Answer to this week's riddle

Travis Hale

Again, the riddle:  

Recent estimates suggest that there are at least 20,000 of what with diameters greater than 330 feet? 

Answer:  Near Earth Objects – asteroids and microplanets that could imperil the Earth, should they collide.

Source: http://www.foreignpolicy.com/articles/2012/12/12/paging_bruce_willis