The U.S. space agency, NASA, said there was no chance that the asteroid would collide with Earth or the moon.
The size: it has a diameter estimated at 400 meters, or about a quarter of a mile. 2005 YU 55 is the biggest asteroid to make a close pass by Earth since 1976.
YU55's trajectory was taken as an opportunity for astronomers to do research into the asteroid's composition.
The last time a large cosmic interloper came that close to Earth was in 1976, and experts say it won't happen again until 2028.
Here's a video of the asteroid YU55.