The 8 - 18 metre wide asteroid 2008 GY21 will make a close pass (51.4 LD, 0.1321 AU) travelling at 15.00 km/second, to the Earth-Moon system on the 12th September 2015 @ 00:27 UT ±1 day 14:16.
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The Lunar Distance (LD), the distance between Earth and the Moon, equals 384,401 km, (or 0.00256 AU).