The asteroid 2005 YU55 may not look like much of a threat, but on Nov. 7, 2011, it came within 860,000 miles of Earth. You can watch a video of the asteroid from NASA here. This was the closest encounter it has had with Earth in the last 200 years. Roughly the size of the Empire State Building, the asteroid's orbit regularly brings it close to Venus and Mars. NASA extensively studied the asteroid from the Deep Space Network antenna at Goldstone, Calif. Using radar, NASA found “evidence for concavities, a ridge near the asteroid's equator, and numerous features that may be large boulders.” NASA studies these asteroids to determine if any might present a hazard to Earth.