Asteroid 2010 WC9 will safely pass by us at about half the moon's distance today at about 3pm (Pacific Time) this afternoon. Estimated size ranges from 197-427 feet, making this asteroid one of the closest approaches ever observed by an asteroid this size.
The Catalina Sky Survey in Arizona first detected the asteroid on November 30, 2010, hence the name, and then it became so faint that astronomers lost track of it. They didn't have enough observations of it to track it's path in its entirety to predict its return. Almost 8 years later, astronomers discovered an asteroid and gave it a temporary designation ZJ99C60 until they realized it was asteroid 2010 WC9 making a return.
Asteroid 2010 WC9 is an Apollo type rock meaning that it has an Earth crossing orbit. At no time will this asteroid be visible to the naked eye as it moves past Earth at a speed of 28,655 miles per hour.
The last time an Apollo type asteroid crossed an Earth orbit in 2013 it did collide with Earth. The Chelyabinsk meteor actually entered Earth's atmosphere, breaking windows in six Russian cities and caused more than 1,500 people to seek medical attention. The estimated size of the Chelyabinsk meteor was approximately 65 feet.