (general relativity) lowers this chance to about 1%.

So we're pretty safe from an eventual collision between Venus and Earth, portrayed in the artist's illustration below.

But the chance isn't zero. "The interesting thing is that it can still happen quite a long time after planets have formed," says John Chambers (Carnegie Institution of Washington). "We're sort of changing the idea that planetary systems are formed and then stay the same."

In fact, the solar system is "chaotic" on long timescales. All the planets and smaller bodies gravitationally influence each other, so that any imprecision in today's trajectories becomes multiplied by a factor of ten every 10 million years. Therefore, astronomers will never be able to predict the positions of the planets beyond a few hundred million years out--even if the imprecision started out less than a trillionth of a millimeter. In the long run, we only have probabilities.

[ILLUSTRATION OMITTED]

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Title Annotation:News Notes
Publication:Sky & Telescope
Article Type:Brief article
Geographic Code:1USA
Date:Oct 1, 2009
Words:229
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