The Earth just had a close encounter with an asteroid about 200 to 400 ft (50 to 120 m) across that missed us by a cosmic whisker. On May 15 at 3:04 pm PDT, asteroid 2010 WC9 came within 120,000 miles of the Earth's surface while clocking a speed of 29,000 mph.
Tuesday's flyby of an asteroid well within the orbit of the Moon may seem frighteningly close, but NASA says that there was no danger of a collision. To put things into perspective, remember that, as Douglas Adams said, space is big, really big. So, although 120,000 miles (200,00 km) may seem like an ear grazing, the distances and sizes are such that it's like trying to shoot a large orange and missing by 200 ft (61 m).
According to the space agency, this was the closest pass that 2010 WC9 will make for the next two centuries and it poses no threat to the Earth in the foreseeable future. The asteroid was first discovered on November 30, 2010 by the Catalina Sky Survey near Tucson, Arizona when it made its last pass inside lunar orbit and was tracked by the NASA-sponsored project until December 10, 2010 when it became too dim to see.
Calculations made at the time indicated that 2010 WC9 posed no threat of impact with the Earth, but its precise trajectory could not be calculated and it was put down as lost until last week when it was recovered by the Mount Lemmon Survey.
Source: NASA
