Earth followed through space by asteroid

ASTRONOMY
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The Independent Online
As it orbits the sun, the Earth is being preceded through space by an asteroid the size of Islington - though scientists are confident there is no risk of it crashing in to us.

Known simply as 3753, the asteroid is about 5km (3.1 miles) across, and on an orbit inclined at 20 degrees to that of the sun and planets. It was discovered a decade ago, but scientists have now worked out that its orbit is locked to the Earth's. As it moves ahead of us, its path relative to the Earth looks like a kidney bean, so that it actually takes 150 years to return to exactly the same point in space. The closest approach to Earth happens every 385 years, when it is 40 times further away than the Moon.

"This behaviour is not unusual in itself," say the researchers, led by Paul Weigert at York University in Ontario, Canada, in the science journal Nature. "What distinguishes 3753 from other near-Earth asteroids is its behaviour as it approaches Earth: our planet's gravitational pull acts to increase the asteroid's period from slightly below to slightly above one year."

The effect of that slowing effect on the asteroid's path is also to keep us alive: our planet's gravitational pull makes the asteroid begin to fall behind the Earth's orbit, and it therefore moves away from Earth, avoiding a collision.

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