An unusually large asteroid that just skimmed by Earth had its own moon, Nasa said Tuesday as the US space agency released its first radar images of the flyby.
The asteroid known as 2004 BL86 made its closest approach late Monday 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 500 feet (150 metres) smaller than expected, and measured about 1,100 feet (325 metres) across.
The asteroid's small moon was approximately 230 feet (70 metres) across.
Canadian astronaut Chris Hadfield called the discovery "strangely delightful," and wondered on Twitter: "Who gets to name it?"
Asteroid 2004 BL86 which flew near-Earth. (Nasa)
The asteroid was already unusual because it was about 10 times bigger than most near-Earth objects, which range from 50 to 100 feet (15 to 30 metres) in diameter.
Nasa said that about 16 per cent of asteroids in 2004 BL86's size category - meaning those that are 655 feet (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.