If looked up at the sky this week to see the passing of a remarkably large asteroid, you had a decent chance of actually seeing it; even a good pair of binoculars was enough to watch the mountain-sized space rock fly just past the moon. However, what you probably couldn’t see with amateur equipment is the even smaller bodies orbiting around this asteroid.
That’s right: our good friend 2004 BL86 has a moon!
Asteroids sometimes do have “moons,” or satellites smaller than themselves, and this makes them so-called binary asteroids. In fact, astronomers have even discovered three-member asteroid systems, as it’s assuming that even more elaborate setups exists somewhere out there. What makes this binary asteroid so interesting is what has always made 2004 BL86 so interesting: its proximity. By viewing the system this close-up, astronomers hope to learn far more about how such seemingly improbable objects could exist.
Conceptually, it’s not that difficult to imagine how such a thing might happen, though. An oblique impact between two large bodies would throw off debris of all sizes in a wide spray — some of these objects would be released at too shallow an angle and settle back on the surface of the asteroid, while some of the rest would move at too steep an angle and achieve escape velocity. Some portion of the material in the middle, though, would be released at the correct angle for its speed and mass, settling into a stable orbit as it falls endlessly along this near-perfectly balanced vector. With no atmosphere to drag against, this thin sliver of the debris spray can orbit for a very long time — long enough to run into us, for instance.
As to how an object as small as an asteroid could have enough mass to hold even a small moon through gravity, many scientists believe that the orbiting satellite rocks are so-called “rubble pile” bodies, Swiss-cheesed with enormous holes that make them very light for their size. These low-density rocks would require less of a pull to stay orbiting a body, and their porous nature doesn’t slow them down as it would in air. 2004 BL86 is about 325 meters across, while its moon is about a fifth as large at 70 meters. It’s weird to think that “debris” could constitute a significant fraction of the planet it came from, but remember that Pluto’s moon Charon is actually more than half the dwarf planet’s size.
However, it has previously been noted that binary asteroids are far more common than expected, nearer to the Sun. One proposed explanation for this is that solar radiation causes the asteroids to spin rapidly, which causes more material to be thrown outward from the equator, which creates more chances for debris to fall into a stable orbit. As asteroids leave this area and their rotation slows, imperfect satellites start to crash down or spin off without being replaced at the same rate, and the occurrence of asteroid-babies goes down.
All this uncertainty will hopefully be lessened as astronomers take the time to pour over information collected from 2004 BL86, a near-miss binary asteroid that should be making news for at least a few months to come.