Multiplicity in the Kuiper Belt: The First Discovery of a Binary Trans-Neptunian Object

Although it was generally assumed that some fraction of asteroids should have binary or multiple components, it wasn't until recently that the first binary asteroid was unambiguously discovered. Since then, a few more main belt binary asteroids have been found, as well as three binary near earth asteroids. Another reservoir of minor planets is the Kuiper belt, a large population of small bodies beyond Neptune that are believed to be the sources for short-period comets. Currently, over 400 of these trans-Neptunian Objects (TNOs) are known, which one may also include Pluto and its moon Charon as not only the first TNOs discovered, but the first known binary TNO system. Due to the Kuiper belt's distance from Earth, it seemed unlikely that other binary TNOs could be discovered at the present. However, we report here on the discovery and orbit of the ``first'' binary TNO, 1998 WW31, based on ground-based and Hubble Space Telescope observations. From the observed orbital parameters, we can estimate physical properties of the components.