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On this night in 1781, William Herschel made history by discovering a new planet: Uranus, the seventh planet from the Sun. And six years later, Herschel discovered the planet's two largest moons.
Today, we know that Uranus has at least 27 moons. But one of these moons may someday escape.
The wayward moon is named Margaret. It's a bare chunk of rock that's only a few miles across.
Margaret is so far from Uranus that it takes four and a half years to complete a single orbit around the planet. And it follows an odd path. For one thing, its orbit is tilted at an odd angle.
For another, its orbit is highly elliptical -- it's stretched out. On average, at its closest, Margaret is three million miles from the planet -- more than 10 times as far as our moon is from Earth. At its farthest, though, Margaret voyages 15 million miles from its master -- more than 60 times as far as the Moon is from Earth.
But Uranus may not be Margaret's master forever. That's because the shape of the little moon's orbit changes. Sometimes the orbit is less elliptical; other times, it's even more elliptical.
When that's the case, Margaret ventures even farther from the planet -- and may eventually escape its grasp completely.
If that happens, Margaret will transform itself from a moon of Uranus into an asteroid orbiting around the Sun, most likely in the same orbit as Uranus, its former master.
Tomorrow: lining up the planet Mars.
Script by Ken Croswell, Copyright 2010