2003 UB313 has a satellite

Oh boy! Brown using Keck and the adaptive optics there managed to detect an object moving in the same direction as 2003 UB313 and about a half an arcsecond away. When further observations are made and a period found for the satellite, the mass of the parent body will be known! MPML for a forwarded announcement or all the press stories.

The Keck 10m scope can easily beat the resolution of the Hubble Space Telescope when it is using the adaptive optics system, but it only beats Hubble over a very small patch of the sky at a time. It's great for looking at singular near point-source objects.

How does knowing a moon's orbit give you the mass of the parent? Here is what you know: You know the angular distance between the moon and the parent, which gives you the actual distance away from the planet, since you also know the distance to the planet from the Earth. Using Newton's formula of F=G*M₁*M₂/R^{^2}, and that the apparent centripetal force for the moon is F=M₂v^{^2}/R, the two forces are equal, so you get that the velocity is equal to the square root of (G*M₁/R).
The period is equal to 2pi*R/v.

So, the period is equal to 2pi*R/(square root of (G * M₁/R).
Or you can say the Mass of the planet is equal to 4pi * R^{^3}/G * T². I think. Okay, I went back and checked some sites, and my derivation looks correct. This site has a good look at it.