Moons of Minor Planets: Revisiting an old visualization

With the news that the Near-Earth Asteroid 1998 QE2 has a small natural satellite of its own percolating through the 'net, I thought it might be a good time to re-visit one of my first animated visualizations. During my PhD, I studied a sample of very widely-separated binary objects in the Kuiper Belt - these are like distant asteroids with moons of their own, but in many cases the moon (the 'secondary') is nearly as large is the main object (the 'primary'). One of the objects I was studying was 2006 CH69, which at the time had a temporary designation of L5c02. After a series of very careful observations, I measured the orbit of the secondary of 2006 CH69 around the primary. It turned out that the system was remarkably eccentric, with e=0.90±0.02 - making it the most eccentric binary minor planet with a well-measured orbit. Given the other properties of the orbit, this meant that when they were at their closest, the two bodies in the system were only separated by about 2,800 kilometers, while at the other extreme of their orbit about each other they were separated by over 52,000 kilometers. This huge swing between the two extremes takes place over half an orbital period, or roughly 2 years.

When I discovered this, I wondered what it would be like to stand on the surface of the primary of 2006 CH69 and look up at the secondary over the course of their mutual orbit. Using a rough calculation of the size of the secondary body, I determined its apparent size as seen from the surface of the primary, and compared it to the apparent size of the full moon (our 'secondary') as seen from the surface of the Earth (our 'primary').

At the time, I was just learning the ropes of how to make effective visualizations, but I put the following animation together:

On the right you see the apparent size of the secondary - here referred to with its earlier designation of L5c02b - as seen from the surface of the system primary. The animation starts with the system at their most widely separated, with the secondary appearing roughly 1/5th the size of the full moon. At their closest half an orbit later, L5c02b grows to appear over 3 times the size of the full moon on the sky!

I took some liberties with this animation - we don't know what the rotation period of L5c02b is, nor do we know if it has any surface markings, or even if it has a round shape! Future, more detailed studies will be needed to refine our understanding of the system.

I later made an animation showing the motion of six of the systems I studied, as seen from the Earth, and the observations we collected of them over the course of a decade.

Mutual Orbits of Ultra-Wide Trans-Neptunian Binaries from Alex Parker on Vimeo.

Click through to read the details of the animation. You can also learn more about these systems in an article I wrote for the Gemini Observatory Newsletter [pdf, page 23].