There's no chance the asteroid will hit Earth, scientists say, but at 777,000 miles away -- 3.25 times the distance from the Earth to the moon -- it's a relatively close call.
The asteroid is about 10 to 20 times bigger than the one that injured 1,000 people last year in Chelyabinsk, Russia, Bob Berman, of the Slooh community observatory said. That asteroid was 55 feet wide.
The asteroid was first spotted April 23 by NASA's Wide-Field Infrared Survey Explorer.
"What's disconcerting is that a rocky/metallic body this large, and coming so very close, should have only first been discovered this soon before its nearest approach," Berman said. "If it were impact us, the energy released would be measured not in kilotons like the atomic bombs that ended World War II, but in H-bomb type megatons."