When asteroid 2004 BL86 whizzed by Earth on Monday, NASA got a close look at it. It turns out the 1,100-foot asteroid has a small moon orbiting it. The moon is just 230 feet in diameter, or a little less than the length of a football field.

Asteroids with moons encircling them aren't rare; NASA estimates that about 16 percent of asteroids that are 200 meters (655 feet) or larger have one or even two moons orbiting them. However, it's not every day we get to see one of these systems so close up.

The binary asteroid kept a safe distance from Earth. At its closest it was about 745,000 miles away, or three times farther than the Moon.