If you've been watching the skies for the spectacle of the Perseid meteor shower over the past few days, you've probably thought about some of the larger rocks up above. A few are downright scary compared to shooting stars. This graphic from NASA shows the orbits of more than 1,400 Potentially Hazardous Asteroids (PHAs), objects measuring at least 460 feet across that come within 4.7 million miles of Earth's orbit. It looks like a giant Spirograph drawing with Earth near the center, but NASA assures us that these giant boulders of rock and ice are not "a worrisome threat" over the next 100 years. Of course, 4.7 million miles is really, really far away -- about 20 times the distance to the moon. But not all PHAs have been found. As one looks further into the future, the orbits of known PHAs become less predictable. Read Tim Hornyak's story on CNET.
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