Hubble Highlights a Riddle: What’s the source of quick-return comets?

New observations from the Hubble Space Telescope are demonstrating that scientists don’t know where a major class of comets comes from.

Until recently, most planetary scientists had assumed that comets that take less than 20 years to orbit the sun originally resided in the Kuiper belt, a doughnut of icy material left over from the formation of the planets 4.5