The Unusual Pluto-Charon Binary Orbit

Pluto Animation - Click for Full SizeThe New Horizons spacecraft is just two months away from Pluto, so I find myself checking online compulsively for photographs. As of now, they are still pretty crummy: it’s still just about ten pixels on a side. I know: it’s small, it’s dark. But I’m excited. I’ve been waiting close to fifty years to get a good look at this over-hyped piece of space debris. I would think that we would get better and better pictures each day. But instead, I assume they aren’t taking pictures all the time because it takes power and they want to save as much of that as they can. The picture there on the left is about the best we have, which dates from last month.

But I learned something really interesting from an article by Alan Stern, Pluto: The Last Picture Show. Pluto and its relatively large moon Charon constitute an actual binary system. Now up until now, I’ve always been bugged by this term. The reason is that all planets with a single moon are technically binary systems. Take, for example, the Earth and Moon. The Moon does not orbit around the Earth. Rather, both the Earth and Moon orbit around the center of mass of the two objects. (Actually, it is far more complicated than that because technically, every other object in the universe is effecting the orbit — but don’t worry about that.)

With the Earth and Moon, the system is so dominated by the Earth that the center of mass is inside the Earth. The Moon has a mass of only a bit more than 1% of that of the Earth. But they are far apart: roughly 400,000 km. That puts the center of mass about 2,000 km inside the surface of the Earth. Charon, in contrast, is really large compared to Pluto: almost 12% its mass. On the other hand, it is close to Pluto: less than 20,000 km, or 20 times closer than the Moon is to Earth. But this puts the center of mass about one Pluto radius above the surface of the dwarf planet. That’s why you see Pluto in the image above moving in a circle. The same kind of image of the earth would show the same kind of thing, but the Earth would just be circling around itself — displaced, not really orbiting.

I don’t actually know of any other pair of objects in our solar system that exhibit this behavior. The truth is that the Moon with 1.2% of the mass of the Earth is highly unusual. Moons are usually far smaller than the planets that they orbit. For example, Phobos is just 0.00000002 times the mass of Mars. So the fact that Pluto managed to grab onto something as large as Charon is incredible. Of course, it probably was not the case that Charon was just sailing past and got captured by Pluto. It might be something more like how the Earth and Moon formed with a collision that resulted in two bodies. Or something else entirely.

Alan Stern summed up my feelings about this whole thing, “So, we’re just two months out — it’s nearly show time. What will we find? Not to tweak you, but I don’t know. No one does. That’s what makes this distant exploration so very exciting, so suspenseful, and so wonderful!” The truth is that we know surprisingly little about Pluto. And in just two months, New Horizons is going to come within 10,000 km of Pluto. That’s closer than Charon ever gets to the dwarf planet. It ought to be very exciting indeed!

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