That's no Moon! Oh, wait...

All the worlds doomsayers have been on high alert for the last week as the asteroid 2004 BL86 passed close to the Earth. Of course, when we say close we mean in space scale, which was 1.2 million kilometres, or around 3 times the separation of the Moon and the Earth. The asteroid unsurprisingly passed safely past, as expected (there are many organisations that focus on predicting the paths of asteroids, to watch out for any dangerous ones).

After the panic period, many people were able to spot the asteroid in the night sky last night, which was relatively bright. Such bright asteroids are a rather rare sight, so its always a treat to be able to spot one. But whilst many of us were trying to spot the asteroid with our eyes or binoculars, the team at NASA's Deep Space Network antennae was taking some incredible radio images of the asteroid. Radio waves are reflected off the asteroid which can be seen by the antennae, allowing scientists to see the asteroid even if it isn't illuminated by visible light form the Sun.

http://www.jpl.nasa.gov/images/asteroid/20150126/2004BL86-640.gif

The radio images have allowed scientists to measure the size of the asteroid and study some of the details of its surface. But it was one unique feature that they didn't expect to see. The asteroid has a moon! Just as the Earth has a large body orbiting around it, our very own Moon, the asteroid 2004 BL86 also has a piece of rock orbiting around it. It's had to think of a body 1/40th the diameter of the Earth having enough gravity to have its own visible moon. You can see a time lapse of the asteroid above, where its moon can be seen as the bright spot moving down from the top of the image.

But whilst it was a surprise to the scientists that this particular asteroid has a moon, it's not overall a surprise, as many moons have been found orbiting around asteroids in our Solar System already. Still, it's not something we get to see very often, let alone around an asteroid that we can see clearly in the sky.