Is Phoebe A Kuiper Belt Object?[T]he Kuiper Belt binaries might have formed more directly through low velocity collisions between KBOs. When the relative velocities are smaller than or comparable to the gravitational escape speeds from the colliding objects, some fraction of the collisions will "stick", resulting in peanut shaped contact binaries. Others will bounce, with some ejection of mass (and energy) allowing binaries to form. Still others will collide but not lose enough energy in order to become bound. In this scenario, the binary KBOs are products of low velocity (100 m/s) collisions in the Kuiper Belt.
by David Jewitt
Last updated Feb 2004
One problem is that collisions between 100 km sized KBOs are currently very rare: too rare to account for the inferred number of binaries. Independent evidence suggests that the Kuiper Belt was once about 100 times more massive than now, so this suggestion is not completely out of line. But another problem is that the known Kuiper Belt binaries tend to have components of comparable mass. Binaries produced collisionally are most usually highly asymetric in the masses of their components...
These ideas all require a much denser Kuiper Belt in order to be effective, and thus make the binaries "primordial" features of the Belt. Petit and collaborators have shown that the stability of wide binaries is marginal on timescales comparable to the age of the solar system. They argue that the existing binaries represent just a faction of those initially present.
Whoops. Forgot to fix the title, which is:
"Binary Kuiper Belt Objects"