Gateway

University of Arizona/Heather Roper

In the outer solar system, between the orbits of Jupiter and Neptune, are a group of small bodies known as the centaurs. Just as the mythical centaurs are hybrids of horse and man, these centaurs have hybrid properties between comets and asteroids.

It is thought that centaurs began their life as icy bodies in the Kuiper belt, well beyond the orbit of Neptune. As small gravitational disruptions shifted their orbit, they would swing inward toward the Sun. The tug of the outer planets’ gravity would then throw them into a centaur orbit.

But the orbits of centaurs are relatively unstable. Most centaurs have orbits that cross the orbits of one or more outer solar system planet, and it is only a matter of time before they swing close enough to a planet for their orbit to shift dramatically. Over cosmic time they would tend to be thrown toward the inner solar system, becoming what are known as Jupiter-Family Comets (JFCs).

Because of this, it’s been a bit of a mystery as to how centaurs are replenished. As some are thrown out of the centaur region, new icy bodies must join their fold. To study the dynamics of these bodies, a team recently modeled the orbits of centaur-like bodies.¹ They found a dynamical gateway where objects from the Kuiper belt enter the centaur region and are then thrown into the inner solar system. This dynamical region of the centaurs is near the orbit of 29P/Schwassmann-Wachmann 1 (SW1), which is a strangely active centaur.

Studying the orbit of SW1 was the original goal of the team, and what they found was that rather than being a strange member of the centaurs, it is a body in the process of transitioning to the inner solar system. In time it will join the Jupiter-Family Comets.