Do you remember when Jupiter vacuumed up the pieces of the disrupted comet Shoemaker–Levy 9 in 1994? These impacts were a grim reminder of the danger faced by Earth from comet and Asteroid collisions.
We now know that this massive planet may cause orbital and velocity changes in any solar body that strays too close.
Jupiter's gravity is capable of throwing comets close to the Earth, but it is also equally adept at cleaning up its mess and removing dangerous comets from the solar system. Were Jupiter only to have one-fifth of its real mass, the balance between hurling comets towards us and then removing them would be lost; Jupiter would still be able to destabilize comets and send them our way, but it would lose the ability to remove many of them.
Jupiter's role seems confused. It definitely sends asteroids and comets our way and, in any given year, more than 90 percent of all objects crossing Earth's orbit are asteroids, so the protection Jupiter provides us from long period comets, or by eventually removing short period comets, is of lesser importance. Hence Jupiter is not the friend that it has been perceived to be. However, things could be far worse: were Jupiter to have a mere 20 percent of its mass, the impact rate would skyrocket.
Picture illusrates comet collisions on Jupiter in 1994
Impacts are the size of Earth!
Click picture for story