The difference in how Jupiter affects different groups of small bodies has to do with Jupiter’s role in each kind of object’s movements. Oort Cloud comets tend to stay in the far reaches of the Solar System, but sometimes they can be nudged into orbits that take them closer to the Sun. This can happen because of the close passage of a star or giant interstellar molecular cloud near the Solar System, or because of the gravitational forces, called disk tides, exerted by the mass of the galaxy as a whole. The forces are from very distant sources, but are mighty enough to affect the movement of small icy bodies.
When this does happen, Jupiter is well positioned to be our defender. It might suck a comet right into its own body, or might pull it in just enough to slingshot it out again, even right out of the Solar System.
With asteroids and short-period comets, Jupiter’s influence is likely what causes a small body to leave its current orbit and head off in a strange new direction. Simulations suggest that without a planet like Jupiter in the Solar System, there would be less gravitational disruption of asteroids’ and comets’ orbits, potentially causing fewer impacts with Earth and other inner planets.
Further work is needed to better understand Jupiter’s exact influence on impact rates on Earth. For now, the gas giant’s reputation as protector of the inner Solar System is not clear-cut.
Our best defense? Ourselves.
Jupiter may or may not be our ally in planetary defense, but ultimately, it’s our responsibility to defend ourselves. That’s why The Planetary Society is committed to advancing efforts to find, track, characterize, and deflect potentially dangerous asteroids and comets.