Miami - An unusually large asteroid that just skimmed by Earth had its own moon, Nasa said on 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 on 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 150m smaller than expected, and measured about 325m across.
The asteroid's small moon was approximately 70m 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 10 times bigger than most near-Earth objects, which range from 15 to 30m in diameter.
Nasa said that about 16% of asteroids in 2004 BL86's size category - meaning those that are 200m 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.