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Near-Earth asteroid has its own little moon in tow

When asteroid 2004 BL86 passed near Earth on January 26, it wasn't alone. A small moon came along for the ride.

Vesta as seen by Dawn Mission.
The giant asteroid Vesta is much larger than 2004 BL86, but doesn't have its own moon. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It's not terribly often we get a visiting asteroid that comes close enough for people on Earth to see it with a strong set of binoculars. That's what happened on January 26 when asteroid 2004 BL86 swooped through our space neighborhood. Scientists and amateur astronomers alike had their eyes on the object, and NASA was especially thrilled to notice it brought some company.

Asteroid 2004 BL86 has a tiny moon in tow. The main asteroid is 1,100 feet across. Its mini-me moon is just 230 feet across. It's unusual, but not unheard of. NASA notes that about 16 percent of near-Earth asteroids larger than 655 feet have a moon, or even two moons.

For comparison, look to the asteroid Vesta, which is located in an asteroid belt 150 million miles away from Earth. NASA's Dawn Mission got a good look at Vesta, which is around 330 miles across. That makes 2004 BL86 look like an ant. When the Dawn Mission approached Vesta in 2011, scientists were on the lookout for a moon, but didn't find one.

The asteroid passed by with plenty of room to spare. The closest it came was 745,000 miles away from our planet. It was still a special event since we'll have to wait until 2027 for another asteroid of this size to come within space-spitting distance of Earth (asteroid 1999 AN10, if you want to write it on your calendar). It will take a couple of centuries before 2004 BL86 and its moon will pass by as close as it did this time around.

The asteroid's moon was discovered by researchers using NASA's Deep Space Network antenna to capture radar images. Scientists put together a movie using the data that shows the dainty moon zipping along with 2004 BL86.