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Double Impact

September 30, 2012

You don’t have to knock down all the bowling pins with a single roll — you can always try to pick up the spare. And the same thing may apply to Uranus, the solar system’s third-largest planet.

Instead of standing more or less upright, as the other planets do, Uranus lies on its side, so its axis aligns roughly with the plane of its orbit around the Sun.

The consensus has been that Uranus was knocked over by a single large impact — a collision with another planet-sized body. But one recent study suggests the planet’s orientation is the result of a few smaller impacts, not a single giant one.

The study was conducted by a team of European scientists. They found that if Uranus had suffered a single impact early in its history, then its moons would all orbit backwards — in the opposite direction from Uranus’s rotation — but they don’t. When the scientists used two or more smaller impacts, though, their simulations matched what we see today. That suggests that Uranus is the equivalent of a spare, not a strike — a planet that was knocked over by more than one planetary bowling ball.

Uranus is putting its best showing of the year now — it rises around sunset and is in the sky all night. It’s brightest for the year, too. But you need dark skies and binoculars to find it. Tonight, it’s well to the upper right of the Moon. But it’ll be easier to see in the nights ahead, as the Moon leaves the planet behind.


Script by Damond Benningfield, Copyright 2012

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