An unusually large asteroid that just skimmed by Earth had its own moon.
The asteroid, known as 2004 BL86, made its closest approach overnight at a distance about three times further than Earth's own Moon.
Radar images from NASA's Deep Space Network antenna in Goldstone, California show that the asteroid itself was about 150 metres smaller than expected, and measured about 325 metres across.
The asteroid's small moon was approximately 70 metres across.
Canadian astronaut Chris Hadfield called the discovery "strangely delightful," and wondered on Twitter: "Who gets to name it?"
The asteroid was already unusual because it was about ten times bigger than most near-Earth objects, which range from 15 to 30 metres in diameter.
NASA said that about 16% of asteroids in 2004 BL86's size category - meaning those that are 200 metres or larger - are "binary," meaning there is a primary asteroid with a smaller asteroid moon orbiting it.
Some even have two moons.
The odd couple of 2004 BL86 and its moon will not come this way again for another two centuries, astronomers say.
The next big space rock known to be heading this way is asteroid 1999 AN10, which NASA said should fly past Earth in 2027.
