The comet HIGHWAY

WHEN TALKING ABOUT COMETS, we often refer to them as visitors from the outer Solar System (or beyond). But not all comets hail from the same place. Some originate from the Solar System’s most distant region, the spherical cloud of debris loosely bound to the Sun called the Oort Cloud. Such objects include the memorable recent visitor Comet NEOWISE (C/2020 F3), which will not return for another 7,000 years. Many other comets, though, originate in the relatively closer Kuiper Belt, a population of small, icy bodies beyond Neptune’s orbit. These comets become more frequent repeat visitors to the inner Solar System, with orbital periods ranging from a few to about 200 years.

Before we knew about the Kuiper Belt, a group of these short-period comets puzzled astronomers. Referred to as

Jupiter-family comets (JFCs) due to Jupiter’s dominant role in controlling their orbital evolution, these visitors have distinct properties. They generally take less than 20 years to complete a trip around the Sun, spending much of their time within the region encircled by Jupiter’s orbit. A key trait, however, is the orientation of their orbits. While long-period comets zip through the inner Solar System from essentially all directions, JFCs overwhelmingly orbit the Sun on paths tilted less than about 30° from the planetary plane, and they travel in the same direction as the major planets, too. What could channel comets into the inner Solar System like this? And where did these objects come from originally?

We now know the answers to these questions. Pristine bodies from the Kuiper Belt slowly feed